Acids, bases and salts class 10

 Introduction :-

Properties of acids :-

• Produce hydrogen ions [H+] ions in water.
• Sour taste
• Turn blue litmus red
• Act as electrolytes in Solution.
• React with several metals releasing hydrogen gas.
• React with carbonates releasing CO2 (g)
• Destroy body tissues.
• Corrode metal surface quickly.

On the basis of origin, acids are classified as:- 

Organic acids :- 

• Acids derived from living organisms like plants and animals.
• For example :- 
• Citric acid is present in fruits e.g. Citrus reticulate (orange)  and Citrus lemon (lemon)
• Acetic acid present in vinegar.
• Oxalic acid present in tomato.
• Tartaric acid present in tamarind.
•  Lactic acid present in sour milk and curd.

Mineral acids :- 

• Also called inorganic acids .
• They are dangerous.
• For example :- Sulphuric acid [H2SO4] , Hydrochloric acid [HCl]

On the basis of their strength, acids are classified as:- 

Strong acids and weak acids.

Strong acids :- 

• Complete dissociation into its ions in aqueous solutions.
• Example:- Nitric acid [HNO3] ,
• Sulphuric acid [H2SO4],
• Hydrochloric acid [HCl] etc.

Weak acids:- 

• Acids which do not completely dissociate into its ions in aqueous.
• Example:- Sulphurous acid [H2SO4] , carbonic acid [H2CO3], Acetic acid [CH3COOH]

On the basis of their concentration, acids are classified as :- 

Dilute acids :- have a low concentrations of acids in aqueous solution.

Concentrated acids:- have a high concentration of acids in aqueous solutions.

On the basis of number of hydrogen ion, acids can be classified as:- 

Monoprotic acids :- 

Acids which produces one mile of H+ ions per mole of acid. Example:- HCl , HNO3

Diprotic acid:- 

Acids which produces two miles of H+ ions per mole of acid. Example:- H2SO4.

Triprotic acid:- 

Acids which produces three moles of H+ ions per mole of acid. Example:- H3PO4 (phosphoric acid)

Polyprotic acid:- Acids which produces more than three moles of H+ ions per mole of acid.

Chemical properties of acids:- 

(1) Acids react with active metal to give hydrogen gas.

• Acid + metal -- > salt + hydrogen gas.
• HCl (l) + Zn (s) --- > ZnCl2 (s) + H2(g)
• H2SO4 (l) + Zn (s) --- > ZnSO4 (s) + H2 (g)

(2) Acid react with metal carbonate and metal hydrogen carbonate to give carbon-di-oxide.

• Metal carbonate / Metal hydrogen carbonate + Acid -- > salt + water + carbon dioxide
• Na2CO3 (s) + 2HCl (aq) --- > 2NaCl(aq) + H2O (l) + CO2
• NaHCO3 (s) + 2HCl (aq) --- > 2NaCl(aq) + H2O (l) + CO2

(3) Acids react with bases to give salt and water . This reaction is called as neutralization reaction.

• NaOH + HCl --> NaCl + H2O
• Base + acid --> salt + water
• (4) Acids react with metal oxides to give salt and water.
• CuO + H2SO4 -- > CuSO4 + H2O

Properties of base :- 

• Produce hydroxide ions [OH-] in water.
• Water soluble base are called alkalies.
• Bitter taste 
• Corrosive in nature.
• Turn red litmus blue.
• Act as electrolyte in solution.
• Neutralize solutions containing H+ ions.
• Have a slippery, "soapy" feel.
• Dissolve fatty material.

On the basis of their strength, bases are classified as :- 

(a) Strong bases :-

•  bases which completely dissociate into its ions in aqueous solution.
• Example:- sodium hydroxide [NaOH]
• Potassium hydroxide [KOH] ,
• Lithium hydroxide [Li(OH)],
• Calcium hydroxide [Ca(OH)2]

(b) Weak bases :- 

• Bases which donor completely dissociate into its ions in aqueous solution. 
• Example:- Ammonium hydroxide [NH4OH] , Magnesium hydroxide Mg(OH)2

On the basis of their concentration, bases are classified as :-

(a) Dilute bases :- Have a low concentration of alkali in aqueous solution.

(b) Concentrated bases:- have a high concentration of alkali in aqueous solution.

Chemical properties of bases:- 

(a) Reaction with metals :- 

• Certain reactive metals such as zinc, aluminium and tin react with alkali solution on heating and hydrogen gas is evolved
• Example:- Zn + NaOH -- > Na2ZnO2 + H2

(b) Reaction with acids :-

• Base react with acids to form salt and water. 

• Example :- NaOH + HCl -- > NaCl + H2O
• KOH + HCl -- > KCl + H2O

(c) Reaction with non-metallic oxides:- 

• Non metallic oxides are generally acidic in nature .
• Non metal  oxide+ base -- > salt + water
• CO2 + NaOH -- > Na2CO3 + H2O

Strength of acid or base solutions :- 

• A. Scale for measuring hydrogen ion concentration in a solution , called pH scale has been developed.
• The p in PH stands for "potenz " in German, meaning power . So p = potential or power H = hydrogen.

pH = 7 i.e. neutral solution i.e H3O+ = OH-

pH > 7 i.e basic solution i.e H3O+<OH-

pH >7 i.e Acidic solution i.e H3O+ >OH-

pH of some common substances.

• Gastric juice = 1.2pH
• Lemon juice = 2.2 pH
• Wine = 4.0 pH
• Rain = 5.5pH
• Pure water, human blood = 7.5 pH
• Baking soda = 8.5 pH
• Detergent , Milk of magnesia = 10.0 pH
• Bleach = 12.5pH
• Sodium hydroxide solution= 14pH

Importance of pH in every day life:-

(1) pH sensitivity of plants and animals:- 

• Human body works in a narrow range of pH 7.0 to 7.8 
• When acid rain [pH<5.6] flows into rivers, it lower the pH of river water.
• Thus survival of aquatic life in such river becomes difficult.
• So acidity can be lethal for plants and animals.

pH in human digestive system:-

• Stomach secretes HCl to kill bacteria in food.
• Mucus prevent the inner lining of stomach from corrosive nature of HCl.
• During indigestion the stomach produces too much acid and cause pain and irritation.
• To get rid of this pain , people use antacids such as magnesium hydroxide (milk of magnesia) ; a base to neutralize excess acid.

pH and tooth decay:- 

• Tooth enamel is the hardest substance in body; made up of calcium phosphate.
• It does not dissolve in water but corroded when pH of mouth is lower than 5.5 
• This acidic medium is produced by degradation of sugar and food particles present in mouth.
• To prevent tooth decay we clean our mouth after eating.
• Also clean teeth by toothpaste ( basic) to neutralize excess acid.

(4) pH as self defence mechanism by plants and animals:-

• Certain animals like bee and plants like nettle secrete highly acidic substances for self defence. 
• Both secrete methanoic acid.

Addition of acids or bases to water:-

• The process of dissolving an acid , specially nitric acid [HNO3] or sulphuric acid [H2SO4] or a base in water is a highly exothermic.
• As a rule :- Always add acid to water not water to acid.
• Acid must be added to water with constant stirring.
• Water must not be added to acid because heat generated cause the mixture to splash out and cause burns.

Indicators:-

• Indicators are those chemical substances which behave differently in acidic and basic medium and help in determining the chemical nature of the substances.
• Acid Base indicators indicate the presence of an acid or a base by changes in acidic or basic medium.

Indicators can be natural or synthetic.

Olfactory indicators:- Those indicators whose odour changes in acidic or basic medium.

Onion:- smell of onion diminished in a base and remain as it is in an acid.

Vanilla :- odour of vanilla essence disappears when it is added to a base.

Odour of vanilla essence persists when it is added to an acid.

Turmeric:- In acids , yellow colour of turmeric remains yellow.

In bases, yellow colour of turmeric turns red.

Phenolphthalein:- Phenolphthalein remains colourless in acids but turn pink in bases 

Methyl orange:- methyl orange turns pink in acids . It becomes yellow in bases.

Litmus:- 

• Litmus is a natural indicator. It's solution is a purple dye which is extracted from lichen.
• Acids turn blue litmus red.
• Bases turn red litmus blue.
• Water is essential for acids and bases to change colour of litmus paper.

Note:- litmus paper will act as an indicator only if either the litmus paper is moist or the acid or base is in the form of aqueous solution

Reason:- Bases and acids release OH- and H+ ions respectively in aqueous solutions.

Summary of indicators:- 

Indicator->acidic med ->basic med
Litmus solution --> Red --> Blue
Methyl orange--> pink --> yellow
Phenolphthalein-> colourless-> pink
Turmeric-->yellow-->red

Properties of salts:-

• Salts forms by combination of acid and base through neutralization reaction.
• The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction.
• The most common salt is sodium chloride NaCl or table salt which is forms by combination of sodium hydroxide and hydrochloric acid.
• NaOH + HCl -- > NaCl + H2O
• Other examples include Epsom salts [MgSO4] used in both salts , ammonium nitrate [NH4NO3] used as fertilizers and baking soda [NaHCO3] used in cooking.
• The pH of salts solution depends on strength of acid and base combined in neutralization reaction.

Common salt :- A raw material for chemicals such as 

• Sodium hydroxide
• Baking soda
• Washing soda
• Bleaching powder etc.

Sodium hydroxide:-

• When electricity is passed through an aqueous solution of sodium hydroxide (brine) ,it decompose to form sodium hydroxide.
• 2NaCl (aq) + 2H2O (l)---> 2NaOH (aq) + Cl2(g) + H2(g)
• The method is called chlor alkali process.
• Chlorine gas is given off at anode.
• Hydrogen gas at cathode.
• Sodium hydroxide solution is formed near cathode.

Uses:-

H2 GAS:- 

• used as fuel.
• Ammonia is used for fertilizer.

Cl2 GAS:- 

• used in water treatment,in swimming pool.
• Pesticides, CFCs , disinfectants etc. 
• H2 + Cl2 --> HCl.

Use of HCl :- 

• Used in cleaning steel
• In manufacturing of NH4Cl
• In medicine and cosmetics

Uses of NaOH:-

•  In de-graeasing metals, soaps and detergents.
• In paper making
• In artificial fibres.

Bleaching powder:- CaOCl2

• Produced by the action of Chlorine on dry slaked lime [Ca(OH)2]
• Ca(OH)2 + Cl2 -- > CaOCl2 + H2O

Uses:- 

(1) For bleaching

• Cotton and linen in textile industry.
• Wood pulp in paper factories.
• Washed clothes in laundry.

(2) as an oxidizing agent in many chemical industries.
(3) for disinfecting drinking water to make it free of germs.

Baking soda:- NaHCO3

• It is produced using NaCl , H2O , CO2 and NH3 as raw material.
• NaCl + H2O + CO2 + NH3 -- > NH4Cl + NaHCO3
• When heated
• 2NaHCO3 + heat --> Na2CO3 + H2O + CO2.
• It is a mild base.

Uses:- 

• Used in kitchen for making crispy pakoras and for faster cooking.
• For making baking powder (mixture of baking soda and mild edible acid such as tartaric acid)
• When baking powder is heated and mixed with water following reaction takes place:- 
• NaHCO3 + H+ (from any acid) --> CO2 + H2O  + sodium salt of acid
• CO2 produced during reaction causes bread or cake to rise making them soft and spongy.
• Used as antacid .
• Used is soda acid fire extinguishers.
• Used in industrial processes.

Washing soda:- Na2CO3. 10H2O

• Produced by heating Baking soda and then recrystallization of sodium carbonate.
• Na2CO3 + 10H2O -- > Na2CO3.10H2O

Uses:- 

• used in glass , soap and paper industries 
• Used in manufacture of sodium compounds such as borax.
• Used as a cleaning agent for domestic purpose.
• Used for removing permanent hardness of water.

Are the crystals of salts really dry?

• No because they contain water of crystallization.
• When heated the crystals, water is removed and colour changes .
• If moisten the crystals again with water, colour of crystals reappear.
• Example:- hydrated copper sulphate -- > CuSO4.5H2O
• Hydrated sodium carbonate --> Na2CO3.10H2O
• Gypsum CaSO4.2H2O

Plaster of Paris (CaSO4.1/2H2O)

• On heating gypsum at 100°C  ,it loses water molecules and become calcium sulphate hemihydrate (CaSO4.1/2H2O)
• This is called plaster of Paris.
• Uses :- used by doctors for supporting fractured bones.
• Used for making toys , materials for decoration for making surface smooth.

Characteristics:-

• white powder 
• On mixing with water, it changes to gypsum once again giving a hard solid mass.
• Ca SO4.1/2H2O + 3/2 H2O -- > CaSO4.2H2O

Class 10 Ques ans

Page number 6

Question 1
Why should a magnesium ribbon be cleaned before burning in air ?
Answer:
Magnesium gets covered with a layer of magnesium oxide when kept in air for a long time. This layer hinders the burning of magnesium. Hence, it is to be cleaned before burning.

Question 2
Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H2 + Cl2 → 2HCl
(ii) 3 BaCl2 + Al2(SO4)3 → BaSO4 + 2 AlCl3
(iii) 2Na + 2H2O → 2NaOH + H2↑

Question 3
Write a balanced chemical equation with state symbols for the following reactions :
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water

Answer:
(i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H2O(l)

Page Number: 10

Question 1
A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer:
(i) The substance whose solution in water is used for white washing is calcium oxide (or quick lime). Its formula is CaO.

CaO (s) + H2O (l) -- > Ca(OH)2 (aq)

Question 2
Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas. [CBSE 2015 (Delhi)]
Answer:
In Activity 1.7, water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode.

2H2O(l) → 2H2(g) + O2(g)
Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.

Page Number: 13

Question 1
Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
OR
An iron nail is dipped in the solution of copper sulphate for about 30 minutes. State the change in colour observed. Give reason for the change. [CBSE 2015 (Delhi)]

Answer:
When an iron nail is dipped in copper sulphate solution, the displacement reaction takes place. The colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate.

Fe + CuSO4 --- > FeSO4 + Cu

Question 2
Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT Text Book).
Answer:
Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.

NaOH (aq) + HCl (aq) -- > NaCl (aq) + H2O (l)

Question 3
Identify the substances that are oxidised and the substances which are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO (s) + H2(g) → Cu (s) + H2O(l)
Answer:(i) Substances oxidised is Na as it gains oxygen and oxygen is reduced.

(ii) Substances reduced is Cu as hydrogen is oxidised as it gains oxygen.

NCERT Solutions for Class 10 Science Chapter 1 Textbook Chapter End Questions

Question 1
Which of the statements about the reaction below are incorrect ?
2 PbO(s) + C(s) → 2Pb (s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced

(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All
Answer:
(i) (a) and (b)

Question 2
Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a

a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Answer:
(d) Displacement reaction.

Question 3
What happens when dilute hydrochloric acid is added to iron filings ? Tick the correct answer :

a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer:
(a) Hydrogen gas and iron chloride are produced.

Question 4
What is a balanced chemical equation ? Why should chemical equations be balanced ?
Answer:
A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
The chemical equations should be balanced to satisfy the law of conservation of mass.

Question 5
Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:
(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

Question 6
Balance the following chemical equations :
(a) HNO3 + Ca (OH)2 → Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

Question 7
Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer:
(a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

Question 8
Write the balanced chemical equation for the following and identify the type of reaction in each case :
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer:
(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
Type : Double displacement reaction

(b) ZnCO3 (s) → ZnO (s) + CO2 (g)
Type : Decomposition reaction

(c) H2 (g) + Cl2 (g) → 2HCl(g)
Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Type : Displacement reaction

Question 9
What does one mean by exothermic and endothermic reactions ? Give examples.
Answer:
Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions. 

An exothermic reaction is indicated by writing “+ Heat”on the products side of an equation.

Example :
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions. 

An endothermic reaction is usually indicated by writing “Heat” on the product side of a chemical equation.
Examples :
(i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

Question 10
Why is respiration considered an exothermic reaction ? Explain.
Answer:
Respiration is an exothermic process because during respiration glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.

C6H12O6 + 6O2 --- > 6CO2 + 6H2O + energy.

Question 11
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer:
In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.
For example:

2H2O (l) --- > 2H2 + O2

While, in a combination reaction, two or more substances simply combine to form a new substance.
For example:

2H2 + O2 --- > 2H2O

Question 12
Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity.
OR
Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity. [CBSE 2015 (Delhi)]
Answer

Question 13
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:
In displacement reactions, a more reactive metal displaces a less reactive metal from its solution. For example,
Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)
This is a displacement reaction where iron displaces copper from its solution.
In double displacement reactions, two reactants in solution exchange their ions. For example,
AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq)
This is a double displacement reaction where silver nitrate and sodium chloride exchange Cl– and NO3– ions between them.

Question 14
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer:

AgNO3 + Cu --- > Cu(NO3)2 + Ag

Question 15
What do you mean by a precipitation reaction ? Explain by giving examples.
Answer:
A reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
Example : When a solution of iron (III) chloride and ammonium hydroxide are mixed, a brown precipitate of iron (III) hydroxide is formed.

FeCl3 + NH4OH --- > Fe(OH)3 + NH4Cl

Question 16
Explain the following in terms of gain or loss of oxygen with two examples each:
(a) Oxidation and
(b) Reduction.
Answer:
(a) Oxidation : The addition of oxygen to a substance is called oxidation.
Example :
(i) S(s) + O2(g) → SO2(g) (Addition of oxygen to sulphur)
(ii) 2Mg(s) + O2 (g) → 2MgO(s) (Addition of oxygen to magnesium)

(b) Reduction : The removal of oxygen from a substance is called reduction.
Example: (i) CuO + H2 Heat−→−− Cu + H2O
Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.

(ii) ZnO + C → Zn + CO
Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.

Question 17
A shiny brown colored element ‘X’ on heating in the air becomes black in color. Name the element ‘X’ and the black coloured compound formed.
Solution:
The shiny brown colored element X is copper metal (Cu). When copper metal is heated in air, it forms a black colored compound copper oxide. So, the black colored compound is the copper oxide or copper (II) oxide, CuO.

Question 18
Why do we apply paint on iron articles?
Solution:
Rust is a soft and porous substance, which gradually falls from the surface of an iron object, and then the iron below starts rusting. Thus, rusting of iron (or corrosion of iron) is a continuous process which, if not prevented in time, eats up the whole iron object. So, when we apply paint on iron articles it reduces the rusting of iron.

Question 19
Oil and fat containing food items are flushed with nitrogen. Why?
Solution:
Packaging fat and oil containing foods in nitrogen gas can prevent rancidity. When the fat and oil present in food materials gets oxidised (in air), their oxidation products have an unpleasant smell and taste. When it is surrounded by unreactive gas, nitrogen, there is no oxygen (of air) to cause its oxidation and make it rancid.

Question 19
Explain the following terms with one example each.
a. Corrosion
b. Rancidity
Solution:
a. Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface. Corrosion is caused mainly by the oxidation of metals by the oxygen of air.
Example: Rusting of iron is the most common form of corrosion. When an iron object is left in damp air for a considerable period of time, it gets covered with a red-brown flaky substance called ‘rust’. This is called rusting of iron.
b. The condition produced by aerial oxidation of fat and oil in food which is marked by an unpleasant smell and taste is called rancidity.
Example: Rancidity can be retarded by keeping food in a refrigerator.
The refrigerator has a low temperature inside it. When the food is kept in a refrigerator, the oxidation of fat and oil in it is slowed down due to low temperature. So, the development of rancidity due to oxidation is retarded.

Class 9 chapter 1 "Matter in our surroundings"

 

Topic:- Introduction

Definition:-Anything that occupy space and have mass is called matter.

Early Indian philosopher classified matter in the form of five basic elements- Panch tatva - earth, sky, air, fire and water.

Modern day scientists have evolved two types of classification of matter based on their physical and chemical nature.

In this chapter we shall learn about matter based on its physical properties.

Physical Nature of Matter

Matter is made up of particles.

Size of particles is beyond our imagination.

Characteristics of Particles of Matter.

*Particles of matter have space between them.

*Particles of matter are continuously moving.

*Particles of matter attract each other

Diffusion:-Movement of particles from higher concentration region to lower concentration region.

Rate of diffusion is higher in gases.

States of Matter

three states.

1 solid

2 liquid

3 gas

SOLID STATE:-

*Fixed shape

*Fixed volume

*Distinct boundary

*Strong force of attraction between particles,

*very small intermolecular distance

*Rigid

*negligible compressibility

 *Don't flow

*show diffusion

*rubber band have special property of elasticity .

*sponge have minute holes through which air expell out when compress.

#LIQUID STATE:-

*NO Fixed shape

*Fixed volume

*NO Distinct boundary

*week force of attraction between particle

*have intermolecular distance

*Non-Rigid

* compressibility

* flow

*show diffusion

GASES:-

*No Fixed shape

*No Fixed volume

*No boundary

*weakest force of attraction between particles,

*very large intermolecular distance

*non- Rigid

*highly compressible

*flow

*show diffusion

Can Matter Change it's state?

Yes

Factors which can change the state of matter:- 

1. Effect of change of temperature

2. Effect of change of pressure.

Effect of Change Of Temperature:-

On increasing the temperature of solids, the kinetic energy of particles increases.

Due to increase in kinetic energy, the particles start vibrating with greater speed.

The energy supplied by heat overcomes the forces of attraction between the particles.

The particles leave their fixed position and start moving more freely.

A stage is reached when the solid melts and is converted into liquid.

Melting point:- The minimum temperature at which solid melts to become a liquid at the atmospheric pressure is called melting point.

mp of water(ice) is 0°C.

Melting point of a solid indicate the strength of forces of attraction between its particles.

Latent heat of fusion:- Amount of heat energy required to convert 1 kg of solid into liquid at atmospheric pressure at its melting point is known as latent heat of fusion.

Boiling point:- The temperature at which a liquid starts boiling at the atmospheric pressure is known as it's boiling point.

Boiling is a bulk phenomenon.

Particles from the bulk of the liquid gain enough energy to change into the vapour state.

Sublimation:- the process of change of solid into gas without changing into liquid is called sublimation and direct change of gas to solid without changing into liquid is called deposition.

Examples that show sublimation are:- camphor, ammonium chloride, dry ice (solid CO2)

Note:- Particles of liquid have more energy than particles of solid at its mp . Due to absoption of latent heat of fusion.

So at 273k (0°C) ice is more effective in cooling than water at the same temperature.

Particles of gas have more energy than particles of liquid at its bp . Due to abosption of latent heat of vaporisation.

Effect of change of pressure in state change:- 

On increasing the pressure, the interparticular distance increases.

That's why we can convert gases to liquid and liquid to solid by increasing the pressure.

Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without changing into liquid state. This is the reason that solid CO2 is called dry ice

Evaporation: 

The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

#Factors affecting evaporation:-

    1.Surface area{ larger the surface            area larger will be rate of evaporation.}

    2.Temperature{higher the temp higher will be rate of evaporation.}

    3.Humidity{Higher the humidity, lesser will be the rate of evaporation.}

    4.Wind Speed{Higher the wind speed ,faster will be the rate of evaporation.}

#How does evaporation cause cooling?

-By absorbing heat .

*Desert cooler cool better on a hot dry days because air drops coming out of it absorb heat from surrounding and evaporate and evaporation cause cooling.

*Water kept in an earthen pot become cool during summer because heat transfer from inside water to surface water and this surface water gets evaporate through air pores present in earthen pot. due to evaporation water become cool

* When we put some acetone or petrol or perfume in our palm ,we feel cool because it's substances absorb heat from our palm nd gets evaporate . and evaporation cause cooling.

* We are able to sip hot tea or milk faster from a saucer rather than a cup because by increasing surface area rate of evaporation increases and evaporation cause cooling.

* Cotton cloth should be wear in summer because it absorb moisture and thus water drop come to surface and gets evaporate.thus evaporation cause cooling.

*When ice cold water is pour in tumbler ,water droplets see on outside surface of tumbler because water vapour present in air ,on coming in contact with cold glass of water, loses energy and converted into liquid state.

how to convert °C into K

°C = 273 + K

300k =____°C

573k =____°C

25°C =____°C

373°C = __°C

Ques ans:-

Page no:-  3

Question 1. Which of the following are matter?

Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.

Answer: Chair, air, almonds, and cold-drink.

Question 2. Give reasons for the following observation:

The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close.

Answer: The smell of hot sizzling food reaches severed meters away, as the particles of hot food have more kinetic energy and hence the rate of diffusion is more than the particles of cold food.

Question 3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Answer: A diver is able to cut through water in a swimming pool. This shows that the particles of water have intermolecular space and has less force of attraction

Question 4. What are the characteristics of the particles of matter?

Answer. The characteristics of the particles of matter are:

(1) Particles have intermolecular space.

(2) Particles have intermolecular force.

(3) Particles of matter are moving continuously.

Page no. 6

Question 1. The mass per unit volume of a substance is called density.

(density = mass/volume).

Arrange the following in order of increasing density: air, exhaust from chimneys, honey, water, chalk, cotton and iron.

Answer: Increasing density:

air < exhaust from chimneys < cotton < water < honey < chalk < iron.

Question 2. (a) Tabulate the differences in the characteristics of states of matter.

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer: (a) Difference in the characteristics of 3 states of matter.

b) Comment on:

(i) Rigidity: The tendency of a substance to retain/maintain their shape when subjected to outside force.

(ii) Compressibility: The matter has intermolecular space. The external force applied on the matter can bring these particles closer. This property is called compressibility. Gases and liquids are compressible.

(iii) Fluidity: The tendency of particles to flow is called fluidity. Liquids and gases flow.

(iv) Filling of a gas container: Gases have particles which vibrate randomly in all the directions. The gas can fill the container.

(v) Shape: Solids have maximum intermolecular force and definite shape.

Whereas liquids and gases takes the shape of container.

(vi) Kinetic energy: The energy possessed by particles due to their motion is called kinetic energy. Molecules of gases vibrate randomly as they have maximum kinetic energy.

(vii) Density: It is defined as mass per unit volume, the solids have highest density.

Question 3. Give reasons

(a) A gas fills completely the vessel in which it is kept.

(b) A gas exerts pressure on the walls of the container.

(c) A wooden table should be called a solid.

(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.

Answer: (a) The molecules of gas have high kinetic energy due to which they keep moving in all directions and hence fill the vessel completely in which they are kept.

(b) A gas exerts pressure on the walls of the container because the molecules of the gas are in constant random motion due to high kinetic energy. These molecules constantly vibrate, move and hit the walls of the container thereby exerting pressure on it.

(c) The molecules/particles of wooden table are tightly packed with each

other, there is no intermolecular space, it cannot be compressed, it cannot flow, all these characteristics are of solid. So wooden table should be called a solid. ‘

(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert. It is because the molecules of air has less force of attraction between them and a very small external force can separate them and pass through it. But in case of solids, the molecules have maximum force of attraction, the particles are tightly bound due to this force. Hence large amount of external force is required to pass through solid.

Question 4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

Answer: Ice is a solid but its density is lower than water due to its structure. The molecules in ice make a cage like structure with lot of vacant spaces, this makes ice float on water.

Page no. 9

Question 1. Convert the following temperature to Celsius scale:

(a) 300 K (b) 573 K

Answer. (a) 300 – 273 = 27°C (b) 573 – 273 = 300°C

Question.2. What is the physical state of water at:

(a) 250°C (b) 100°C

Answer: (a) 250°C = gas (b) 100°C liquid as well as gas

Question 3. For any substance, why does the temperature remain constant during the change of state?

Answer: During the change of state of any matter heat is supplied to the substance. The molecules of this matter use heat to overcome the force of attraction between the particles, at this period of time, temperature remains constant. This extra heat is acquired by the molecules in the form of hidden heat called latent heat to change from one state of matter to the other state.

Question 4. Suggest a method to liquefy atmospheric gases?

Answer: By increasing pressure and decreasing temperature we can liquefy atmospheric gases.

Page no. 10

Question 1. Why does a desert cooler cool better on a hot dry day?

Answer: The outer walls of the cooler get sprinkled by water constantly. This water evaporates due to hot dry weather. Evaporation causes cooling of inside air of cooler. This cool air is sent in the room by the fan.

Question 2. How does the water kept in an earthen pot (matka) become cool during summer?

Answer: The earthen pot is porous with lot of pores on it, the water oozes out through these pores and the water gets evaporated at the surface of the pot thereby causing cooling effect.This makes the pot cold and the water inside the pot cools by this process.

Question 3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Answer: Acetone, petrol or perfume evaporate when they come into contact with air. The evaporation causes cooling sensation in our hands.

Question 4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Answer: Tea in a saucer has larger surface area than in a cup. The rate of evaporation is faster with increased surface area. The cooling of tea in saucer takes place sooner than in a cup. Hence we are able to sip hot tea or milk faster from a saucer rather than a cup.

Question 5. What type of clothes should we wear in summer?

Answer: We should wear light coloured cotton clothes in summer. Light colour because it reflects heat. Cotton clothes because it has pores in it, which absorbs sweat and allows the sweat to evaporate faster thereby giving cooling effect

Page no 12

Question 1. Convert the following temperatures to the Celsius scale.

(a) 293 K (b) 470 K.

Answer: (a) 293 K into °C

293 – 273 = 20°C

(b) 470 K into °C 470 – 273 = 197°C

Question 2. Convert the following temperatures to the Kelvin scale.

(a) 25°C (b) 373°C.

Answer: (a) 25°C into K

25 + 273 = 298 K

(b) 373°C into K 4 373 + 273 = 646 K

Question 3. Give reason for the following observations.

(a) Naphthalene balls disappear with time without leaving any solid

(b) We can get the smell of perfume sitting several metres away.

Answer: (a) Naphthalene balls disappear with time without leaving any solid, because naphthalene balls sublime and directly changes into vapour state without leaving any solid

b) We can get the smell of perfume sitting several metres away because perfume contain volatile solvent and diffuse faster and can reach people sitting several metres away.

Question 4. Arrange the following substances in increasing order of forces of attraction between the particles—water, sugar, oxygen.

Answer: Oxygen —> water —> sugar.

Question 5. What is the physical state of water at—

(a) 25°C (b) 0°C (c)100°C

Answer: (a) 25°C is liquid (b) 0°C is solid or liquid

(c) 100°C is liquid and gas

Question 6. Give two reasons to justify

(a) water at room temperature is a liquid.

(b) an iron almirah is a solid at room temperature.

Answer: (a) Water at room temperature is a liquid because its freezing point is 0°C and boiling point is 100°C.

(b) An iron almirah is a solid at room temperature because melting point of iron is higher than room temperature.

Question 7. Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer: Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the fusion to become water. Hence the cooling effect of ice is more than the water at same temperature because water does not absorb this extra heat from the medium.

Question 8. What produces more severe bums, boiling water or steam?

Answer: Steam at 100°C will produce more severe bums as extra heat is hidden in it called latent heat whereas the boiling water does not have this hidden heat.

Question 9. Name A, B, C, D, E and F in the following diagram showing change in its state

Answer: A —> Liquefication/melting/fusion B —> Vapourisation/evaporation C—>Condensation D—> Solidification E —> Sublimation F —> Sublimation

Thankyou :-)