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They are all chemical changes.

i) Gram molecular weight of carbon dioxide = 12 + 2 × 16g = 44 g. 

22.4 litres of CO₂ at STP weighs 44 g. 

The weight of 448 ml of CO₂ at STP =  \(\frac{44}{22400}\) × 448 = 0.88 g 

ii) 44g of CO₂ contains 12 g of carbon. 

The amount of carbon present in 0.88 g of CO₂ = \(\frac{12}{44}\) × 0.88 = 0.24 g 

iii) Weight of hydrogen = Weight of hydrocarbon – Weight of carbon = 0.29 – 0.24 = 0.05 g

1) Pure water is a bad conductor of electricity. By adding a small amount of salt the solution becomes conductor of electricity. 

2) Hydrogen does not support combustion. So it puts off the burning splinter and also it is combustible. So burns with blue flame. 

3) When we pass carbon dioxide through lime water it turns into milky because lime water (calcium hydroxide) reacts with carbon dioxide and forms a white milky substance, i.e. calcium carbonate.

(a) A reaction in which two or more substances combine to form a single substance is called a combination reaction.

(b) A catalyst is a substance which changes the rate of a chemical reaction without undergoing a chemical change.

(c) The formation of gas bubbles in a liquid during a reaction is called effervescence

(d) The reaction between an acid and a base is called neutralization reaction.

(e) Soluble bases are called alkalis.

(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.

(g) In the type of reaction called double decomposition reaction, ions two compounds exchange their positive and negative radicals ions respectively.

(h) A catalyst either increases or decreases the rate of a chemical change but itself remains unchanged at the end of the reaction.

(i) The chemical reaction between hydrogen and chlorine is a combination reaction

(j) When a piece of copper is added to silver nitrate solution, it turns blue in color.

(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called a combination reaction.

A + B → AB

e.g (i) When iron and sulphur are heated together, they combine to form iron sulphide.

Fe + S + heat → FeS

(ii) When carbon bums in oxygen to form a gaseous compound called carbon dioxide.

C + O₂ + heat → CO₂ + heat

(b) Decomposition reaction: A reaction in which a compound breaks up due to the application of heat into two or more simple substances is called decomposition reaction.

AB + heat → A + B

e.g. (i) Mercuric oxide when heated, decomposes to form two elements mercury and oxygen

2HgO(s) + heat → 2Hg(s) + O₂(g)

(ii) CaCO₃, when heated, decomposes to calcium oxide and carbon dioxide.

CaCO₃ + heat → CaO + CO₂

(c) Displacement reaction: A reaction in which a more active element displaces a less active element from a compound is called displacement reaction.

AB + C → CB + A

e.g. (i) Zinc, displaces copper from copper sulphate solution.

Zn + CuSO₄ (aq) → ZnSO₄ (aq) + Cu

(ii) Iron piece when added to copper sulphate solution, copper is displaced.

Fe + CuSO₄ → FeSO₄ + Cu.

(d) Double decomposition reaction: A chemical reaction in which two compounds in their aqueous state exchange their ions to form new compounds is called a double decomposition reaction.

AB + CD → CB + AD

e.g. (i) AgNO₃ + HCl AgCl + HNO₃(aq)

(ii) NaOH (aq) + HCl (aq) NaCl (aq) + H₂O.

We know that, oxidizing agent undergoes reduction and reducing agent undergoes oxidation. Hence, we need to know whether the substance has undergone oxidation or reduction. As X undergoes a chemical change by losing hydrogen atom, it undergoes oxidation. Hence, it is a reducing agent. To the product Y, hydrogen is added, it undergoes reduction. Hence, Y acts as an oxidising agent.

Characteristics of Chemical Reactions:

When a chemical reaction takes place, it is accompained by one or more of the following characteristics:

Evolution of a gas:
A gas may evolve during a chemical reaction, if one of the reactants is a liquid or in solution form. Evolution of gas is confirmed by appearance of effervescence (bubbling) in the liquid. The gas can be identified by testing it. In a chemical equation, the evolution of a gas is shown by putting an arrow pointing upwards
(↑).
Example:
When zinc reacts with dilute sulphuric acid, hydrogen gas is given off. The gas can be tested with the help of burning matchstick when the burning matchstick is taken near the test tube containing hydrogen gas, hydrogen burns with a pop sound.
Zn + H₂SO₄ →  ZnSO₄ + H₂↑
Change of colour:
One of the product formed during a chemical reaction may have a different colour than that of the reactants. In such case, change of colour accompanies the chemical reaction.
Example:
When white crystal of lead nitrate are heated strongly, yellow-coloured lead monoxide is formed. A reddish – brown gas, nitrogen dioxide is also given off.
2Pb (NO₃)₂ → 2PbO + 4NO₂ ↑+ O₂ ↑
Lead nitrate (white) Lead monoxide (yellow) Nitrogen dioxide (reddish brown) oxygen.

In chemical combination or synthesis reactions, two or more elements or substances combine to form a single product.

For example:

H₂ + Cl₂ → 2HCl

When hydrogen combines with chlorine in the presence of diffused sunlight, it forms hydrogen chloride.

When fats and oils are oxidized they become rancid. So their smell and taste change.

Catalyst is a substance which does not take part in chemical reaction, but change the rate of a reaction.

The chemical changes that we observe in our daily life are

(a) rusting of iron
(b) colour change in food items
(c) photosynthesis
(d) burning of paper

An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

In a chemical reaction whenever one substance is oxidised, another substance is reduced. This means that oxidation and reduction reaction always occur simultaneously and known as redox reaction.

Daily life examples : 

1. Combustion of fuels. 

2. Corrosion of metals. 

3. Change of colour of fruits like apples, bananas, when they are cut. 

4. Burning of crackers.

5. Rancidity of food material. 

6. During rainy season the power supply to our home from the electric pole will be interrupted due to formation of metal oxide layer on the electric wire. 

7. Rising of dough with yeast depends on oxidation of sugars to carbon dioxide and water. 

8. Bleaching of coloured objects using moist chlorine. 

9. Respiration.

A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

• A chemical equation gives information about the reactants and products through their symbols and formulae.
• It gives the ratio of molecules of reactants and products. 
• As molecular masses are expressed in unified masses, the relative masses of reactants and products are known from the equation. 
• If the masses are expressed in grams, then the equation also gives the molar ratios of reactants and products. 
• If gases are involved, we can equate the masses to their volumes. 
• Using molar mass and Avogadro’s number we can calculate the number of molecules and atoms of different substances from the equation.

a) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

b) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

c) H₂ + Cl₂ → 2HCl

d) NH₄NO₃ → N₂O + 2H₂O

1) A chemical equation in which the number of atoms of different elements on the reactants side (left side) are same as those on product side (right side) is called a balanced chemical equation. 

Ex : Zn + 2HCl → ZnCl₂ + H₂ ↑

2) All the chemical equations must balance because atoms are neither created nor destroyed in chemical reactions.

3) The number of atoms of each element before and after reaction must be the same. 

4) According to the law of conservation of mass, the total mass of the substances that are taking part in a chemical reaction must be the same before and after the reaction.

A chemical equation is a statement that represents a chemical change in terms of symbols and formula.

A lot of information can be obtained from the balanced chemical equation.

1. It emphasizes the fact that matter can neither be destroyed nor be created.

2. Simple calculation can be done on the basis of chemical equation.

3. Problems based on percentage calculation can be solved.

Strong electrolytes are compounds which dissociate completely in their aqueous or fused state. It allows electricity to flow through them. e.g. all strong acids and alkalis.

Weak electrolytes are chemical compounds which dissociate partially in their aqueous or fused state. They are not good conductors of electricity, e.g. weak acids like carbonic acid, weak alkalis like ammonium hydroxide.

(i) Hydrogen 

(ii) Ammonia 

(iii) Carbon

hydrogen is a reducing agent.

Reduction reaction is the exact opposite of oxidation reaction.

1) The black coatings on silver.

 4 Ag + 2H₂S + O₂ → 2 Ag₂S + 2H₂O 

2) Green coating on copper. 

2 Cu + O₂ → 2 CuO

1. What is alloying? 

2. How are alloys prepared? 

3. What are the qualities of alloys? 

4. Which quantity prevents the corrosion in alloys? 

5. Can we prevent corrosion by alloying a metal ?

(c) precipitation reaction

(c) zinc atom

Original substances participating in a chemical reaction are known as the reactant and the new substance formed are known as the products.

1) Reactants : 

The substances which undergo chemical change in the reaction are called ‘Reactants’. 

2) Products: 

The new substances formed in a chemical reaction are called ‘Products’. 

Ex : Zn + 2HCl → ZnCl₂ + H₂ ↑ 

3) In above reaction Zn and 2HCl are called reactants, ZnCl₂ and H₂ are called products.

1. Solid state is indicated by the symbol (s) 

2. Liquid state is indicated by the symbol (l) 

3. Gaseous state is indicated by the symbol (g)

 4. Aqueous solution is indicated by the symbol (aq)

1. Physical state, 

2. Heat changes (exothermic and endothermic reactions), 

3. Gas evolved, 

4. Precipitate formed.

Yes, it is a balanced equation.

Iron displaces copper from copper sulfate but copper cannot displace iron. Iron is more reactive than copper. Hence the more reactive element iron displaces the less reactive element copper.

2 NaNO₂ + O₂ → 2 NaNO₃ 

Here O₂ is element and NaNO₂ is compound.

1) Sodium sulphate solution on mixing with barium chloride solution forms a white precipitate of barium sulphate and soluble sodium chloride.

Na₂SO₄ + BaCl₂ → BaSO₄ + 2 NaCl 

2) Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water. 

NaOH + HCl → NaCl + H₂O 

3) Sodium chloride spontaneously combines with silver nitrate in solution giving silver chloride precipitate. 

NaCl + AgNO₃ → AgCl + NaNO₃ 

4) Mix lead nitrate solution and potassium iodide solution to form a yellow precipitate of lead iodide and potassium nitrate. 

Pb(NO₃)₂ + 2Kl → PbI₂ + 2KNO₃

5) Calcium hydroxide reacts with nitric acid to form water and calcium nitrate.

 Ca(OH)₂ + 2 HNO₃ → 2H₂O + Ca(NO₃)₂

6) Magnesium chloride reacts with potassium hydroxide to form magnesium hydroxide and potassium chloride. 

MgCl₂ + 2 KOH → Mg(OH)₂ + 2 KCl

Yes. The atoms of each element in left side are equal to the atoms of their corresponding element.

(c) more reactive element displace less reactive element

1) i) CaCO_3(s) → CaO_(s) + CO_2(g) . It is a chemical decomposition reaction. 

ii) 2Ag Br_(s) → 2Ag_(s) + Br_2(g) . It is also a chemical decomposition reaction. 

2) 2Ag Br_(s) → 2Ag_(s) + Br_2(g) . It is a photochemical reaction. Because, this reaction takes place in the presence of sunlight only.

This reaction is chemical displacement. ‘C’ is more reactive than ‘A’. So C displaces ‘A’ from AB and occupies its place.

1) Calcium carbonate on heating decomposes to calcium oxide and carbon dioxide.

CaCO → CaO + CO₂  

2) On electrolysis, water decomposes to water and hydrogen. 

2 H₂O → 2 H₂ + O₂ 

3) Silver bromide decomposes to silver and bromine in sunlight. 

2 AgBr → 2 Ag + Br₂ 

4) Silver chloride decomposes to silver and chlorine. 

2 AgCl → 2 Ag + Cl₂ 

5) Glucose decomposes to ethanol and carbon dioxide. 

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

6) Sodium bicarbonate decomposes to sodium carbonate, water and carbon dioxide. 

2 NaHCO₃ + Heat → Na₂CO₃ + H₂O + CO₂ 

7) On heating lead nitrate decomposes to lead oxide, oxygen and nitrogen dioxide.

2Pb(NO₃)₂   → 2PbO + 4 NO₂ + O₂

• Take two iron nails and clean then by rubbing with sand paper. 
• Take two test tubes and mark them ‘A’ and ‘B’. 
• Pour copper sulphate solution in the test tube ‘A’ and Zinc Chloride solution in the test tube ‘B’. 
• Dip iron nails in both test tubes. 
• Keep them without disturbing for 20 min.

Observation : 

1.  The nail which is dipped in Copper Sulphate solution becomes brownish colour. 
2.  The nail which is dipped in Zinc Chloride solution doesn’t change.

Reaction : 

1. In test tube ‘A’: CuSO₄ + Fe → FeSO₄ + Cu 
2. In test tube ‘B’: ZnCl₂ + Fe → No reaction.

Conclusion : 

1.  Iron is more reactive than copper. So it displaces copper from Copper Sulphate solution. 
2.  Iron is less reactive than zinc. So, it doesn’t displace zinc from ZnClr

Exothermic reactions

A reaction in which insoluble substance in water is formed as product is called precipitation reaction.

• Nitrogen gas reacts with oxygen gas in the presence of heat energy and forms nitric oxide gas. 
• It is an endothermic reaction. 
• This is an example for chemical combination reaction. 
• In this reaction, the reactants are gases and products are also gases.

By stopping the oxidation it prevents the rancidity of food material.

The spoilage of food can be prevented by adding preservatives like vitamin C and vitamin E.

• Ferrous reacts with oxygen in the air and form iron oxide.                      2Fe + O₂ → 2FeO 
• This reaction is called corrosion. It spoils the iron articles by rusting. 
• Corrosion of iron articles can be prevented or minimized by shielding the metal surface from oxygen and moisture. 
• It can be prevented by applying paint on the articles.

1. oxidation

2. endothermic reaction 

3. decomposition 

4. displacement 

5. reactants 

6. direction