Exemplar Questions and Solutions

Important CBSE Questions – 2 Mark Questions

1) What do you understand by precipitation reaction? Explain with suitable examples.

Ans) When two reactants react and product formed remains insoluble and settles as a solid it is called a precipitate. Such reactions in which precipitate is formed are called precipitation reactions. For example,

(i) when aqueous solution of sodium sulphate is mixed with the aqueous solution or barium chloride, barium sulphate comes in the form of white precipitate

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(↓) + 2NaCl(aq)

(ii) When aqueous solution of sodium chloride is mixed with the aqueous solution of silver nitrate, silver chloride comes in the form of white precipitate.

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

2) What is the relationship between oxidation and oxidising agent in a redox reaction? Write an example of a redox reaction showing the relationship of the same:

Ans) Oxidation is a process in which there is either loss of hydrogen or addition of oxygen wheras an oxidising agent is a substance that that accepts electrons and causes other substances to lose electrons. It underges reduction and oxidises the other substance. In other words, oxidizing agents add oxygen to another substance or remove hydrogen from it.

For example,

Fe₂O₃ + 3Mg → 2Fe + 3MgO

Oxidation of magnesium (by addition of oxygen) occurs and reduction of iron oxide (removal of oxygen) takes place. So, magnesium is the reducing agent and iron oxide is the oxidizing agent.

3) Give any two applications of displacement reactions:

Ans) In a displacement reaction, a more reactive metal will displace a less reactive metal from its solution. Some applications of displacement reaction are

a) It is used in the refining of silver

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

b) It is used in antacid that causes relief from acid indigestion. An antacid(calcium hydroxide) neutralizes stomach acid(hydrochloric acid)

Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

4) Give any two industrial application of combination reaction: 

Ans) Two important industrial application of combinations are-

a) Reaction of quick lime with water to form slaked lime used in white washing

CaO(s) + H₂O(l) → Ca(OH)₂(s)

b) Manufacture of HCl gas from H₂ and Cl₂

H₂(g) + Cl₂(g) → 2HCl(g)

5) Is corrosion an oxidation reaction?

Corrosion is a natural process through which a refined metal is converted into a more chemically-stable form, such as its oxide, hydroxide, or sulfide. In other words, corrosion also involves the oxidation of substances by oxygen. Oxidation is the process of reaction of any metal or non -metal with oxygen to form the oxide of that metal or non-metal.

4Fe + 3O₂ → 2Fe₂O₃

The metal atoms (Fe) must lose electrons when they form the rust compound (Fe₂O₃.nH₂O) called metal oxide. This is written as:

2Fe + O₂ → 2Fe₂₊ + 2O_2-

While taking into consideration only the reaction of iron, we get

2Fe → 2Fe₂₊ + 2e_–

Thus oxidation is one form of corrosion of metals to form their oxides.

6) An Aluminium can is used to store ferrous sulphate solution. It is observed that in a few days, holes started to appear on the can. Explain the observation and write the chemical equation to support your answer:

Ans) As Al is more reactive than Fe, it displaces iron from Ferrous sulphate and form Aluminium sulphateand Fe is precipitated out. The holes appear on the can because of the above displacement reaction taking place between Aluminium and ferrous sulphate.

3FeSO₄ + 2Al → Al(SO₄)3 + 3Fe

7) Give any three examples of double displacement reaction in our everyday life:

Ans) a) While baking a cake, we add baking soda to the batter, this baking soda (NaHCO₃) reacts with acidic substances(HX) in cake batter in a double replacement reaction

NaHCO₃ + HX → NaX + H₂CO₃

H₂CO₃(carbonic acid) decomposes into water and CO₂. The CO₂ causes the cake batter to rise.

b) The easiest way to combat excess stomach acid is to ingest an antacid. An antacid (Calcium hydroxide) neutralizes stomach acid(Hcl) Ca(OH)₂ + 2 HCl → CaCl₂ + 2H₂O

c) Lime (calcium) when mixed with water produces calcium hydroxide(slaked lime) with evolution of hydrogen gas.

Ca(s) + 2H₂O (l) → Ca(OH)₂ (aq) + H₂ (g)

This Calcium hydroxide has widespread uses, including applications in food manufacturing, hair care products, dental work, leather production, an important ingredient in cement, plaster and mortars. etc.

8) Why cannot a chemical change be normally reversed?

Ans) A chemical change occurs when the atoms are rearranged resulting in the formation of an entirely new substance having different properties. Therefore it normally can’t be reversed. For example, the ashes and the smoke got by burning a log of wood cannot be put back together to form a log of wood again.

9) Explain the term Corrosion with examples:

Ans) When a metal is attacked by substances around it such as atmospheric oxygen, moisture or sulphides present in the air, an oxidation reaction takes place on the surface of the metal. This oxidation reaction results in the formation of oxide, carbonate and sulphide layer on the surface of the metal and the metal corrodes. The process is called corrosion.

10) Why do marble statues corrode when kept in open for long time?

Marble Statues are made up of Calcium Carbonate. The gaseous oxides (CO₂, NO₂ and SO₂) released from factories and vehicles gets mixed with the atmospheric air and causes air pollution. These gases mix with water in air to form acids such as Carbonic {Ca(HCO₃)₂}, sulfuric (H₂SO₄)or nitric acid(HNO₃). These are components of acid rain that will react with the calcium carbonate in the marble causing the corrosion of the statues.

11) Why decomposition reactions are called the opposite of combination reactions? Explain with equations of these reactions.

Ans) In a combination reaction, two or more substances are combined to form new substance.

For example,

CaO + CO₂ → CaCO₃ + energy —- combination reaction

In a decomposition reaction, one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.

For example,

Thus, from the above given examples, we can say that decomposition reaction is the opposite of combination reaction.

12) What is the rule for displacement reaction?

Ans) a) If the pure metal is higher in the reactivity series than the metal in the compound then displacement will happen.

b) When a metal is heated with the oxide of a less reactive metal, it will displace the metal from it.

E.g., The reaction between iron and copper oxide, mixed and heated strongly.

CuO + Fe → FeO + Cu

Iron being more reactive than copper, it displaces the copper from the oxide. Iron acts as a reducing agent by removing oxygen from the other metal.

c) If a less reactive metal is added to a salt solution of more reactive metal, nothing will happen. For example,

2Na + 2HCl → 2NaCl + H₂

2K + H₂SO₄ → K₂SO₄ + H₂

13) What is the difference between chemical and physical change?

Chemical change is any change that results in the formation of new chemical substances whereas in a chemical change, no new substances are formed. E.g., Rusting of iron (iron oxide is formed) is a chemical change whereas dissolving sugar in water is a physical change (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.

14) What is a chemical reaction? Give one example:

Ans) The process in which one or more substances interact to form a new substances having entirely new properties is called a chemical reaction. For example, when iron and oxygen react, they change to a new substance, iron oxide (rust). Iron oxide (Fe2O3) has different chemical properties to iron and oxygen.

4Fe + 3O₂ → 2Fe₂O₃.

15) Define combination reaction. Give two examples:

Ans) A chemical reaction in which two or more substances combine to form a new substance is called a combination reaction.

We can represent these reactions using the general equation

A + B → AB

E.g.,

C + O₂ → CO₂

H₂ + O₂ → 2H₂O

16) What is galvanised iron? How does galvanizing protect iron from rusting?

Ans) Galvanization is the process of applying a protective zinc coating on iron or steel, to prevent rusting. Galvanized iron is a type of iron that has been coated in a layer of zinc to protect it from corrosion. Zinc is used because it is more electrochemically active than iron (or steel). Thus, Zinc will get oxidized and will protect the iron sacrificially. The Zinc oxide formed is sticky in nature and will protect the inner layer of iron from oxidation.

17) State one characteristic each of a chemical reaction which take place when:

a) Dilute hydrochloric acid is added to sodium carbonate.

b) Lemon juice is added gradually to potassium permanganate solution.

c) Dilute sulphuric acid is added to barium chloride solution.

d) Quicklime is treated with water.

e) Wax is burned in the form of candle

Ans) a) Dilute hydrochloric acid is added to sodium carbonate- The chemical reaction between Sodium carbonate and dilute hydrochloric acid is characterized by the evolution of carbon dioxide gas.

Na₂CO₃ + 2 HCl → 2NaCl + CO₂ + H₂O

b) Lemon juice is added gradually to potassium permanganate solution- The chemical reaction between lemon juice(citric acid) and purple coloured potassium permanganate solution is characterized by a change in colour from purple to a colourless solution.

c) Dilute sulphuric acid is added to barium chloride solution- The chemical reaction between dilute sulphuric acid and purple barium chloride solution is characterized by the formation of a white precipitate of barium sulphate.

BaCl₂ + H₂SO₄ → BaSO₄ + 2 HCl

d) Quicklime is treated with water- The chemical reaction between quicklime and water to form slaked lime(Ca(OH)₂) is characterized by a change in temperature. There is a rise in temperature as it is an exothermic reaction.

CaO(s) + H₂O(l) → Ca(OH)₂(s)

e) Wax is burned in the form of candle- When wax is burned in the form of candle, water and carbondioxide are formed. The combustion reaction of candle wax, is characterised by a change in state from solid (wax) to liquid(water) and gas (Carbondioxide).

18) What is the difference between a displacement reaction and a double displacement reaction? Support your answers using examples.

Ans) In a (single)displacement reaction, only one element in a compound is displaced by a more reactive element. In other words, a free element replaces one of the ions in a compound.

Eg., CuSO₄(aq) + Fe(s) → Cu(s) + FeSO₄(aq)

Here, as Fe is more reactive than Cu, it displaces Cu from CuSO4 and forms FeSO₄

In a double displacement reaction, exchange of ions takes place or rather parts of two compounds exchange their places to form two new compounds.

E.g., AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq).

When silver nitrate (AgNO₃) combines with sodium chloride(NaCl), two new compounds namely silver chloride (AgCl )and sodium nitrate (NaNO₃)are formed as sodium and silver exchanged their places.

19)Give two characteristics of chemical reaction which take place when dilute sulphuric acid is poured over Zinc granules:

When dil.sulphuric acid is poured over zinc granules, a displacement reaction takes place and results in the formation of a new substance (zinc sulphate) and hydrogen gas.

H₂SO₄(aq)+Zn(s) → ZnSO₄(aq)+H₂(g)

The reaction vessel becomes hot indicating that the reaction is exothermic.

20)When hydrogen burns in oxygen, water is formed and when water is electrolyzed, then hydrogen and oxygen are produced. What type of reaction takes place: In the first case? In the second case?

Ans) When hydrogen burns in oxygen, water is formed. The given reaction is a clear example of combination reaction as two or more substances (hydrogen and oxygen) combine to form a single substance(water)

2H₂ + O₂ → 2H₂O

When water is electrolyzed, then hydrogen and oxygen are produced. The given reaction is a clear example of decomposition reaction, as a single substance (water) decomposes into two or more substance (hydrogen and oxygen).

2H₂O → 2 H₂ + O₂

21) List five observations which help us to determine whether a chemical reaction has taken place or not

Ans) The five observations which help us to determine whether a chemical reaction has taken place or not are-

1. Change in state.

2. Change in colour.

3. Evolution of a gas.

4. Change in temperature

5. Formation of new substance

22) Why do we add dilute sulphuric acid during the electrolysis of water?

Ans) Dil. Sulphuric acid is added during the electrolysis of water because water is a poor conductor of electricity and dil. Sulphuric acid is used to increase the conductivity of water. The acid dissociates to produce ions which in turn increase conductivity.

23) What is a skeletal equation? Give an example:

Ans) An unbalanced chemical equation is called the skeletal chemical equation.

When a chemical equation is written using symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to number of atoms of the same elements towards the side of the products, is called a skeletal equation or an unbalanced equation.

Eg., H₂ + O₂ → H₂O

CO₂ + H₂O(l) → C₆H1₂O₆ + O₂(photosynthesis)

24) Why should we balance a skeletal equation?

Ans) According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation in order to satisfy the law of conservation of mass.