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