Exemplar Questions and Solutions

Important CBSE Questions – Short Answer Questions

16) Why do we store silver chloride in dark coloured bottles?

Ans) When silver chloride is exposed to light, it breaks down into elemental silver and chlorine gas:

2AgCl → 2Ag + Cl₂

The white colour of silver colour changes into grey colour during the chemical reaction. We store silver chloride in dark coloured bottles because the dark coloured bottles interrupt the path of light and it almost becomes impossible for the light to reach silver chloride in the bottles and thus, its decomposition is prevented. Thus, to increase the shelf life of silver chloride, by preventing a photochemical decomposition reaction, substances like silver chloride are kept in dark bottles in the laboratory.

17) Balance the following chemical equations and identify the type of chemical reaction.

a) Mg + Cl₂ → MgCl₂

Ans) The original equation is already balanced

Mg + Cl₂ → MgCl₂

It is a combination reaction because two or more substances (Mg and Cl₂) combine to form a single substance (MgCl₂), are called combination reactions.

The above given reaction is also a redox reaction. Magnesium loses two electrons and gets oxidized. It is therefore, the reducing agent while and chlorine gains two electron and is reduced. Therefore, chlorine is the oxidizing agent.

The above given reaction is an example of thermal decomposition reaction because decomposition {a single substance(HgO) decomposes to two or more simpler substances (Hg and O)}is carried out by heating.

It is a combination reaction as two or more substances (Sodium and Sulphur) are combined to form new substance(Sodium sulphide).

d) TiCl₄ + Mg → Ti + MgCl₂

Ans) TiCl₄ + 2Mg → Ti + 2MgCl₂

It is a displacement reaction because Magnesium displaces titanium (Ti) from Titanium tetrachloride(TiCl₄) as Mg is more reactive than Ti.

e) CaO + SiO₂ → CaSiO₃

Ans) CaO + SiO2 → CaSiO3

The above reaction is a combination reaction as two or more substances(CaO and SiO₂) are combined to form new substance namely Calcium silicate(CaSiO₃).

The above reaction is a decomposition reaction as a single substance namely Hydrogen peroxide(H₂O₂) decomposes to form two substances(water and oxygen)

18) A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

a) Write the chemical formulae of X and Y.

Ans) The chemical formula of X is MgO (Magnesium oxide) and Y is Mg₃N₂(Magnesium nitride)

b) Write a balanced chemical equation, when X is dissolved in water.

Ans) As we know, X is Magnesium oxide. When Magnesium oxide dissolves in water, it gives Magnesium hydroxide.

MgO + H₂O → Mg(OH)₂

19) Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Ans) The activity series of metals is very useful in predicting the products formed during a displacement reaction and also the reactivity of metals in other reactions. While taking a look into the position of zinc and copper in the reactivity series, we can find that zinc is placed much above copper. This means that the metals above hydrogen are more reactive than hydrogen and the ones below it. These metals can easily displace hydrogen from acids or water and liberate hydrogen gas whereas those below hydrogen are less reactive than hydrogen and they do not replace hydrogen from water or acid.

So, zinc will react with hydrochloric acid to produce zinc chloride and hydrogen gas.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Zinc will reduce the hydrogen to diatomic hydrogen gas and get oxidized in the process.

In case of copper, no reaction takes place when copper is placed in hydrochloric acid as copper is less reactive than hydrogen and thus can’t displace hydrogen from its compound.

Cu(s) + HCl(aq) → NO REACTION

20) A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and gives its chemical formula.

Ans) a)The atmospheric air is made of moisture and gases like nitrogen, carbon dioxide, oxygen, argon, etc along with air pollutants such as sulphur dioxide, hydrogen sulphide, oxides of nitrogen, chlorides etc. When silver is exposed to air, the silver starts reacting with the oxygen present in the atmosphere and forms silver oxide. In the presence of moisture and hydrogen sulphide, it combines with sulfur and forms silver sulfide. Silver sulfide is black solid and when this silver sulfide forms a thin coating on the surface of silver, it darkens the silver. This phenomenon is called tarnishing of silver. Reaction involve in this is given here:

2Ag + H₂S → Ag₂S + H₂

(b) The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag₂S.