Exemplar Questions and Solutions

Important CBSE Questions – Important Long Answer Questions

1) What is rancidity? Write different methods of preventing rancidity.

Ans) Foods containing fats, oil or butter start to impart foul smell and change in taste due to oxidation is called rancidity

 METHODS TO PREVENT RANCIDITY:

(i)  By filling nitrogen in the food storage bags.

(ii)   The food materials should be placed at very low temperatures.
(iii)    Addition of antioxidants also prevents rancidity process.

(iv) Keeping food item in air tight containers


2) Name the type of reactions in the following cases:

     a. Garbage producing foul smell

     b. Burning of natural gas.

     c. Carbon dioxide gas passed through lime water.

Ans) a)Garbage producing foul smell  is a decomposition reaction wherein organic wastes are decomposed by microorganisms to produce methane gas (CH₄) and Hydrogen sulphide(H₂S) gas. It is an exothermic reaction.

b) Burning of natural gas is an oxidation reaction and is also an exothermic reaction.

c) Carbon dioxide gas passed through lime water- When carbon dioxide gas is passed through lime water, it turns milky due to the formation of calcium carbonate(white precipitate) which is insoluble in water. So it is a precipitation reaction and a combination reaction. The reaction is exothermic

3)What are the different kinds of decomposition reaction?

Ans) The different kinds of decomposition reactions are-

a) Thermal decomposition– When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition’.

Example: Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction

b)Electrolytic decomposition- When a decomposition reaction is carried out by using electric current, it is called electrolytic decomposition.

Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.

c)Photolytic decomposition- When a decomposition reaction is carried out by using light energy, it is called photolytic decomposition

Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

4)A light sensitive compound X of silver is used in photography. Once exposed to sunlight, the colourof the Compound X changes to grey.

a)Identify X

b)Write a chemical equation to express the above change

c) Identify the type of chemical reaction

Ans) The light sensitive compound X is Silver bromide

c) It is a clear example of photolytic decomposition reaction as it involves the decomposition of Silver bromide using light

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 precipitate of barium sulphate.

6)List three effects of oxidation in our daily life. Are these effects useful or harmful. Justify:

Ans)The three effects of oxidation that takes place in our daily life are-

a) Corrosion- Corrosion is the atmospheric oxidation of metals. That means that oxygen combines with the metal and forms a new layer. Corrosion can be useful as well as harmful. Rusting of iron makes iron weak and can cause hazards when used in building constructions, ship building etc. But, metals like Aluminium, Chromium, Zinc etc corrode more readily than iron. But their oxides form a coating on the metal that in turn protects the metal from further attack.

b) Combustion of fuels- This is useful and is caused due to oxidation of fuels. They provide us huge amounts of heat energy.

Rancidity- It is caused by the oxidation of foods rich in fats and oils. They make the food inedible, so is considered to be harmful.

7) What are the important rules for writing word equations?

Ans) a) Always write the names of the reactants on the left hand side with a plus sign(+) in between the reactants(if there are more than one reactant).

b) Always write the names of the products on the right hand side with a plus sign(+) in between the products(if there are more than one product).

c) Put an arrow mark between the reactants and products. The arrowhead points towards the products, and shows the direction of the reaction.

For example,

When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.

The word-equation for the above reaction would be –