Science 3 Marks Question

Crystals of copper sulphate are prepared by the method of crystallization. The process is as follows. A cupful of water is taken in a beaker. Few drops of dilute sulphuric acid are added to this. The water is then heated and when it starts boiling, copper sulphate powder is added with stirring. Copper sulphate powder should be added on till the solution becomes saturated. It is then filtered into a china dish and allowed to cool. The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.

When a candle burns, both physical and chemical changes take place.

Physical change → Melting of wax

Chemical change → Burning of wax

Eating of food is another example where both physical and chemical changes occur simultaneously

Physical change → Breaking down of larger food particles into smaller particles

Chemical change → Digestion of food

When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. The bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change.

In this change, citric acid contained in lemon juice reacts with sodium hydrogen carbonate which results in the evolution of carbon dioxide gas.

Lemon juice( Citric acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + Other substances

When we burn wood, a new substance, coal, is formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance is formed. Therefore, it is a physical change.

Both air and moisture are required for rusting to take place. In coastal areas, the quantity of moisture present in air is more than that in deserts. In desert areas, the amount of moisture in air is even lower. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

1. Galvanisation: Depositing a layer of zinc or chromium on iron to prevent it from rusting is known as galvanization. Doing this prevents direct contact of iron with air and oxygen and thus prevents it from rusting.
2. Crystallisation: Some substances can be obtained in pure state from their solutions by the process of crystallisation.

A change in which the composition of a substance is altered is called as chemical change. As a result, the original properties get changed and one or more new substances are formed. Burning of paper, rusting of iron, cooking of food. All new substances are formed as a result of chemical reactions, a medicine is the end product of the chain of chemical reaction, photosynthesis, digestion is the chemical reaction, useful materials like plastics, detergents and many more products are formed by chemical reactions only, thus we can say chemical changes are very important in our life.

A part from new products, many other things accompany a chemical change, those things are:

1. Heat, light or any other radiation may be given off or absorbed.
2. Change in smell may take place.
3. Sound may be produced.
4. Change in colour may take place.
5. A gas may be formed.

In addition to new products, the following may accompany a chemical change:

1. Heat, light or any other radiation (e.g. ultraviolet) may be given off or absorbed.
2. Sound may be produced.
3. A change in smell may take place or a new smell may be given off.
4. A colour change may take place.
5. A gas may be formed.

1. Sea water is collected in shallow pits. It is then allowed to evaporate in the sun. As, the water evaporates, the salt solution becomes supersaturated (concentrated). This supersaturated solution cannot hold the excess salt. Thus, it separates out in the form of salt crystals. These salt crystals are collected and are redissolved in water and filtered to remove insoluble impurities. The clear solution is again evaporated to obtain the crystals of pure salt.
2. Salt is obtained from sea water by the process of evaporation.
3. The values shown by Karan are curious, aware and intelligence.

When a candle burns, both physical and chemical changes take place.

• Physical change → Melting of wax
• Chemical change → Burning of wax

Eating of food is another example where both physical and chemical changes occur simultaneously.

• Physical change → Breaking down of larger food particles into smaller particles
• Chemical change → Digestion of food

Once the curd is formed, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.