Notes Science Grade 7
PHYSICAL AND CHEMICAL CHANGES (Unit 7)
1. What is meant by physical change? Give some examples of physical changes.
Ans: A Physical change is one in which only the physical properties of a substance change and its chemical composition remains the same. It is a temporary change.
( Size, shape color, etc. are physical properties of a substance.)
Freezing of water, melting of wax, cutting fruit into pieces, and evaporation of water are examples of physical changes.
2. What is meant by chemical change? Give some examples of chemical changes.
Ans: A Chemical change is one in which a new substance is formed. It is a permanent change. Burning of paper or wood, rusting of iron, and turning of milk into yogurt are examples of chemical changes.
3. Give an example to show that people change the environment.
Ans: People change the environment through their activities. The excessive use of fertilizers is a cause of water pollution. The smoke released from vehicles and industries is the main cause of air pollution. Water and air pollution are changing the environment badly.
4. What is hydrogenation?
Ans: When hydrogen is passed through the vegetable oil in the presence of nickel, it converts into solid fat. This process is called hydrogenation.
5. What is plastic?
Ans: Plastics are very large molecules made from many smaller molecules called monomers. That is why plastics are called polymers. Polyethylene and polyvinyl chloride (PVC) are examples of plastics.
6. What is fertilizer?
Ans: A substance that adds minerals to the soil is called fertilizer. It is used to increase the fertility of the soil.
1. Explain with examples that a chemical change brings a change in the properties of a substance.
Ans: Rusting iron is a chemical change. Iron turns into a yellowish-brown substance.
It is different from iron.
Photosynthesis is a chemical process. In this process, carbon dioxide gas reacts with liquid water in presence of sunlight and chlorophyll. In this process, glucose is formed which is solid. Oxygen is also released during this process which is different from carbon dioxide. Products are different substances.
2. Write a brief note on plastics.
Ans: Plastics are also the result of chemical changes. It can be molded into any form. Plastics are very large molecules made from many smaller molecules called monomers. That is why plastics are called polymers. Polyethylene and polyvinyl chloride (PVC) are examples of plastics. Toys, bottles, utensils, etc. are made of plastics. Plastics do not decay therefore are a cause of pollution.
3. Write a brief note on “change of vegetable oil into fat.
Ans: A chemical process of hydrogenation changes vegetable oil into solid fat called Banaspati ghee. When hydrogen is passed through the vegetable oil in the presence of nickel, it converts into solid fat. This process is called hydrogenation. A large amount of heat is used to bring about this chemical change.
4. How fertilizers are useful and harmful for us?
Ans: Useful Effects of Fertilizers: Repeated cultivation of crops decreases the fertility of the soil. Farmers use chemicals to increase the fertility of the soil. Most fertilizers supply nitrogen (N), phosphorous (P), and potassium (K) elements to the soil known as NKP.
Harmful Effects of Fertilizers: In the case of excessive use of fertilizer, some components are not absorbed by the plants. These fertilizers may reach into canals causing water pollution.
5. Explain reversible and irreversible changes with examples.
A change that can go forwards or backward is called a reversible change. It is a temporary change. We can get the same thing again.
Examples: Melting of ice into liquid water.
Mixing salt in water.
Wetting a dry cloth.
It is a permanent change. We cannot get the things in their original form.
Examples: Turning milk into yogurt.
Mixing of plaster of Paris with water.
Burning of paper and wood.
Tests of this unit are also available on this site.