Physical and Chemical Changes is Chapter 6 taken from JKBOSE and NCERT textbook of Science for students of Class 7th. In the previous post, I provided you with Questions and Answers for Chapter 5 Acid Bases and Salts. This post is about Physical and Chemical Changes Class 7th Question Answers. Before we proceed to our Question Answers section of the chapter, you need to have basic information about the chapter you have studied in your classroom reading. So here is a brief overview of the chapter:
Physical and Chemical Changes Class 7th Question Answers
Overview of Chapter
- PHYSICAL CHANGES.
- CHEMICAL CHANGES.
- RUSTING OF IRON.
- CRYSTALLISATION.
PHYSICAL CHANGES: The changes in which physical properties of a substance undergoes change are termed physical changes. The properties like shape, size, state and colour are physical properties. It means when a substance undergoes a change in any of these properties it is termed a physical change. No new substance is formed in these changes. For example, Cutting or tearing of paper, melting of ice and freezing of water, boiling of water and condensation of steam etc. are all physical changes.
CHEMICAL CHANGES: The changes in which new substances are formed after the reaction of two or more substances are termed chemical changes. They are also called chemical reactions. Chemical changes are permanent in nature and usually are not reversed to form the original substance. The chemical reactions either absorb or release a lot of energy through heat light and sound. For example, the souring of milk, rusting of iron, cooking of food, photosynthesis, burning of magnesium ribbon etc are all chemical changes.
RUSTING OF IRON: The formation of a thin film of brownish substance on the surface of iron objects when they are exposed to moist air is called rusting of iron and a thin layer of brownish substance is called rust. The presence of moist air or oxygen and water/water vapours is essential for rusting of iron. Rusting of iron eats away iron objects and makes them useless. Rusting of iron can be prevented by painting, applying grease, applying oil, galvanising, chromium plating and alloying etc.
CRYSTALLISATION: The process of cooling hot concentrated solutions of a substance to obtain its crystals is called crystallisation. Solid substances are usually purified by the process of crystallisation. It is an example of physical change.
EXERCISES
- Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f ) Digestion of food
Ans.
(a) Photosynthesis—-Chemical Change.
(b) Dissolving sugar in water—-Physical Change
(c) Burning of coal—–Chemical Change
(d) Melting of wax——Physical Change
(e) Beating aluminium to make aluminium foil——Physical Change
(f ) Digestion of food——Chemical Change
- State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Ans.
(a) False, cutting a log of wood into pieces is a physical change.
(b) False, Formation of manure from leaves is a chemical change.
(c) True
(d) Iron and rust are two chemically different substances.
(e) True
- Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
(b) The chemical name of baking soda is sodium bicarbonate or sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are painting and greasing.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.
- When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Ans. When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), a chemical reaction takes place. In this reaction, new substances are formed along with the evolution of carbon dioxide and heat.
- When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both chemical and physical changes take place.
Ans. The physical and chemical changes which take place in the burning of a candle are as under:
Physical Change: The melting of candle wax on heating and its solidification on cooling is called a physical change.
Chemical Change: The burning of wax vapours near a flame to give carbon dioxide, heat and light is a chemical change.
Burning of LPG in our home is another example in which both physical and chemical changes take place. The LPG gets changed into a gaseous state before burning is a physical change. Burning of LPG to form carbon dioxide, heat and light is a chemical change.
- How would you show that the setting of curd is a chemical change?
Ans. Curd is an entirely different substance from milk as it has different taste, smell and chemical properties. The change which takes place in milk during the setting of curd is permanent in nature and cannot be reversed back. So, the setting of curd is a chemical change.
- Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Ans. The burning of wood is a chemical change while cutting wood into small pieces is a physical change. Burning of wood produces gases and ash which are entirely different substances from wood so it is a chemical change. The cutting of wood into smaller pieces is a physical change because small pieces of wood have the same properties and no new substance is formed in this process.
- Describe how crystals of copper sulphate are prepared.
Ans. Preparation of crystals of copper sulphate.
- Take a cup of water in a beaker and add a few drops of dilute sulphuric acid to it.
- Heat the water.
- When water starts boiling, add copper sulphate powder to it while stirring continuously.
- Continue adding copper sulphate powder till the solution gets saturated (no more powder gets dissolved in the solution)
- Filter the solution.
- Now allow the solution to cool down without disturbing it.
- The crystals of copper sulphate solution can be seen at the bottom of the beaker after some time.
- Explain how the painting of an iron gate prevents it from rusting.
Ans. The rusting of iron is caused when the surface of the iron is exposed to moist air. When we apply paint on the iron gate the surface of the iron got covered with paint and hence there is no contact between moist air and the surface of the iron. So, painting of iron gate prevents it from rusting.
- Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans. The rusting of iron takes place in presence of moist air or both oxygen and water vapours. The coastal areas have a high amount of water vapours in the air while deserts have dry air. So rusting of iron objects takes place faster in coastal areas than in deserts.
- The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Ans. Process – B is a chemical change because gas burns to form carbon dioxide, heat and light in this process.
- Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Ans. (iii) Both these processes A and B are chemical changes because new substances are formed in both these changes.
Choose the correct one
- Rust is
(a) Carbon dioxide
(b) Iron
(c) Oxygen
(d) Iron oxide.
Ans. (d) Iron Oxide.
- Which of the following is not a physical change?
(a) Rusting of iron
(b) Melting of ice
(c) Freezing of water
(d) Dissolving sugar in water
Ans. Rusting of Iron.
That’s all about Physical and Chemical Changes Class 7th Question Answers. Hope it has helped you with your queries. Do share your views about this post in the comment section.
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