Question 1. 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
Solution 1.
| chemical change | Physical change |
| Photosynthesis
|
Dissolving sugar in water |
| Burning of coal | Melting of wax |
| Digestion of food
|
Beating aluminium to make aluminium foil |
Question 2. (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. False
Solution 2. (a)Cutting a log of wood into pieces is a chemical change. False
(b) Formation of manure from leaves is a physical change. False
(c)Iron pipes coated with zinc do not get rusted easily. True
(d)Iron and rust are the same substances. False
(e)Condensation of steam is not a chemical change. True
Question 3. 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 __________.
(b) The chemical name of baking soda is ____________.
(c) Two methods by which rusting of iron can be prevented are __________ and __________
(d) Changes in which only ____________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _____________ changes.
Solution 3. a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
(c) Two methods by which rusting of iron can be prevented are painting or greasing and galvanization
(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.
Question 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Solution 4. This type of change is called a chemical change.
The chemical reaction between baking soda and lemon juice can be expressed as.
Lemon juice (citric acid) + baking soda (sodium hydrogen carbonate) = co2 (bubbles) + other substances
Question 5. 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.
Solution 5. Burning of a candle is the process in which both chemical and physical take place
Physical process: melting of wax and vaporization of melted wax
Chemical process: burning of wax vapours to produce carbon dioxide, light and heat.
Another example of the same kind of process where both chemical and physical changes take place is LPG when LPG comes out of the cylinder it changes from liquid to gaseous form this is the physical change. The chemical change occurs when the gas burns in air.
Question 6. How would you show that the setting of curd is a chemical change?
Solution 6. The setting of curd is a chemical change because this process is irreversible as you can not get milk back from curd. Moreover, the texture and taste of curd is different from milk hence it is a chemical change.
Question 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Solution 7. Burning of wood is a chemical process because in this process wood burns along with oxygen and produce charcoal, carbon dioxide, heat and light.
Wood + oxygen –àcharcoal + carbon + heat + light
Cutting of wood into small pieces is a physical change because the size of wood is changed and no new substances are formed.
Question 8. Describe how crystals of copper sulphate are prepared.
Solution 8. Add a few drops of dilute sulphuric acid to a cupful of water in a beaker. Heat the water till it starts to boil and start adding copper sulphate slowly to the solution till no more powder can be dissolved. Now leave the solution after filtering it for cooling. After sometime you will find the crystal of copper sulphate.
Question 9. Explain how painting of an iron gate prevents it from rusting?
Solution 9. Rusting of iron happens when it is exposed to oxygen and moisture. On painting the iron gate, oxygen and moister can not make contact with the iron gate and hence, it is prevented from rusting.
Question 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Solution 10. In coastal areas, the amount of moisture in the air is higher than in the air in deserts. So, in coastal areas, the process of rusting iron is faster than in deserts.
Question 11. 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 bums (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.
Solution 11. (ii) Process B is a chemical change.
Question 12. 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.
Solution 12. (iii) Both processes A and B are chemical changes.