Q1. Make a list of ten changes you have noticed around you.
Ans:
i) Melting of ice. ii) Seasonal changes
iii) Evaporation iv) Making of tea
v) Stretching of rubber band vi) Turning on of t.v.
vii) Lighting of candle viii) Change of plant into tree
ix) Sunrise/sunset x) Blowing of balloon
Activity 6.1 – (perform the activity)
Q1. Is there a change in the property of the paper?
Ans: No, there is no change in the property of the paper.
Activity 6.2 – (Perform the activity)
Q1. Did you recover chalk from the dust?
Ans: Yes.
Activity 6.3 – (Perform the activity)
Q1. Does the water become solid ice once again?
Ans: Yes
Activity 6.4 – (Perform the activity)
Q1. Do you see any droplet of water there?
Ans: Yes. Droplets of water can be seen .
Activity 6.5 – (Perform the activity)
Q1. Does the colour of the tip of the blade change?
Ans: Yes, it changes to red.
Q2. Does it get black its original colour?
Ans: Yes.
Chemical changes : Activity 6.6 – (Perform the activity)
Q1. Does the ash look like magnesium ribbon?
Ans: No
Q2. Does the mixture turn red litmus blue?
Ans: Yes.
Q3. Does the mixture turn blue litmus red?
Ans: No
Q4. On the bases of this test, how do you classify the aqueous solution – acidic or basic?
Ans: Basic
Activity 6.7 – (Perform the activity)
Q1. Do you see any change in the colour of the solution?
Ans: Yes, the colour has changed from blue to red.
Q2. Take out the nail or the blade. Has it changed in any way?
Ans: Yes, a brown deposit can be seen on the nail.
Activity 6.8 – (Perform the activity)
Q1. What happens to the lime water?
Ans: The lime water turns milky.
Q2. Can you name the substance formed in this reaction?
Ans: The new substance formed is Calcium carbonate.
Q3. When the food gets spoilt it produces a foul smell, shall we call this change a chemical change?
Ans: Yes.
Q4. Is neutralization a chemical change?
Ans: Yes.
Q5. Can we call the breaking down of ozone a chemical change?
Ans: Yes.
Q6. We learnt in chapter 1 that plant produce their food by a process called photosynthesis. Can we call photosynthesis a chemical change?
Ans: Yes, because new substances are formed.
6.3. Rusting of iron :
Q1. How do we prevent rusting?
Ans: Rusting can be prevented by:
a) applying a coat of paint or grease regularly .
b) depositing a layer of chromium or zinc on iron.
6.4 Crystallization – Activity 6.9
Q1. Can you see the crystals of copper sulphate?
Ans: Yes.
Q2. Try to identify changes that you observe around you as physical or chemical changes.
Ans: Changes Nature
- Melting of ice physical
- Evaporation physical
- Lighting of candle physical
- Making of tea chemical
- Stretching of rubber band physical
- Change of plant to tree chemical
- Blowing of balloon physical
- Cooking food chemical
TEXT EXERCISES :
Q1. Classify the changes involved in the following processes as physical or chemical changes:
a) Photosynthesis chemical
b) Dissolving sugar in water physical
c) Burning of coal chemical
d) Melting of wax physical
e) Beating aluminum to make aluminum foil physical
d) Digestion of food chemical
Q2. 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. (T/F)
b) Formation of manure from leaves is physical change. (T/F)
c) Iron pipes coated with zinc do not get rested easily. (T/F)
d) Iron and rust are the same substances. (T/F)
e) Condensation of steam is not a chemical change. (T/F)
Ans:
a) False, It is a physical change .
b) False, It is a chemical change .
c) True .
d) False, Iron and rust are different substances .
e) True .
Q3. 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 .
Ans:
a) calcium carbonate b) sodium hydrogen carbonate c) painting, galvanisation
c) painting, galvanisation d) physical e) chemical
Q4. 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: It is a chemical change. A new substance carbon dioxide is formed.
Lemon juice + baking soda Carbon dioxide + other substances .
Q5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Ans: Melting of wax is a physical change. Burning of wax gives out smoke and CO2 which is a chemical change.
Digestion of food:
The breakdown of large food particles into small ones are physical changes. The digestion of food by different secretive juices and its absorption is a chemical change.
Q6. How would you show that setting of curd is a chemical change?
Ans: Curd is obtained when a sour substance is added to milk. It cannot be turned back to milk. The properties of milk and curd are different.
Q7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans: Burning of wood produces ash with liberation of CO2 . It cannot be turned back to wood and so it is a chemical change. Cutting of wood does not change the nature of the wood and no new substance is formed and therefore it is a physical change.
Q8. Describe how crystals of copper sulphate are prepared
Ans:
i) Take a cup full of water in a beaker and add a few drops of dilute sulphuric acid.
ii) Heat the water and when it starts boiling add copper sulphate powder slowly while stirring.
iii) Continue adding the powder till no more of it can be dissolved.
iv) Filter the solution and allow it to cool. Crystals of copper sulphate are formed.
Q9. Explain how painting of an iron gate prevents it from rusting.
Ans: Rusting of iron requires contact with water (moisture) and oxygen. The layer of paint will prevent the contact between iron, oxygen and water (moisture) and prevent it from rusting.
Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: The moisture content of the air is more in coastal areas than in desert areas and hence rusting of iron objects is faster in coastal areas.
Q11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exist 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:
ii) Process – B is a chemical change.
Q12. 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 processes A and B are chemical changes.
Extended learning – Activities :
Q1. Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?
Ans:
i) Rusting of iron : Iron is used to make a number of things like bridges, ships, cars, truck bodies etc. Rusting of iron destroys these things and the monetary loss due to rusting is huge.
Prevention :
Rusting can be prevented by painting the articles and by a process called galvanization which involves depositing a layer of zinc or chromium on iron.
ii) Spoilage of food : When food gets spoilt it is unfit to eat and so we have to throw away the food.
Prevention :
Keep the food in the refrigerator as the micro-organisms that spoil food can’t grow in low temperatures.
Q2. Reform the activity given :
Explain your observation :
(i) The nail in bottle A is rusted more than the nail in bottle B. No rusting is observed in bottle C.
All the things needed for rusting i.e moisture, air and iron are in bottle A. Therefore rusting is observed,where as in bottle B, the water is boiled and hence the dissolved oxygen is removed so rusting observed is loss. In bottle C, the layer of oil prevented the contact with air and hence rusting didn’t take place.