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Page No 90:

Question A:

Solve the following crossword puzzle:
figure

Across
2. the process of coating a thin layer of tin on utensils
4. An alloy of iron
5. A reddish brown, soft, porous, flaky coating on iron articles

Down
1. The common name of acetic acid
3. The colour of crystals of copper sulphates

Answer:



Page No 91:

Question B.1:

Properties of substances usually remain same in a
(a) chemical changes
(b) physical change
(c) periodic change
(d) irreversible change

Answer:

(b) physical change

During a physical change, the properties of substances remain the same.

Page No 91:

Question B.2:

Melting of wax and heating of blade are the examples of
(a) physical change
(b) chemical change
(c) periodic change
(d) irreversible change

Answer:

(a) physical change

Melting of wax and heating of blade are the examples of physical change.

Page No 91:

Question B.3:

Eating away of iron due to rusting is called
(a) corrosion
(b) tinning
(c) alloying
(d) none of these

Answer:

(a) corrosion

Eating away of iron due to rusting is called corrosion.

Page No 91:

Question B.4:

Condition necessary for rusting is
(a) presence of air
(b) presence of water
(c) both (a) and (b)
(d) none of these

Answer:

(c) both (a) and (b)

Both the presence of air and water are necessary for rusting.

Page No 91:

Question B.5:

New substance is formed in this change
(a) physical
(b) chemical
(c) periodic
(d) irreversible

Answer:

(b) chemical

In a chemical change, a new substance is formed. This newly formed substance shows different properties from the original substance.

Page No 91:

Question B.6:

Is an alloy
(a) zinc
(b) tin
(c) stainless steel
(d) gold

Answer:

(c) stainless steel

Stainless steel is an alloy of iron.

Page No 91:

Question B.7:

The metal used for galvanisation is
(a) tin
(b) zinc
(c) lead
(d) copper

Answer:

(b) zinc

Galvanisation is a process of applying a thin coat of zinc on the surface of the iron sheets so as to prevent it from rusting.

Page No 91:

Question C:

Fill in the blanks:
1. ........................ is hydrated ferric oxide.
2. ........................ is used for galvanising iron.
3. We get ........................ by the process of crystallisation.
4. A chemical change is usually ........................ and permanent.
5. All chemical changes involve ........................ or ........................ of energy.

Answer:

1. Rust is hydrated ferric oxide.
2. Zinc is used for galvanising iron.
3. We get pure substances by the process of crystallisation.
4. A chemical change is usually irreversible and permanent.
5. All chemical changes involve absorption or liberation of energy.

Page No 91:

Question D:

Match the items in Column A with the items in Column B:

Column A Column B
1. Ripening of fruit (a) Stainless steel
2. Cutting of wood (b) Breaking a brick
3. Galvanisation (c) Chemical change
4. Alloy (d) Zinc
  (e) Physical change

Answer:

  Column A Column B
1. Ripening of fruit (c) Chemical change
2. Cutting of wood (e) Physical change
3. Galvanisation (d) Zinc
4. Alloy (a) Stainless steel

Page No 91:

Question E:

Write True (T) or False (F) against the following statements in the given brackets:
1. Souring of milk is a physical change. ( )
2. Digestion of food is a physical change. ( )
3. Crystallisation is widely used to prevent rusting. ( )
4. Formation of steam from water on heating is a physical change. ( )
5. Glowing of the filament of a bulb is an example of a chemical change. ( )

Answer:

1. False
Souring of milk is a chemical change.

2. False
Digestion of food is a chemical change.

3. False
Crystallisation is widely used to obtain a compound in its purest form.

4. True

5. False
Glowing of the filament of a bulb is an example of a physical change.



Page No 92:

Question 1:

Why do stainless steel utensils not rust?

Answer:

Stainless steel utensils are alloys of iron. Alloys are the mixture of either two or more same metallic solid solution or different metallic solid solution. When a metal is alloyed with other metal, its resistance towards corrosion increases. Stainless steel is one such alloy; hence, it does not rust.

Page No 92:

Question 2:

Why do people paint the iron gates of their houses during the rainy season?

Answer:

During rainy season, the iron gates are directly exposed to the rain water and air, which causes rusting. Rusting is a continuous process that corrodes the metal and makes it less durable, and it occurs in the presence of air and water. Hence, the iron gates are coated with paint to  prevent corrosion.

Page No 92:

Question F:

Properties of physical and chemical changes are given below. Sort the properties of physical and chemical changes in the correct column.

1. New substances are formed.
2. Precipitation is an example of this.
3. Products have new composition.
4. Composition of substance remains same.
5. It is usually permanent.
6. It is usually temporary.
7. Burning of LPG is an example of this.
8. It is reversible.
 

Physical change Chemical change
1.
2.
3.
4.
5.
6.
7.
8.

Answer:

 Physical change   Chemical change
1. Precipitation is an example of this. 5. New substances are formed.
2. Composition of substance remains same. 6. Products have new composition.
3. It is usually temporary. 7. It is usually permanent.
4. It is reversible. 8. Burning of LPG is an example of this.

Page No 92:

Question G:

Some techniques to prevent corrosion are mentioned in Box 2. Find the actual terms used for them and examples of each technique from the Box 1:

figure

Answer:



(a) Galvanisation : Thin coat of zinc : Roofing
(b) Electroplating : Coating using electric current : Bicycle handles
(c) Tinning : Coating with molten tin : Cooking vessels
(d) Anodising : Coating metals with films of their oxides : Soap cases



Page No 94:

Question A.1:

Write any two features of a physical change.

Answer:

Following are the two features of a physical change:
(i) No new substance is formed.
(ii) The change is not permanent and can be reversible.

Page No 94:

Question A.2:

Write any tow features of a chemical change.

Answer:

Following are the two features of a chemical change:
(i) A new substance is formed and has different properties from the original substance.
(ii) Energy in the form of heat and light is either absorbed or released during the change.

Page No 94:

Question A.3:

How are alloys made? Name one alloy.

Answer:

Alloys are made by combining two or more metals or metals with other non-metals. Stainless steel is an alloy of iron made by combining iron and carbon.

Page No 94:

Question A.4:

What is the harmful effect of corrosion?

Answer:

Iron is used as a main component in the construction of bridges, buildings, rails, etc. Corrosion of iron may damage these iron structures and may result in accidents or disasters. Corrosion causes great economic loss.

Page No 94:

Question A.5:

Why is stainless steel preferred to make utensils?

Answer:

Stainless steel is an alloy of iron and does not corrode easily, even when continuously exposed to water and air. Hence, stainless steel is preferred to make utensils.

Page No 94:

Question A.6:

Define electroplating.

Answer:

Electroplating is a process of covering the surface of the metal with another thin layer of metal using electric current (electrolysis). Electroplating is a method to prevent corrosion of metals.

Page No 94:

Question A.7:

Growing of seed into a plant is which type of change?

Answer:

Growing of seed into a plant is a chemical change because the product (plant) formed is different from the original (seed). Further, the change of a seed into a plant is not reversible.

Page No 94:

Question A.8:

Why are tools and machine parts coated with greases?

Answer:

Tools and machine parts are coated with grease to prevent corrosion.

Page No 94:

Question B.1:

Formation of compost is a chemical change. Explain how.

Answer:

During the formation of compost, the wastes are decomposed into nutrient-rich organic substances. The newly formed substance exhibits completely different properties from the original substance. This process is irreversible because it cannot be converted back to the original substance by reversing the process. Hence, it is a chemical change.

Page No 94:

Question B.2:

What are the conditions for rusting?

Answer:

Rusting of metal takes place under following conditions:
(i) Presence of atmospheric oxygen or air
(ii) Presence of water or moisture

Page No 94:

Question B.3:

What is crystallisation and in what way is the process useful to us?

Answer:

Crystallisation is a process of obtaining pure solid crystals of a compound from hot saturated solution on cooling.
Crystallisation helps to remove impurities from a solid substance or to purify the solid substances.

Page No 94:

Question B.4:

Why do ornaments of gold and silver look dull after using for some time?

Answer:

The dullness of the gold and silver ornaments over a period of continuous use is due to corrosion. When the metals are continuously exposed to air or moisture, their surfaces react with air and water, thus forming their respective salts (nitrates or sulphides). Therefore, the metal loses its lustre.

Page No 94:

Question B.5:

What do you understand by corrosion of metals?

Answer:

When metals are continuously exposed to air and moisture, their metal surfaces react with the oxygen and other gases present in the air and form their respective oxides (hydroxides and carbonates). This process is called corrosion of metals.

Page No 94:

Question B.6:

Why do we give coating of tin metal on utensils made of brass?

Answer:

Brass utensils are coated with tin metal to prevent them from corrosion. This process is called tinning.

Page No 94:

Question B.7:

What is tinning?

Answer:

Tinning is a process of coating a thin layer of molten tin over a metal surface to prevent the metal from corrosion. Cooking vessels made of copper and brass are coated with tin.

Page No 94:

Question C.1:

Explain how burning of a paper is different from tearing it.

Answer:

Tearing of paper does not result in the formation of any new substance. The resulting pieces of paper will have the same properties as that of the original paper. Hence, this is a physical change. Whereas when the paper is burnt, it liberates heat and light and forms ashes. These ashes exhibit different properties from that of the original paper. Also it is a irreversible process. The change is permanent; hence, it is a chemical change.

Page No 94:

Question C.2:

How will you prepare crystals of alum? Explain.

Answer:

Prepare a supersaturated solution of alum in a china dish by dissolving the powdered alum in water. Add the alum to the water and dissolve it until it reaches its saturation point. Heat the solution. After some time, the water starts to evaporate. Heat the solution until the entire water in the solution gets evaporated. Switch off the flame and observe. The residue of crystalline substance can be seen. Allow the dish to cool to recover the crystals of alum.

Page No 94:

Question C.3:

Explain four methods by which rusting can be prevented.

Answer:

The following methods are used to prevent rusting:

(i) Coating of oil and grease to machinery parts and other tools.
(ii) Painting of metals used in railway coaches, bridges, furniture, automobiles, etc.
(iii) Coating of non-corrosive metals.
(iv) Alloying of metals with other metals.

Page No 94:

Question C.4:

Classify the following changes as physical and chemical changes: digestion of food, making sugar syrup, burning of wood, cutting of fruits, boiling of egg, grinding wheat into flour, and stitching fabric into garments.

Answer:

 Physical Changes Chemical Changes
 Making sugar syrup  Digestion of food
 Cutting of fruits  Burning of wood
 Grinding of wheat into flour  Boiling of egg
 Stitching fabric into garments  



Page No 95:

Question C.5:

When magnesium burns in air, a while powder is formed. Explain.

Answer:

When magnesium burns in the air, it reacts with the oxygen present in air and burns with a dazzling light, thus producing dense white fumes. These white fumes, on cooling, form a powder called magnesium oxide that exhibits completely different properties from that of magnesium. This change is irreversible; hence, it is a chemical change.

Page No 95:

Question C.6:

What happens when baking soda is added to vinegar? Write a word equation for this reaction.

Answer:

When baking soda is added to vinegar in a test tube, a hissing sound is heard, followed by the bubbles of a gas escaping the test tube. At this stage, if lime water is passed through the test tube, the lime water turns milky, indicating that the gas liberated during the process is carbon dioxide gas. Also, a new substance calcium carbonate is formed.

This can be represented by the following equation:
Vinegar + Baking soda → Carbon dioxide + Other substances
Lime water + Carbon dioxide → Calcium carbonate + Water



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