Materials: Metals and Non-metals
Science Class Eight
NCERT Exercise Solution
Question (1) Which of the following can be beaten into thin sheets?
(a) Zinc
(b) Phosphorous
(c) Sulphur
(d) Oxygen
Answer (a) Zinc
Explanation Metals are malleable. Malleability is a characteristic property of metals because of which metals can be beaten into thin sheets.
In the give option only (a) Zinc is the metal and rest are non-metals. Thus option (a) Zinc is the correct answer.
Question (2) Which of the following statements is coreect?
(a) All metals are ductile
(b) All non-metals are ductile
(c) Generally, metals are ductile
(d) Some non-metals are ductile
Answer (a) All metals are ductile
Explanation Ductility is a characteristic property of metals due to which a metal can be drawn into a thin wire. However since, mercury exists in the liquid state at room temperature, thus mercury cannot be drawn into thin wire at room temperature.
Thus, option (a) All metals are ductile is the correct answer.
Question (3) Fill in the blanks
(a) Phosphorous is a very ________ non-metal.
Answer (a) Phosphorous is a very reactive non-metal.
Explanation Phosphorous is very reactive non-metal. It catches fire when left in air. And generally non metals do not react with water. This is the cause that phosphorous is stored in water.
(b) Metals are ______ conductors of heat and ______.
Answer (b) Metals are good conductors of heat and electricity.
Explanation Conductivity is a characteristic property of metals. All metals are good conductors of heat and electricity. This is the cause that electric wires and cooking utensils are made of metals.
Non-metals are bad conductor of electricity and heat. But graphite, which is a form of non-metal carbon, is a good conductor of electricity and it is an exceptional non-metal.
(c) Iron is ________ reactive than copper.
Answer (c) Iron is more reactive than copper.
Explanation Iron is more reactive than copper metal. This is the cause that when iron nails is dipped in the aqueous solution of copper sulphate, iron replaces copper from the solution of copper sulphate and iron sulphate is formed. Such reactions are called Displacement Reactions.
(d) Metals react with acids to produce _______ gas.
Answer (d) Metals react with acids to produce hydrogen gas.
Explanation Most of the metals produce hydrogen gas when react with acid. For example when zinc metal reacts with hydrochloric acid it produces hydrogen gas and zinc chloride.
Question (4) Mark 'T' is the statement is true and 'F' it is false.
(a) Generally, non-metals react with acids.
Answer: F
Explanation Generally non-metals do not react with acid while metals form hydrogen gas when react with acids.
(b) Sodium is very reactive metal.
Answer T
Explanation Sodium is very reactive metal. Sodium reacts with water vigorously and catches fire producing sodium hydroxide and hydrogen. Sodium reacts with moisture present in air also. This is the cause that sodium metal is preserved in kerosene oil.
(c) Copper displaces zinc from zinc sulphate solution.
Answer F
Explanation Copper is less reactive metal than zinc. Because of this copper does not displace zinc from the aqueous solution of zinc sulphate. On the other hand zinc displaces the copper from the aqueous solution of copper sulphate.
(d) Coal can be drawn into wires.
Answer F
Explanation: Non-metals are not ductile. Ductility is a characteristic property of metals because of which a metal can be drawn into wire. Since coal is a form of carbon which is a non metal. Thus, coal cannot be drawn into wires.
Question (5) Some properties are listed in the following Table. Distinguish between metals and non-metals on the basis of these properties.
| Properties | Metals | Non-metals |
|---|---|---|
| Appearance | ||
| Hardness | ||
| Malleability | ||
| Ductility | ||
| Heat conduction | ||
| Conduction of electricity |
Answer
| Properties | Metals | Non-metals |
|---|---|---|
| Appearance | Shiny | Dull |
| Hardness | Hard | Brittle |
| Malleability | Malleable | Non-malleable |
| Ductility | Ductile | Non-ductile |
| Heat conduction | Good conductor of heat | Bad conductor of heat |
| Conduction of electricity | Good conductor of electricity | Bad conductor of electricity |
Question (6) Give reasons for the following:
(a) Aluminium foils are used to wrap food items.
Answer A thin layer of aluminium oxide is formed when it reacts with oxygen. This thin layer of aluminium oxide prevents the further reaction of aluminium with any other substance.
This is the cause that aluminium foils are used to wrap food items to keep the food items safe for longer time.
(b) Immersion rods of heating liquids are made of metallic substances.
Answer Immersion rods of heating liquids are made of metallic substances because metals are good conductor of electricity.
(c) Copper cannot displace zinc from its salt solution.
Answer Copper is less reactive than zinc metal. This is the cause that copper cannot displace zinc from its salt solution.
(d) Sodium and potassium are stored in kerosene.
Answer Sodium and potassium are very reactive metals and reacts vigorously with water. Sodium and potassium even reacts with moisture present in air.
This is the cause that sodium and potassium are stored in kerosene oil to prevent reacting them with moisture present in air.
Question (7) Can you store lemon pickle in an aluminium utensil? Explain.
Answer No, lemon pickle cannot be stored in aluminium utensils.
Metals produce hydrogen gas when react with acid. And aluminium is a metal and lemon contains acid. Thus, if lemon pickle is stored in an aluminium utensil, it reacts with aluminium and will spoil the lemon pickle.
Question (8) Math the substances given in Column A with their uses given in the Column B
| Column A | Column B |
|---|---|
| (i) Gold | (a) Thermometers |
| (ii) Iron | (b) Electric wire |
| (iii) Aluminium | (c) Wrapping food |
| (iv) Carbon | (d) Jewelry |
| (v) Copper | (e) Machinery |
| (vi) Mercury | (f) Fuel |
Answer
| Column A | Column B |
|---|---|
| (i) Gold | (d) Jewellery |
| (ii) Iron | (e) Machinery |
| (iii) Aluminium | (c) Wrapping food |
| (iv) Carbon | (f) Fuel |
| (v) Copper | (b) Electric wires |
| (vi) Mercury | (a) Thermometers |
Question (9) What happens when
(a) Dilute sulphuric acid is poured on a copper plate?
Answer
Copper is a less reactive metal and do not react with extremely dilute acids. But copper reacts with less dilute sulphuric acid, i.e. dilute acid with relatively more concentration of acid content.
But when less dilute sulphuric acid that is with high concentration of acid is poured on a copper plate, hydrogen gas is evolved and copper sulphate is formed.
Copper (Cu) + Sulphuric acid (H2SO4) ⟶ Hydrogen (H2) + Copper sulphate (CuSO4)
However, copper reacts with dilute sulphuric acid slowly and copper does not react with dilute hydrochloric acid even on heating.
(b) Iron nails are placed in copper sulphate solution?
Answer When iron nails are placed in copper sulphate solution, iron displaces copper and forms iron sulphate.
Iron nails (Fe) + Copper sulphate (CuSO4) ⟶ Iron sulphate (FeSO4) + Copper (Cu)
In this reaction the blue colour of copper sulphate solution turns into light green because of formation of iron sulphate. It is a displacement reaction.
Question (10) Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.
(a) How will she find the nature of the gas?
(b) Write down word equations of all the reaction taking place in this process.
Answer
(a) How will she find the nature of the gas?
She will find the nature of the gas by bringing moist litmus paper near the mouth of the test tube.
When a moist blue litmus paper is brought near the mouth of the test tube, in which gas is collected, the blue litmus paper turns to red. This indicates that the collected gas is acidic in nature.
(b) Write down word equations of all the reaction taking place in this process.
When charcoal is burnt in air, carbon dioxide gas is produced.
Charcoal (Carbon) + Air (oxygen) ⟶ Carbon dioxide (CO2)
When a moist litmus paper is brought near the mouth of the test tube, the water present in the moist litmus paper react with carbon dioxide gas and carbonic acid is formed.
Carbon dioxide (CO2 + Water (H2) ⟶ Carbonic acid (H2CO3)
This carbonic acid turns blue litmus paper red.
Question (11) One day Reeta went to a jeweller's shop with her mother. Her mother gave an old gold jewellery to the goldsmith to polish. Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason the loss in weight?
Answer
Aquaregia is a very strong acid and is used in cleaning of jewelleries. Aquaregia is the mixture of nitric acid and hydrochloric acid in the ration of 1:3.
Aquaregia dissolves gold in it which can be recovered latter using chemical process. Aquaregia should be applied in very little amount to clean jewelleries. But some cunning goldsmith uses more aquaregia in cleaning so that some of the gold could be stolen.
Thus, Reeta's mother found a slight loss in the weight of her gold jewellery due to using of more aquaregia in order to stolen some of the gold by cunning goldsmith while cleaning it.
Reference: