Class 8 Basic Science (English Medium) Chapter 07 Metals - Questions and Answers
Textbooks Solution for Class 8th Basic Science (English Medium) ലോഹങ്ങൾ | Text Books Solution Chemistry (English Medium) Chapter 07 Metals
Class 8 Chemistry - Questions and Answers
Chapter: 07 - Metals
* PROPERTIES OF METALS
1. Malleability
Metals can be beaten into thin sheets by hammering. This property of metals is called malleability. Gold is the most malleable metal
2. Ductility: Metals can be drawn into thin wires. This property is known as ductility. Platinum is the most ductile metal.
3. Hardness: Most metals are hard substances. It is due to their hardness, metals like iron are used for making tools, engine parts, rails etc. But there are a few metals that are soft. Lithium, sodium and potassium are examples of soft metals.
4. Metallic lustre: The surface of metals has a shining appearance. It is called lustre. The lustre of gold is one of the reasons for using it for making ornaments.
5. Thermal and electrical conductivity: All metals are conductors of heat and electricity. Copper, Aluminium and silver are good conductors. Silver is the best electric and thermal conductor. It is due to good electrical conductivity, we commonly use aluminium and copper for making electrical wires. Similarly
aluminium is widely used for making cooking vessels because of its good thermal conductivity.
6. Sonority: Metals have the ability to produce sound when it is tapped with hard materials. It is due to the sonority of metals, we use metals for making cymbals and bells.
7. High density: The density of most metals is very high. But for Lithium, sodium and potassium are metals having a low density.
Melting point: The melting point is the temperature at which solid changes into a liquid.
For example, the melting point of Iron is 1538°C.
Boiling point: It is the temperature at which a liquid boils and changes into a gas.
8. High melting point.
The melting points of most metals are very high. But Gallium and caesium are metals of low melting point.
Mercury is a metal that exists in a liquid state at room temperature.
* CHEMICAL PROPERTIES OF METALS.
1. Reaction with atmosphere.
When metals come into contact with the atmosphere, they react with gases present in the atmosphere like oxygen, carbon dioxide and water vapour. This is the reason for losing lustre on the metallic surface and getting tarnished.
2. Reaction with water.
Certain metals like sodium, potassium, calcium etc react with water to form hydrogen.
The equation of the reaction of sodium with water is given.
Sodium + water →Sodium hydroxide + hydrogen (2Na + 2H₂O →2NaOH + H₂)
3. Reaction with Acids.
Most of the metals can react with acids to form hydrogen gas.
The equations of reaction between a few metals with dilute hydrochloric acid are given below.
i. Zn + 2HCl → ZnCl₂ + H₂
ii. Mg+ 2HCl → MgCl₂ + H₂
iii. Fe + 2HCl → FeCl₂ + H₂
iv. 2Al + 6HCl → 2AlCl₃ + 3H₂
Lemon and most of the fruits contain acids. That is why we prefer steel knives to iron knives for cutting these. For the same reason, it is not wise to keep buttermilk in aluminium vessels.
Corrosion of Iron: Iron gets rusted due to the reaction with oxygen and moisture present in the atmosphere.
The presence of salts and acids increase the rate of corrosion. That is why the window bars in houses close to the seashore corrode faster. There are metals other than iron that undergo corrosion.
Corrosive metals can be protected by painting, electroplating etc.
Gold and platinum are non-corrosive metals.
METALS: PRACTICE QUESTIONS & ANSWERS.
1. List out the common features of metals.
Answer: Ductility, malleability, sonority, good thermal conductivity, good electrical conductivity, high density, high melting point, hardness.
2. Which is the most ductile metal?
Answer: Platinum.
3. Name the substance used for making filament?
Answer: Tungsten.
4. Why do we prefer copper and aluminium for making electrical wires?
Answer: Because they are good conductors of electricity.
5. Which metal is the best electric and thermal conductor?
Answer: Silver.
6. Name the property of metals made use of when it is used to make bells and cymbals?
Answer: Sonority.
7. Find the odd one from the following sets using the hint given.
a. Iron, platinum, copper, sodium. (Hint: hardness)
Answer: Sodium.
b. Lithium, silver, copper, gold. (Hint: density)
Answer: Lithium.
c. Gallium, platinum, aluminium, iron (Hint: Melting point)
Answer: Gallium.
8. When we use metallic articles, we make use of particular properties of metals. In this respect, fill the table given below.
A – good thermal conductivity.
B – Hardness.
C – Ornaments.
D – Malleability.
9. Bells are made of metals. What feature of metals is mainly made use of here?
Answer: Sonority.
10. Name a metal that is seen in a liquid state at room temperature.
Answer: Mercury.
11. Most of the metals are denser substances. Give a few examples for less dense metals.
Answer: Lithium, Sodium, Potassium.
12. When Gold is heated up to 1062°C, it is liquefied. This temperature is called ......
Answer: Melting point.
13. Gold is usually used for making ornaments. Give two features of gold in favour of this.
Answer: Attractive lustre, Non-corrosive.
14. Why it is said that it is not good to keep buttermilk in the aluminium vessel.
Answer: The vessel gets damaged due to the reaction with the acid present in the buttermilk.
15. Give two favourable situations for corrosion of iron.
Answer: Presence of moisture and acid.
16. Usually only newly formed metallic surfaces show metallic lustre. Why?
Answer: Because the compounds formed by the reaction of the metal with the atmosphere tarnishes the surface of the metals.
17. Give a few examples for metals that can react even with cold water.
Answer: Sodium, Potassium.
18. Metals like Sodium, potassium react with cold water. Identify the gas formed due to this reaction.
Answer: Hydrogen.
19. Write down the balanced equation of the reaction between sodium and water.
Answer: 2Na + 2H₂O →2NaOH + H₂
20. What are the products obtained when Zinc reacts with dilute hydrochloric acid.
Answer: Zinc chloride and hydrogen.
21. Name two metals that do not have a reaction with dilute acid.
Answer: Gold and platinum.
22. When zinc reacts with dilute hydrochloric acid, hydrogen gas is released. How will you confirm the presence of hydrogen?
Answer: Hold a glowing stick in the gas. If it burns with an explosion, the presence of hydrogen can be confirmed.
23. Balance the following equations.
i. Mg+ HCl → MgCl₂ + H₂
ii. Fe + HCl → FeCl₂ + H₂
iii. Al + HCl → AlCl₃ + H₂
iv. Na + H2O →NaOH + H₂
Answer:
i. Mg+ 2HCl → MgCl₂ + H₂
ii. Fe + 2HCl → FeCl₂ + H₂
iii. 2Al + 6HCl → 2AlCl₃ + 3H₂
iv. 2Na + 2H₂O →2NaOH + H₂
23. Why stainless steel knife is preferred to iron knife for cutting lemon?
Answer: Iron knife reacts with the acid present in the lemon. But stainless steel doesn’t react with acid.
24. Rahul is conducting an experiment to find out favourable situations for rusting of iron.
i. List out the materials he has to collect for this.
ii. What might be the conclusion of his project?
Answer:
i. Iron nail, cotton soaked in water, dilute hydrochloric acid, saline water, quick lime, test tubes and cork.
ii. The presence of moisture, salt and acid are favourable situations for the corrosion of iron.
25. In a project for finding out the situations that influence the corrosion of iron, quick lime is used. What feature of quick lime is made use of here?
Answer: Quick lime can absorb moisture.
26. The arrangement in respect of an experiment conducted in a classroom is
pictured here.
b. What is seen in the first test tube in addition to the iron nail.
c. What is added in the closed test tube? What is its purpose?
d. What might be the result of the project?
Answer:
a.To find out the factors which influence the corrosion of iron.
b. Cotton soaked in water.
c. Quick lime. It can absorb moisture.
d. The presence of moisture, salt and acid are favourable situations for corrosion of iron.
27. Sodium and potassium are stored in kerosene. Why?
Answer: Sodium and potassium are highly reactive metals. So it is to be kept away from the atmosphere.
28. Give reasons for the following.
a. Painted window bars do not undergo corrosion.
b. In coastal regions, copper nails are preferred to iron nails.
c. Copper wires are widely used for making electrical wires.
d. Aluminium is used for cooking vessels.
e. Tamarind is not kept in the aluminium vessel.
f. Apply oils on iron tools when it is kept for a long time.
Answer:
a. It blocks the contact between the iron bar and the atmosphere.
b. The presence of moist and salty air increases rusting of iron.
c. Because copper is a good electrical conductor.
d. Because Aluminium is a good thermal conductor.
e. The acid present in the tamarind reacts with aluminium.
f. The oil layer on the surface of the metal blocks the contact between metal and atmosphere.
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