A. Fill in the blanks
- The property, due to which a metal piece can be hammered into thin sheets, is known as malleability.
- Bronze is an alloy of copper and tin.
- Gold is mixed with silver and/or copper to make it hard.
- Iodine is the only non-metal to have a natural lustre.
- Phosphorus is a non-metal which catches fire in air.
- The only metal, which exists in the liquid state at room temperature, is mercury.
B. Match the following
| Column I | Column II |
|---|---|
| 1. Hardest substance | (d) diamond |
| 2. Aluminium ore | (c) bauxite |
| 3. A non-metal, normally existing in the liquid state | (a) bromine |
| 4. A good electrical conductor | (b) graphite |
| 5. A metalloid | (e) arsenic |
C. Tick (✓) the correct option
1. The two most abundant metals, found in the earth’s crust, are—
✅ aluminium and iron
2. The metals, used in making the alloy ‘german silver’, are—
✅ copper + zinc + nickel
3. When a metal reacts with oxygen, the resulting oxide of the metal—
✅ is a basic oxide
4. Out of the metals, gold, silver, copper and aluminium, the one that is most ductile, is—
✅ gold
5. Two of the metals, that cannot be stored either in air, or under water, are—
✅ potassium and sodium
D. Answer the following questions in brief
1. Name (i) one metal and (ii) one non-metal which normally exists in the liquid state.
(i) Metal:
Mercury is the only metal that normally exists in the liquid state at room temperature.
(ii) Non-metal:
Bromine is the only non-metal that normally exists in the liquid state at room temperature.
These substances are unique because most metals and non-metals are found in the solid state under normal conditions.
2. State the fraction of gold present in an 18-carat gold piece.
Pure gold is 24-carat gold.
Therefore, the fraction of gold present in an 18-carat gold piece is:2418=43
Thus, an 18-carat gold piece contains 3/4 (75%) pure gold, while the remaining 25% consists of other metals such as silver or copper that make the gold harder and more durable.
3. State the property of copper wires that makes them useful for electrical fittings.
Copper wires are used for electrical fittings because copper is a good conductor of electricity.
It allows electric current to pass through it easily with very little resistance. Copper is also ductile, which means it can be drawn into thin wires without breaking.
Because of these properties, copper is widely used in homes, industries and electrical appliances.
4. Define an alloy.
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal, prepared by melting and mixing them together.
Alloys are generally made to improve the properties of metals such as strength, hardness and resistance to corrosion.
Examples:
- Brass = Copper + Zinc
- Bronze = Copper + Tin
- German Silver = Copper + Zinc + Nickel
Thus, alloys are often more useful than pure metals for various purposes.
5. List any two uses of non-metals.
Non-metals have many important uses in our daily life.
Any two uses are:
(a) Oxygen
- Oxygen is essential for respiration in living organisms.
(b) Chlorine
- Chlorine is used for disinfecting drinking water and swimming pools by killing harmful microorganisms.
Other examples:
- Nitrogen is used in fertilizers.
- Iodine is used as an antiseptic.
- Graphite is used in making electrodes and pencil leads.
These uses show that non-metals are extremely important for human life, health and industry.
E. Answer the following questions
1. Write the balanced chemical equations for the following reactions:
(a) Sodium reacts with oxygen
When sodium reacts with oxygen, it forms sodium oxide.
Balanced Chemical Equation:
4Na+O2→2Na2O
Sodium is a highly reactive metal. It reacts vigorously with oxygen present in the air and forms sodium oxide. This is why sodium is stored under kerosene to prevent its contact with air.
(b) Aluminium reacts with dilute hydrochloric acid
When aluminium reacts with dilute hydrochloric acid, aluminium chloride and hydrogen gas are formed.
Balanced Chemical Equation:
2Al+6HCl→2AlCl3+3H2
This reaction shows that metals react with acids to produce a salt and hydrogen gas. The hydrogen gas evolved can be identified by the characteristic ‘pop’ sound when a burning splint is brought near it.
2. Why are gold and platinum suitable for making jewellery?
Gold and platinum are highly valued for making jewellery because of the following properties:
- They possess a bright natural lustre and attractive appearance.
- They are highly malleable and can be shaped into beautiful designs.
- They are ductile and can be drawn into fine wires.
- They do not corrode or rust easily.
- They remain shiny for a long time.
Because of these qualities, gold and platinum are considered ideal metals for ornaments and decorative items. Their beauty, durability and resistance to corrosion make them precious metals throughout the world.
3. ‘Sodium and potassium are stored under kerosene.’ Give reason.
Sodium and potassium are highly reactive metals.
They react vigorously with:
- oxygen present in air,
- moisture present in the atmosphere,
- and even water.
During these reactions, a large amount of heat is produced, which may cause them to catch fire.
To prevent accidental reactions and ensure safe storage, sodium and potassium are kept immersed under kerosene oil. Kerosene acts as a protective layer and prevents their contact with air and moisture.
This demonstrates how some metals require special precautions because of their extreme reactivity.
4. Why is bronze preferred over copper metal for making statues?
Bronze is an alloy of copper and tin.
It is preferred over pure copper for making statues because:
- Bronze is harder and stronger than copper.
- It is more resistant to corrosion.
- It can withstand weather conditions for a longer period.
- It retains fine details when moulded into artistic shapes.
- Statues made of bronze are more durable and long-lasting.
Therefore, bronze is considered a better material than pure copper for sculptures, monuments and statues.
5. Explain why metals are generally used in making cooking utensils.
Metals are commonly used for making cooking utensils because they possess several useful properties:
Good conductors of heat
Metals transfer heat quickly and uniformly, allowing food to cook efficiently.
High melting points
Most metals can withstand high cooking temperatures without melting.
Strong and durable
Metal utensils are strong and can be used for a long time.
Easy to shape
Metals can be moulded into different sizes and shapes according to need.
Examples of metals used in utensils include aluminium, stainless steel and copper.
Thus, the excellent heat-conducting ability, strength and durability of metals make them ideal materials for cooking utensils.
Conclusion
Metals and non-metals play an important role in our daily lives. Their unique properties help us use them in various fields such as cooking, construction, transportation, jewellery making and industry. Understanding these properties helps us appreciate how science is closely connected to everyday life.
Also Read: Chapter 1DAV Class 8 Science Chapter 1 The Cell Its Structure and Functions Solutions
Chapter 2DAV Class 8 Science Chapter 2 Microorganisms: Friend and Foe Solutions – Questions and Answers
Class 8th Social Science Home: Tata Dav S.St solutions
Class 7Science solution :CLASS 7
Class 6 Science SolutionClass 6th