7. Metals and Nonmetals
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Eight General Science.
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Exercises
| Property of metal | Use in everyday life |
|---|---|
| i. Ductility | Making electric wires |
| ii. Malleability | Making aluminium foil and metal sheets |
| iii. Conduction of heat | Making cooking utensils |
| iv. Conduction of electricity | Making electric cables |
| v. Sonority | Making school and temple bells |
(a) Gold, silver, iron, diamond Diamond
Diamond is a nonmetal. The others are metals.
(b) Ductility, brittleness, sonority, malleability Brittleness
Brittleness is a property of nonmetals. The others are properties of metals.
(c) Carbon, bromine, sulphur, phosphorus Bromine
Bromine is a liquid. The others are solids.
(d) Brass, bronze, iron, steel Iron
Iron is an element. The others are alloys.
(a) The stainless steel vessels in kitchen have copper coating on the bottom.
Copper is a good conductor of heat. It spreads heat quickly and evenly. So food is cooked faster and fuel is saved. Hence, steel vessels have copper coating at the bottom.
(b) Copper and brass vessels are cleaned with lemon.
A greenish layer forms on copper and brass vessels due to corrosion. Lemon contains citric acid. The acid dissolves this layer. So the vessels become shiny again.
(c) Sodium metal is kept in kerosene.
Sodium is a very reactive metal. It reacts with oxygen and moisture in air and catches fire. Sodium does not react with kerosene. Hence, sodium is kept in kerosene.
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(a) What is done to prevent corrosion of metals?
- A layer of oil, grease, paint or varnish is applied on the metal.
- Iron is coated with zinc. This is called galvanising.
- Tin plating, chromium plating or nickel plating is done.
- Metals are converted into alloys like stainless steel.
(b) What are the metals that make the alloys brass and bronze?
- Brass: Copper and zinc.
- Bronze: Copper and tin.
(c) What are the adverse effects of corrosion?
- Metal articles become weak and break.
- Bridges, railway lines, ships and machines get damaged.
- Metals lose their lustre.
- A lot of money is spent on repairing corroded articles.
(d) What are uses of noble metals?
- Gold, silver and platinum are used to make ornaments.
- Gold and silver are used to make medals.
- Silver is used in some medicines.
- Gold and silver are used in electronic devices.
Three test tubes used to study the conditions needed for rusting.
(a) Why is the nail in the test tube 2 not rusted?
The water in test tube 2 is boiled. So there is no air in it. The layer of oil stops air from entering. Hence, the nail does not rust.
(b) Why is the nail in the test tube 1 rusted highly?
In test tube 1, the nail is in contact with both air and water. Hence, the nail rusts highly.
(c) Would the nail in the test tube 3 get rusted?
No. Calcium chloride absorbs the moisture in test tube 3. Only dry air is present. Hence, the nail does not rust.