Give reasons. (ii) Ionic compounds have high melting points.

Ionic compounds have high melting points because there is a strong electrostatic force of attraction between the oppositely charged ions and hence a large amount of energy is required to break the strong bonding force between ions.

Give reasons. (i) Platinum, gold and silver are used to make jewellery.

Platinum, gold and silver are used to make jewellery because- These are low reactive metals. So, they rarely corrode and hence do not lose their shine and lustre. They are highly malleable and ductile so can be transformed into any shape or design. Gold and platinum are not readily oxidized by oxygen or other nonmetals, they do not tarnish readily.

State two ways to prevent the rusting of iron.

The two ways to prevent rusting of iron are as follows- (i) Galvanization: Articles that are exposed to extreme moisture such as roof sheets and pipes are protected from rusting by galvanizing. Galvanization is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The iron coated with zinc is called galvanized iron or simply G l . As zinc is more reactive than iron, it reacts with oxygen in the presence of moisture to form an invisible layer of basic zinc carbonate Z n C O 3 . Z n O H 2 which protects it from further rusting. The galvanized iron articles remain protected from rusting even if zinc coating is broken. This is because zinc is more reactive than iron. (ii) Painting: In materials like railings, window, grills, lamp posts, iron gates, iron bridge, railway coaches, bodies of cars, buses and trucks, etc., painting the metal surface does not allow them to come in contact with the moist air and thus, prevents rusting.

Define the term- (i.) Ore (ii.) Gangue

Ore- the mineral from which metal can be extracted easily and profitably is called an ore. Ex: Bauxite is the ore of Aluminium. All minerals are not considered as ores but all ores are also minerals. Gangue- Ores mined from the earth are naturally contaminated with sand, rocky materials. There are impurities present in the ore which are known as gangue.

What is heat treatment of steel? Explain.

Steel is an iron alloy that must be treated before it can be cut and shaped into the final product. Steel is now heated and cooled to change its physical and mechanical properties without changing its shape. This is referred to as heat treatment. This method is used to strengthen materials and to modify some mechanical properties such as formability, machining, and so on. Meanwhile, the tough steel alloy goes through three basic heat treatment steps: annealing, quenching, and tempering.

Write any five physical properties of metals.

The five physical properties of metals are i) Hardness: Most of the metals are hard. Alkali metals like sodium, potassium are an exception to this, they are soft metals. ii) Strength: Most of the metals are strong and have high tensile strength (Except Sodium and potassium which are soft metals). iii) State: Metals are solid at room temperature except for mercury which is a liquid at room temperature. iv) Sound: Metals produce ringing sound, they are sonorous in nature. Thus, metallic wires are used in making musical instruments. v) Conduction: Metals are good conductor of heat and electricity. Therefore, electric wires are made of metals like copper and aluminium.

Define corrosion and write two ways to prevent the corrosion.

Corrosion is the process by which metals are destroyed progressively by chemical action. Rust is the brown flaky substance formed on iron surface when it comes in contact with water and oxygen which destroys iron fastly whereas patina is the thin layer that forms over copper, silver, brass and other semi-reactive metals as their outer layer undergoes a chemical reaction, this process takes a much longer time to destroy copper. Two ways to prevent the corrosion: 1. Galvanization - It is the process of depositing a thin layer of zinc on iron to prevent rusting. 2. Painting - The coating of paint acts as a barrier and protects the metal from corrosion by preventing the flow of electrochemical charge.