Iron is a lustrous material. 'Lustrous' means to shine and the surface of the iron shines. Hence, it is a lustrous material.

The substance sand is not soluble in water, as when sand is stir in water, we find that sand settles down at the bottom of the beaker.

The material insoluble in water would either sink or float in water. For example coin which is insoluble in water would sink in water because it is heavier than water. The wax, which does not dissolve in water would float on it as it is lighter than water.

The leaf would float on water as it is lighter than water.

The copper is not used to make cover of switches because copper is a good conductor of electricity. We would get an electric shock when we touch the cover of switch. The copper is also used to make coins. The coins are made of copper because the coins made from copper stay around for many years.

The material iron is not used to make coins. It is because iron can easily rust on exposure to humid air. Silver is also used to make coin of higher value. Special coins are made, but not currency coins.

The mustard oil is immiscible in water as it does not mix with water and forms a separate layer. The mustard oil floats on top of water  because it is less dense than water. 

Since the metals like iron, copper, aluminium are hard and cannot be pressed easily, they are long lasting. Hence, these metals are used to make utensils.

The wood is not made in factories, but is obtained directly by nature from trees.

Oxygen gas dissolves in water.

The glycerine is miscible in water. The glycerine gets competely mixed in water and does not form a separate layer. The glycerine is a liquid. The liquids that get completely mixed in water are called as miscible in water.

We cannot press or twist coin. As coin is made of metal and the metal is a hard material, it cannot be pressed.

 The chalk powder does not dissolve in water.

It is very soft metal. It creates alloy with many metals and then the hardness increases.

The substance isinglass is not obtained from plants, but from fish.