Mirror, glass, cellophane and marble have a smooth surface but coal and rock have an uneven or rough surface. 

Aquatic animals are those who remain under the surface of water and respire there only. All animals take in oxygen and release carbon dioxide gas during respiration. These two gases are exchanged between water and animals because of their solubility in water. 

To make an object, the selection of materials is done by following two criteria. Both, properties of the material and purpose of the object are important. 

Iron, copper and gold are metals, all of which have a lustrous surface. Metals are good conductors of heat and electricity and are opaque. 

Metals, when kept open in the atmosphere, react with atmospheric air and water vapours and gradually lose their shine. 

Water-insoluble substances do not get dissolved in water, e.g. chalk powder, wood particles, stone, etc. 

The objects through which light cannot pass are known as opaque objects. Thus, cardboard is opaque as light cannot pass through it. 

Transparent objects are the ones through which light can easily pass, e.g. water, air, etc. 

Raw materials are the ones which are used to make other useful materials, e.g. wood is used to make furniture, plastic is used to make toys and buckets, paper is used to make books, cotton is used to make quilts, etc. Books, belts and newspapers are the final products prepared by using paper, leather, etc. as the raw materials. 

Materials can be classified into soluble or insoluble substances, on the basis of their solubility in water or any other given solvent. They may be grouped as conductors or insulators, on the basis of their conductivity. They may be classified as transparent, translucent or opaque on the basis of transmission of light. However, mass is not an appropriate property to classify the materials. 

Transparent objects are the ones through which light can easily pass, e.g. air, water, glass beaker, plastic, etc. but cardboard, cupboard, plywood, etc. are the examples of opaque objects. 

Vinegar, lemon juice, dettol, acids, alcohol, etc. are all water-soluble and do not form a layer when dissolved in water. On the other hand, kerosene oil and mustard oil are not soluble in water and form a separate layer. Oil generally being lighter, forms the upper layer. 

Metals are good conductors, malleable, ductile and lustrous. So, they are used in making jewellery and wires.

Hardness, lustre and transparency can be identified by simply observing.

Glass, wood and thermocol are raw materials used for making objects.