Sorting Materials into Groups-I

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. 

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. 

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. 

Rubber, wool, cotton, paper etc. cannot be used for making utensils as they do not have much of strength. Glass and plastic are the materials used for such purposes. 

Sand is heavier than water and settles down at the bottom. Vinegar and sugar both are water-soluble, but oil is lighter than water and floats on the surface.  

Though cotton, wool, talcum and flour are soft substances, but out of these talcum is the softest.

Lustrous means shiny. Iron, copper and silver are metals and all metals have shiny surface. Diamond, glass and steel are also shiny. Wood and coal are the substances which appear dull and non lustrous. 

Diamond is the hardest known substance. Cornflour, sponge and fur are quite soft substances, their shape can be changed easily by applying little force. 

Out of wood, iron, stone and glass, only iron is a metal (element), whereas the rest are not. 

Cotton, sponge, wool are soft, non-lustrous and can be compressed easily by applying pressure. So, only option 2 is correct. 

Sugar, common salt and copper sulphate are compounds or pure substances which have a definite formula. For example: sugar - C₆H₁₂O₆, common salt - NaCl and copper sulphate - CuSO₄. Sand does not have a fixed formula and is an impure substance.  

Toys are generally made of plastic, wheels from rubber, tumblers from plastic and notebooks are made from paper. 

One ca see through water and glass. Such substances are said to be transparent and the phenomenon is known as transparency. 

The objects through which light cannot pass and also cannot be seen through, such objects are known as opaque objects, e.g. cardboard. 

Only those materials sink in water which are insoluble and are heavier than water, e.g. sand, soil etc. Oil and sawdust are lighter than water and remain at the surface. 

There are some objects which are neither completely transparent nor completely opaque. They are translucent. One cannot see clearly through translucent objects, e.g. butter paper, frosted glass and oil patch etc. 

Mentioning “rough” indicates the texture of surface irrespective of the shine/luster or softness/hardness of the materials. So, stone is rough, glass is smooth. 

Substances soluble in water disappear in water. The softness or the hardness of the substance do not play any role in this.  

Vinegar, salt and sugar get easily dissolved in water and are said to be water soluble. Sawdust remains separate when added to water and is said to be water insoluble. 

Salt and sugar get dissolved in water and are said to be soluble, whereas wax and oil do not get dissolved in water and are said to be insoluble.