Sorting Materials into Groups

We use several objects in our day-to-day lives. Pencils, erasers, books, notebooks etc., are objects that you must be using a lot. All objects are made of some substances called materials.

1.0Introduction

It is very important to use the right material for making each object. For example, a chalk made of materials like wood or plastic would be useless because it cannot be used to write on the blackboard. Therefore, it is important for us to know the properties of different kinds of materials around us.

2.0Objects around us

Objects around us have different shapes, colours and uses. They are made up of one or more materials such as paper, glass, plastic, cloth, wood, metal, mud, soil, cotton, etc.

Classification of materials

The act of dividing things into category, according to some common relations or affinities is called classification.

3.0Some physical properties of materials

We generally use a tumbler to keep a liquid. Therefore, would it not be silly, if we were to make a tumbler out of cloth. What we need for a tumbler is glass, plastics, metal or other such material that will hold water. Similarly, it would not be wise to use paper-like materials for cooking vessels. We see then, that we choose a material to make an object depending on its properties, and the purpose for which the object is to be used.

Different types of materials have different properties such as appearance, solubility, transparency, conductivity and behaviour towards magnet etc. Lets study these properties one by one.

• Appearance: Materials usually look different from each other. Mud looks different from wood. Wood looks different from plastics. Plastics look different from copper. Copper looks different from iron or aluminium. However, there might be some similarities between copper, iron and aluminium that are not in the plastics, wood or mud. Lustrous materials
  
  Non-Lustrous materials
  
  However, in case of thin wires of copper, aluminium and iron, the sand-papered surface is shining. The special kind of shine present on natural materials or shine appears when they are freshly cut or sand papered is called lustre. The materials which have natural shine on their surface or the shine appears when they are freshly cut or sandpapered are called lustrous materials.

• Examples of lustrous materials (a) Among the solids, all metals have a lustre. Generally, this lustre is silvery white, except gold and copper which have a yellow and reddish lustre respectively. Graphite (a physical form of carbon) and iodine crystals
  
  Cullinan, a diamond from Africa, is the largest diamond ever found. have lustre. Similarly, gems and pearls are lustrous. (b) Amongst the liquids, mercury (it is a liquid metal) is lustrous. (c) All gases are non-lustrous. Diamond shines due to reflection of light.

• Hardness You must have experienced that if you press a piece of foam with your thumb it gets pressed, but a piece of stone or wood does not get pressed. You can say that a piece of wood or stone is harder than a piece of foam. However, how shall we find which amongst wood or stone is harder? The best way of finding it out is to scratch the wood against the stone. If the wood gets scratched by stone, then the stone is harder than the wood. When the surfaces of two different materials are rubbed against each other, then the surface which scratches is said to be hard and the property is called hardness of the material. Conversely, the surface which gets scratched is said to be soft as compared to the scratching surface. For example, (a) Diamond is the hardest naturally occurring substance. Among the different kinds of diamonds, black diamond is the hardest. (b) Glass is also a hard substance, still diamonds can be used to scratch glass and hence are used for cutting glass. (c) Talc obtained from soapstone is the softest substance. Soapstone is a magnesium rich metamorphic rock from which talc is obtained.
• Soluble or insoluble in water It is a common experience that when we put a spoonful of common salt or sugar into a tumbler filled with water and stir the contents, the sugar or the common salt completely disappears. We say that sugar or common salt dissolves in water. Such substances which dissolve in water are known as soluble substances. Solubility: The property of substance due to which it dissolves in water is called solubility. Insoluble substances: The substances which do not dissolve in water are called insoluble substances. For example, sand, chalk, etc.

• Solubility of liquid substances in water Most of the liquids are insoluble in water. However, liquids such as alcohol, vinegar, lemon juice etc., dissolve in water. The liquids which dissolve in water are called miscible liquids. The liquids which do not dissolve in water are immiscible liquids. Liquids such as kerosene oil, coconut oil, petrol, ether, benzene, etc., are not miscible with water. However, they are easily miscible with one another. For example, coconut oil is easily miscible in kerosene oil, petrol, ether and benzene.
• Solubility of gases in water Most of the gases are insoluble in water. For example, nitrogen, hydrogen and carbon monoxide are insoluble in water. However, some gases are soluble in water. Oxygen gas is slightly soluble in water. It is the oxygen dissolved in water which enables the fishes and other aquatic animals to breathe with the help of their gills. Carbon dioxide gas is slightly soluble in water. It is the dissolved carbon dioxide gas in water, which acts as a source of food for water plants. The water plants convert the carbon dioxide gas into their food with the help of chlorophyll and sunlight. Thus, the materials can be classified on the basis of their solubility in water.
  
  Fizzy drinks contain dissolved carbon dioxide, which goes off on opening the bottle, due to its less solubility.

4.0Objects may float or sink in water

Substances like sand and chalk powder sink in water, whereas materials like coconut oil and kerosene oil float on the surface of water. It is because, the substances heavier than water sink in it. Whereas, the substances lighter than water, float in it. Thus, we can group materials on the basis of sinking or floating in water. Let us perform the following to find out few substances which sink or float in water.

• Density means how compact or concentrated something.
• Heavier substances have high density and lighter substance have low density.
• Heavier substances sink in water, whereas lighter substances float on water.

5.0Transparency

Transparency is another property of materials which is used to distinguish them.

Transparent: A material through which you can see clearly is called transparent material. It happens so because light can pass through it.

• For example, (i) Solids such as glass, diamond, gems and certain kinds of plastics are transparent materials. (ii) Most of the liquids, such as water, kerosene oil, alcohol, sea water, etc., are transparent materials. (iii) All gases, such as oxygen, hydrogen, nitrogen, air, etc. are transparent materials.

Transparency: The property of a material through which you can

Opaque: A material through which you cannot see is called opaque material. Through it light cannot pass. For example, Solids, such as metals (all kinds), wood, paper, stones, etc, are opaque materials. Opacity: The property of materials through which you cannot see is called opacity.

Translucent: A material through which you can see partially is called translucent material. Light can pass through it but we cannot see clearly through it. For example, Ground glass, frosted glass, tissue paper, butter paper, oiled paper, muddy water, fog, mist, dust laden air are translucent materials.

Thus, on the basis of seeing through a particular material, the substances can be grouped as transparent, opaque and translucent.

6.0Basic terminology

• Affinities - A similar quality in two or more people or things.
• Hammer - A tool with a heavy metal head that is used for hitting nails, etc.
• Spoke - One of the thin pieces of metal that connect the centre of a wheel to the outside edge (the rim).
• Plough - A large farm tool which is pulled by a tractor or by an animal.
• Bullock cart - Two wheeled/ four wheeled vehicle pulled by oxen.
• Tumbler - A drinking glass/ container without handle.
• Silly - Not showing understanding, foolish.
• Lustre - Brightness that a shiny surface has
• Vessel - A container for liquids (bottle, cup, etc.)
• Exposed - Not covered.
• Conversely - In a way that is opposite to something.
• Vigorously - Very forceful or energetic
• Miscible - That can be mixed together.
• Immiscible - That cannot be mixed together
• Brass - An alloy (mixture of two metals i.e. copper and zinc.)
• Optician - Person whose job is to test eyes, sell glasses, etc.
• Ground glass - Glass that has a rough surface produced by grinding, used for diffusing light.
• Frosted glass - Glass that has a rough surface so that you cannot see clearly through it.
• Mist - A cloud made of very small drops of water in the air just above the ground, that makes it difficult to see.
• Laden - Having or carrying a lot of somethings.
• Transparent - Material through which you can see.
• Translucent - Material through which you can see partially.
• Opaque - Material through which you cannot see.
• Distinguish - To make something different from others.
• Screw - Thin pointed piece of metal used for fixing two things.
• Appearance - The way that something looks.
• Conductivity - Property of allowing heat or electricity to go through something.
• Solubility - The ability to be dissolved, especially in water.
• Gems - A previous stone
• Pearl - hard round object which is shiny and usually creamy white in colour.
• Disappear - To no longer to be able to seen.
• Partially - Not completely.

7.0Memory Map

• Why do we call water the universal solvent? Apparently, it may seem that quite a large number of substances are insoluble in water. However, if the substances are kept in water for a very long time, which may be for a century or two, the water can dissolve anything and everything in itself. It is for this reason that water is called the universal solvent. It is for the same reason that water plays an important role in the bodies of living organisms, because it can dissolve a large number of substances.
• Why jewellers exhibit jewellery in glass show case? Jewellers exhibit the jewellery in glass showcases, so that it is easily visible and yet safe from thieves. Similarly, opticians show various kinds of spectacles and goggles in glass showcases.