ou are familiar with a number of materials like iron, aluminium, copper, etc. Some materials have been given in Table 4.1. Table 4.1 : Appearance and Hardness of materials Object /Material Appearance Hardness (Shiny/Dull) (Very hard/ Not very hard)
similar change if we try to beat a wood log ? Let us find out.
Take a small iron nail, a coal piece, a piece of thick aluminium wire and a pencil lead. Beat the iron nail with a hammer (Fig. 4.1). (But take care that you don’t hurt yourself in the process). Try to hit hard. Hit hard
Sulphur Aluminium Copper ----Fig. 4.1 : Beating an iron nail with hammer
Can you name the materials which are metals? The rest of the materials in Table 4.1 are non-metals. Metals can be distinguished from non-metals on the basis of their physical and chemical properties. Recall that lustre and hardness are physical properties.
also the aluminium wire. Then repeat the same kind of treatment on the coal piece and pencil lead. Record your observations in Table 4.2. Table 4.2 Malleability of Materials Object/ Material Iron nail Coal piece Aluminium wire Pencil lead Change in Shape (Flattens/Breaks into pieces)
4.1 Physical Properties of Metals and Non-metals
Have you ever seen a blacksmith beating an iron piece or an article made up of iron, like a spade, a shovel, an axe? Do you find a change in the shape of these articles on beating? Would you expect a
You saw that the shape of the iron nail and the aluminium wire changed on beating. If they were beaten harder these could be changed into sheets. You might be familiar with silver foil used for decorating sweets. You must also be familiar with the aluminium foil used for wrapping food. The property of metals by which they can be beaten into thin sheets is called malleability. This is a characteristic property of metals. As you must have noticed, materials like coal and pencil lead do not show this property. Can we call these as metals? Can you hold a hot metallic pan which is without a plastic or a wooden handle and not get hurt? Perhaps not! Why? Try to list some other experiences in which a wooden or plastic handle protects you from being hurt while handling hot things. On the basis of these experiences what can you say about the conduction of heat by wood and plastic? You must have seen an electrician using his screw driver. What kind of handle does it have? Why? Let us find out.
the activity with various objects in Class VI. Now, repeat the activity with the materials mentioned in Table 4.3. Observe and group these materials into good conductors and poor conductors. Table 4.3 : Electrical conductivity of materials S.No. Materials Good Conductor / Poor Conductor
1. 2. 3. 4.
Iron rod/nail Sulphur Coal piece Copper wire
You observe that iron rod, nail and copper wire are good conductors while rolled sulphur piece and coal piece are poor conductors.
Recall how to make an electric circuit to test whether electricity can pass through an object or not (Fig. 4.2). You might have performed
Oh! The meaning of recalling our experiences and then of this activity was to show that metals are good conductors of heat and electricity. We learnt this in Class VI.
Fig. 4.2 : Electric tester MATERIALS : METALS AND NON-METALS
Where do you find the use of aluminium and copper wires? Have you seen wires of coal? Definitely not! The property of metal by which it can be drawn into wires is called ductility. Have you ever noticed the difference in sound on dropping an iron sheet/ plate, a metal coin, and a piece of coal on the floor? If not, you can try it now. Do you note any difference in the sound produced? 45
Have you seen wooden bells in temples? Can you give reason? The things made of metals produce ringing sound when struck hard. Suppose you have two boxes similar in appearance, one made of wood and the other of metal. Can you tell...