States of Matter << Learners Hobby

Anything which occupies space and has mass is matter. And that matter is made of tiny particles called atoms. These atoms combine or cling together to form molecules. How matter behaves under different conditions depends on how atoms group together to form molecules and how molecules are arranged. In other words, the  behaviour of matter depends on its structure. In this chapter, we will study the structure of matter in its three states and how matter changes from one state to another.
STATES OF MATTER
    There are two things you must know about molecules before we take a closer look at solids, liquids and gases.
    First, the molecules of matter attract each other. The force of attraction between molecules is called intermolecular force. The strength of this force depends upon the distance between the molecules. The closer the molecules, the greater is the intermolecular force. This force tries to keep the molecules together.
    Second, the molecules of matter are in constant motion. They are, however, kept in place by the force of attraction between them. When the energy of molecules increases (say, because of an increase in temperature), their motion also increases. When they gain enough energy, they can overcome the intermolecular force of attraction and move away from each other.
    The state (solid, liquid or gaseous) of a substance under particular conditions depends upon which factor has a greater effect - intermolecular force or molecular motion. We say 'under particular conditions' because most substance can exist in all three states. We think of nitrogen as a gas or kerosene as a liquid or iron as a solid because these substances exist in these states under the conditions of temperature and pressure existing on the earth. They could exist in other states if we changed the temperature and pressure.
SOLIDS

In a solid, the molecules are arranged very close to each other, so the intermolecular force is strong. This makes it practically impossible for them to get away from each other, just as you cannot get away from your partner in a three-legged race because his leg is tied to yours. Imagine rows and rows of people with their legs tied together. They might be able to jump about a little, but if they really want to move, they will have to move together, keeping their 'arrangement' intact. Similarly, the arrangement of molecules in a solid remains intact. In other words, the molecules of a solid are arranged in a regular pattern. They cannot slip past each other, so a solid does not flow.
    It is not as if the molecules of a solid do not move at all. However, they cannot move much, just as mothballs packed tightly in a carton cannot move much. And just like a carton of mothballs packed tightly together, a solid has a definite shape and size. You cannot really squash a box of mothballs out of shape because the mothballs have nowhere to move. Similarly, the closely packed molecules of a solid cannot move much, so a solid is generally hard, rigid and cannot be compressed.
Porous solids: There are some exception, however. You can, for example, squeeze a sponge or compress it. This is because it has millions of little holes, or pores, which are filled up with air. When you squeeze the sponge, the air gets forced out. If you squeeze it under water, you will see the air bubbling out of it. Under water, the pores in a sponge get filled with water. If you take the sponge out of the water and squeeze it again, the water flows out of the pores.
    A solid which has pores is called porous. The porosity of some solids is a great blessing for us. It is because paper is pores that ink leaves a mark on it. The pores in paper hold the ink, which would otherwise have go wiped off. The pores in woollen clothes trap air and keep us warm. The pores in the soil hold water, which is drawn by the roots of plants. Water seeps out of the pores of earthen pots and evaporates. Evaporation, if you remember, is a change that requires energy. This energy comes from the water in the pot and the water cools.
LIQUIDS
   
    The molecules of a liquid are much farther apart than those in a solid. Thus, the intermolecular force holding them together is weaker, and they can move about more freely and faster than the molecules of a solid. This lets them slip past each other, allowing the liquid to flow and take on the shape of any container it is poured into. The molecules of a liquid are not held together in a regular pattern. However, intermolecular force does bind them to some extent, so they cannot get away from each other altogether. This is why a liquid has a definite volume, though it is free to flow and does not have a particular shape. You can pour 200 ml of water into a jug, a beaker or a pan. It will take on the shape of the container, but its volume will remain the same (200 ml).
GASES
   
    Gas molecules are so far apart that the forces between them are very weak. So weak are these forces that the molecules are quite independent and can move about freely in all directions. They move much faster than liquid molecules, and fill up all the space available to them in almost no time. That is why when someone lights an incense stick at one end of a big room, the fragrance spreads all over the room very rapidly. Since gas molecules move about freely, a gas has no particular shape of volume.
    The movement of solid particles caused by collisions with the constantly moving molecules of a gas (or liquid) is called Brownian movement, after the botanist Robert Brown. He observed the motion of pollen grains in a liquid.
    The large spaces between gas molecules make gases compressible. Of course, you have to apply some pressure in order to compress a gas. You do this when you pump air into a bicycle tube. The gas in an LPG cylinder is also compressed and confined to the volume of the cylinder under great pressure. When a cylinder leaks, the gas is no longer under pressure and it spreads all over the kitchen.
    There is another way in which gases differ from solids and liquids. They do not have a surface. Solid molecules are held together very tightly. So, solids have shapes, sizes and clearly defined surfaces. Liquid molecules are not held so tightly. Still, the forces of attraction are strong enough to pull the molecules into the liquid. This is why liquids also have surfaces. Gas molecules are so independent of each other and so free to move about that gases do not have surfaces.