Solids , gases and liquids

The world around us is made of variety of things. These things can be living or non-living. But all these things have weight and they occupy space. They are called matter.

Anything that has weight and occupies space is called matter. The amount of space matter takes up is called volume. Matter itself is made of small particles called molecules. They are so small that we cannot see them. All substances are made up of these small molecules. Molecules are arranged differently in different substances.

Matter exists in three forms:

• Solid
• Liquid
• Gas

Solids are hard. They have a fixed shape because molecules in a solid are closely packed. They do not have space between them, so they cannot move and have a fixed shape, e.g., bench, table, chair, stone and tree.

However, the shape of some of the solids can be changed by pressing or using a little force, e.g.,

• We can make clay toys by pressing it.
• We can press rubber ball also.

So, we can say:

• Most solids are hard. But some can be soft also.
• Solids have a fixed shape.
• Solids have a definite volume.

In liquids, molecules are loosely packed. They have space between molecules. Therefore, molecules can move from one place to another. That is why liquids can flow. Liquids flow from a higher level to a lower level.

Liquids do not have a definite shape. They take the shape of the container in which they are kept or poured into. They have a definite volume.

Take half liter water and pour it into a glass. Then pour it into a bottle and again pour it into a beaker. Each time water changes its shape and takes the shape of the container, but its volume (half liter) remains the same. Milk, juice, oil, vinegar, water, kerosene and petrol are all examples of liquids.

Now, we can say:

• Liquids do not have a fixed shape. They take the shape of the containers are poured into.
• Liquids have a definite volume.
• Liquids can be poured and they flow from a higher level to lower level.

All metals are solids except mercury. Mercury is a liquid and it is used in thermometer.

In gases, molecules are far apart. They have a lot of space between molecules. So, molecules can move freely from one place to another. That is why gases have a tendency to occupy the whole space available to them. Have you ever lighted an incense stick in your room ? After some time, you can feel the fragrance in the whole room, because the incense stick burns to give out smoke which occupies the space available in the room.

Air around us is a mixture of gases like oxygen, hydrogen, nitrogen, etc.

We can say:

• Molecules in gases are very loosely packed and move around freely to occupy the space available to them.
• Gases do not have a fixed shape.
• Gases do not have a fixed volume.

Water in rivers and lakes has oxygen dissolved in it. Fish breathe the oxygen dissolved in water.

Solid, liquid and gas can be converted from one form to another by heating or cooling. This is called interconversion of matter.

Three forms of water are :

• Ice (solid)
• Water (liquid)
• Vapour (gas)

Take ice cubes and keep them in a pan for some time. They melt to form water. Heat this water, it changes into vapour.

Now, take a plate and keep it over the vapour coming out of the beaker. You can see the vapour condenses to form drops of water. Keep this water in a freezer for some time. You can see that this water changes into ice.

take butter in a bowl and allow it to stay in sunlight for sometime. What do you see ?

You can see that the butter melts and changes into liquid state.

Now, for changing this liquid butter into solid state again. We have to put this butter in the freezer for some time and we will see butter changes into solid again.

While conversion of one state of matter into another, the following processes take place:

1. When liquid changes into vapour on heating, it is called evaporation.
2. When vapor is cooled , it changes into liquid again. This is called condensation.
3. When water is kept in a freezer, it changes into ice(solid). The change of liquid into solid on cooling is called freezing.
4. When ice is kept in a plate for some time, it melts into water. The change of solid into liquid on heating is called melting.

Sometimes, few substances like camphor, naphthalene balls changes from solid to vapour on heating without changing into liquid state. This is called sublimation.

• Anything that has weight and occupies space is called matter.
• Matter exists in three forms- solid, liquid and gas.
• All matter is made up of tiny particles called molecules.
• Molecules are arranged differently in different substances.
• A solid that dissolves in a liquid is called a solute.
• A liquid in which a solute is dissolved is called a solvent.
• A mixture obtained by dissolving solute and solvent is called a solution.
• Three states of matter can be converted from one form to another by heating or cooling.