Whatever you see, touch/feel or smell is matter but fire, light and heat are not matter. Matter can be invisible or so small that you can’t see it.  Most matter can be classified as solid, liquid or gas.

## Change of State of Matter

### Reversible Changes

Matter can change state which might be reversible or irreversible. Matter can change state with a change in temperature and these changes are often reversible. In other words, we can get back to what we started with.

### There are other types of reversible changes.

·         Dissolving solids

Some solids can be dissolved in a liquid. This is a reversible change because we can get the solid back.  The solids dissolve but they do not disappear, they just break into tiny bits that become a part of the liquid.  Using small bits helps solids dissolve faster. Stirring and heating also help in dissolving.

Dissolving solid in a liquid is a reversible change because you can get the solid back by boiling the liquid or letting it evaporate.

Note- It is important to understand that dissolving is not the same as melting.

·         Mixtures

Some mixtures can be separated by filtering the mixture. This change is reversible

Note- Some changes that seem to be completely reversible may not be. Like, melted butter can be cooled so that it turns back into a solid but it will not be the same as the butter that you started with.

## Irreversible Changes

Some changes are irreversible which means that it cannot be brought back to the original state. A new material is formed.
The signs of an irreversible change may include a change in colour, bubbles, heat, flames, a bright light or an explosion.

1. What are the states of matter?
2. Give features of the three states with examples.
3. What is reversible and irreversible change?
4. Give two examples each of reversible and irreversible change.

## Recap

• Matter can be invisible or so small that you can’t see it.
• Matter can be classified as solid, liquid or gas.
• Matter can change state which might be reversible or irreversible.
• Matter can change state with a change in temperature and these changes are often
• Some solids can be dissolved in liquid.
• The irreversible change means that it cannot be brought back to the original state. A new material is formed.