Matter exists in three main forms called states. These are solid, liquid, and gas.

Solid

A solid is firm and hard. It keeps its own shape and does not change when you put it in a different container.

Examples: stones, firewood, cooking pots, books, desks

Liquid

A liquid has no fixed shape. It takes the shape of the container it is placed in. However, a liquid has a fixed amount (volume).

Examples: water, juice, milk, soda, cooking oil

Gas

A gas has no fixed shape and no fixed volume. It spreads out to fill any available space. Gas is usually lighter than solids and liquids.

Examples: air, steam, the air inside a balloon

Let us use water to understand the three states of matter:

When you freeze water, it becomes solid ice. When you heat water to 100 °C, it becomes steam (gas). When steam cools down, it turns back into water, and if water freezes, it becomes ice again.

Matter can change from one state to another when the temperature changes:

• Melting: Solid → Liquid (ice melts to water at 0 °C)
• Freezing: Liquid → Solid (water freezes to ice at 0 °C)
• Boiling/Evaporation: Liquid → Gas (water boils to steam at 100 °C)
• Condensation: Gas → Liquid (steam cools to become water again)

The freezing point of water is 0 °C (32 °F). This is the temperature at which water turns into ice.

The boiling point of water is 100 °C (212 °F). This is the temperature at which water turns into steam.

In everyday life in Tanzania, understanding the states of matter helps us in many ways. For example, when you boil water to make tea or cook ugali, you are changing liquid water into steam (gas). Farmers in Tanzania use ice to keep fish and meat fresh before selling them at the market — ice is water in its solid state, and it keeps food cold because it stays at 0 °C until it melts. Knowing the boiling point (100 °C) also helps people boil water to make it safe for drinking, especially in areas where clean water is not readily available.