Melting is when a solid turns into a liquid by gaining heat, and freezing is when a liquid turns into a solid by losing heat. These are opposite changes — water melts into liquid water when it warms up, and liquid water freezes into ice when it cools down. Every solid has a melting point, the temperature at which it starts to melt. For water, that temperature is 0 degrees Celsius (32 degrees Fahrenheit). Melting and freezing are reversible — you can go back and forth between solid and liquid.
Give each student an ice cube on a plate and have them observe it melting, recording the time and temperature. Then freeze water in small cups overnight and compare. Try melting other solids like butter, chocolate, and crayons in warm water baths to show that different solids melt at different temperatures.
You have already learned that solids hold their shape and liquids flow. But matter does not always stay in one state — it can switch. When a solid gains enough heat, it melts and becomes a liquid. When a liquid loses enough heat, it freezes and becomes a solid. These changes happen all the time in your daily life, from ice cubes melting in your drink to popsicles freezing in the freezer.
The temperature at which a substance melts is called its melting point. For water, the melting point is 0 degrees Celsius. Below that temperature, water stays frozen as ice. Above it, ice begins to turn into liquid water. Different substances have different melting points. Butter melts when you put it in a warm pan, but you would need a much hotter temperature to melt a metal like iron. That is why a metal spoon does not melt when you stir hot soup — the soup is nowhere near hot enough.
Here is an important idea: melting and freezing are reversible changes. That means you can undo them. Take an ice cube out of the freezer, and it melts into water. Put that water back in the freezer, and it becomes an ice cube again. The water itself did not change — no new substance was created. The same particles are there; they just rearranged from a rigid, locked pattern (solid) into a loose, flowing arrangement (liquid), and back again.
During melting, something interesting happens with temperature. While an ice cube is actively melting — when you can see both ice and water together — the temperature stays right at the melting point. All the heat energy goes into breaking the solid apart rather than making it warmer. Once all the ice is melted, the temperature of the water starts rising again. This is a clue that changing state takes energy.
Look for melting and freezing everywhere: snow melting on the sidewalk, a candle's wax pool forming around the flame, frost appearing on a window on a cold night. Each one is matter switching between solid and liquid states, driven by heat flowing in or out.