Solid to Liquid and Back Again: Melting and Freezing

Imagine you take an ice lolly out of the freezer on a hot summer day. You hold it in your hand and watch it drip. What is happening to it? Now think — what would happen if you put the liquid puddle back in the freezer overnight? Would it turn back into a solid lolly shape? Hold those thoughts — this lesson will explain exactly what is going on!

Materials can exist in different states. The two states we look at here are solid and liquid. A solid has a fixed shape — it does not flow. A liquid flows and takes the shape of its container. MELTING happens when a solid is heated and turns into a liquid. Heat gives the tiny particles inside the solid more energy, so they move more freely and the solid loses its shape. Ice is a solid form of water. When you warm ice, it melts and becomes liquid water. The temperature at which ice melts is 0 °C (32 °F). This is called the melting point. FREEZING is the opposite. When a liquid is cooled enough, it loses heat energy, its particles slow down and pack tightly together, and the liquid turns into a solid. Water freezes at 0 °C — the same temperature at which ice melts. This is called the freezing point. These changes are reversible. That means you can go back and forth — melt the solid, then freeze the liquid again, and the material itself does not change. Ice is still water; it has just changed state. A real-world example you can observe: puddles on a very cold winter night. Liquid water in the puddle loses heat to the cold air. When the temperature drops to 0 °C, the water freezes and becomes ice. In the morning sun, that same ice warms up, reaches 0 °C again, and melts back into a puddle.