Everyday changes of state in cooking and freezing
Chemistry · Key Stage 2 · The School
A kitchen is full of changes of state
Butter melts in a hot pan. Ice cubes melt in a warm drink. A frozen pea turns soft as it cooks. An ice lolly freezes solid in the freezer and melts again in your hand. Every one of these is the same water or fat, just changing state.
Which changes come back
Melting butter and freezing water are both PHYSICAL changes. Cool melted butter and it turns solid again. Warm ice and it turns liquid again. No new substance is made, so both changes can be reversed, again and again.
Freezing food to store it
Food is frozen partly to make it last longer, and partly because ice takes up a fixed shape that is easy to stack and store. A frozen block does not spill, drip or spread the way a liquid would. It also does not need a lid to hold its shape.
One tray, watched over an hour
Pour liquid orange juice into an ice lolly mould and put it in the freezer. After thirty minutes it is slushy. After an hour it is completely solid. Take it out into a warm room and, slowly, it melts back to liquid. The juice itself never changed — only its state did, twice. You could put that same juice through the cycle again tomorrow.