Solids, Liquids and the Water Cycle

Have you ever watched an ice cube melt in your hand? 🧊 One moment it is hard and cold, the next it is a puddle of water dripping through your fingers. That same water could turn into invisible steam if you heated it enough. Matter — everything around us — can exist in three forms called states: solid, liquid and gas. Today we find out what makes each state special and how they change into one another.

A solid keeps its own shape. The tiny particles (incredibly small pieces that everything is made of) are packed tightly together and held firmly in place, like people squashed on a full bus — they can wobble a little but they cannot move around. A liquid flows and takes the shape of whatever container it is poured into, because its particles are close but free to slide past each other. A gas spreads out to fill any space, because its particles zoom around very fast with lots of room between them.

States can change when you add or remove heat energy. When a solid is heated enough it melts — its particles get so much energy that they break free from their fixed positions and flow as a liquid. The temperature at which this happens is called the melting point. For water, the melting point is 0 °C. Below 0 °C water is solid ice; above 0 °C it melts into liquid water.

When you heat a liquid, the particles move faster and faster until some escape from the surface and become gas — this is called evaporation. It happens even on a cold day, which is why puddles disappear without boiling! When water boils at 100 °C, evaporation happens very quickly and you see lots of bubbles and steam. The opposite process is condensation — when a gas cools down, its particles slow and clump together to form a liquid again. Think of a cold mirror in a steamy bathroom: the water vapour (water as a gas) touches the cold surface, cools, and forms water droplets.

Now let us follow water on its never-ending journey — the water cycle! The sun heats the surface of the sea, rivers and lakes, causing liquid water to evaporate and rise into the air as invisible water vapour. High up in the sky the air is much colder, so the vapour condenses into tiny water droplets that cluster together to form clouds ☁️. This stage is called cloud formation or condensation in the atmosphere.

As more and more droplets join together inside a cloud, they become too heavy to stay up and fall back to Earth as precipitation — rain, sleet, snow or hail, depending on how cold it is. The water lands on the ground, flows into streams and rivers, soaks into the soil, or collects in lakes and seas ready to be evaporated all over again. The cycle never stops — water just keeps moving between the ground, the sky and everything in between. 🌧️🌊