A Journey Through States of Water

Drop an ice cube into a glass of lemonade and leave it a while: it turns into water. Ice feels hard, cold and keeps its shape; water flows and takes the shape of the glass. It is tempting to think they are two different things, but they are not. Ice and water are the same substance, water, simply in two different states: solid and liquid. A state is just the form matter is in, and the very same water can also exist as an invisible gas called water vapour. Wax, oil and ghee are examples of other substances that also exist as solids or liquids depending on temperature, so this is not a water-only trick, it is how matter in general behaves.

After rain, playgrounds fill with puddles, and by evening many have vanished, without anyone pouring the water away. The same disappearing act happens when you sprinkle water on a hot pan to make dosa, when you rub hand sanitiser on your hands, and when wet clothes or a mopped floor dry by themselves. In every case the water has not vanished, it has changed into an invisible gas, water vapour, and mixed into the air around it. This slow change from liquid to vapour, happening even at ordinary room temperature, without any bubbling, is called evaporation, and it is happening around you continuously, all the time, whether you notice it or not.

Take cold water in a glass tumbler, drop in a few ice cubes, and leave it undisturbed for five minutes: tiny water droplets appear on the outside of the glass, even though you never poured any water there. Where could this water have come from? One guess is that water seeped out through the glass. But look closely at the water level inside the tumbler: it has not gone down at all, and in a tall, narrow bottle even a slight drop in level would be easy to notice. So the water did not seep out. If it was not already inside the tumbler, it must have come from somewhere else: the air around the glass.

The air all around us always contains some invisible water vapour, and when that vapour touches a cold surface, like the outside of a glass holding ice water, it cools down and turns back into visible liquid droplets. This change, from vapour back into liquid, is called condensation, and it is the exact opposite of evaporation. Dew drops on grass and spider webs in the early morning form the very same way: overnight the ground and plants become cool, and water vapour in the air condenses onto them as tiny droplets. You can even prove that condensation, not seeping, causes the tumbler droplets: weigh the icy tumbler every five minutes on a kitchen scale, and its total weight actually goes up over time, exactly as it should if extra water is arriving from the air rather than leaking out from inside.

Put an ice cube in one container, then move it to a different-shaped container: the ice keeps its own shape, it does not flow or spread at all. Now pour water between containers of different shapes: it flows freely and takes on whatever new shape the container has, though the amount of water, its volume, stays exactly the same. Water vapour behaves differently again: being a gas, it spreads out to fill the entire available space, mixing completely into the air of a room, with no fixed shape or fixed volume of its own. That spreading is exactly why you can smell food cooking in the kitchen even from another room: the smell is carried by gases that spread through the air and reach your nose, without anyone carrying them there. So the three states can be told apart by asking two simple questions: does it keep a fixed shape, and does it flow or spread?

Water is the easiest example to see all three states of in daily life, but it is not special in this way; every substance can, in principle, exist as a solid, a liquid or a gas. Stones, wood and glass are solids we see around us all the time. Milk and oil are common liquids, alongside water. Air itself is mostly invisible gases, including oxygen, which we breathe in, and carbon dioxide, which we breathe out; both behave exactly like water vapour, spreading to fill whatever space they are in. Once you start looking, you can find dozens of solids, liquids and gases in your own home.

To change ice into water, and water into water vapour, you must supply heat, which is why ice melts on a warm table and a kettle of water eventually boils away. To go the other way, water into ice, you must remove heat instead, by placing it in a cold environment such as a freezer. The conversion of a solid into a liquid is called melting, and the conversion of a liquid into a solid is called freezing; for a pure substance like water, both happen at exactly the same temperature, 0 degrees Celsius. Water is not the only substance that behaves like this: candle wax melts when heated and re-solidifies on cooling, and in cold winter weather even coconut oil, normally liquid, turns solid in the bottle.

Pour the same small amount of water into a bottle cap and onto a wide plate, and place both side by side: the water on the plate, which has a much larger surface exposed to the air, evaporates noticeably faster than the water in the narrow cap. Comparing a cap of water kept in sunlight against an identical one kept in shade shows sunlight (heat) also speeds evaporation up, and comparing a windy spot against a still one shows moving air does too, which is exactly why clothes dry faster on a hot, windy day. There is a fourth factor: on a rainy day the air already holds a lot of water vapour, a condition called humidity, and when humidity is already high, water evaporates more slowly, since the air has less spare room to hold more vapour, which is why clothes dry slowly on a humid, rainy day.

Sitting under a running fan on a hot day feels cooler because the moving air helps sweat evaporate off your skin faster, and evaporation always cools whatever it is evaporating from. This is exactly why rubbing hand sanitiser feels cold on your skin, and why sprinkling water on a floor or roof in summer cools it down. Many Indian homes use this same effect on purpose: water stored in an earthen matka or a surahi feels far colder than water from a steel container, because the porous clay lets a little water seep through to the outer surface, where it evaporates continuously and cools the water left behind inside. The same idea, scaled up, builds a pot-in-pot cooler: a small clay pot is nested inside a larger one with wet sand packed in the gap between them; as the water in the sand slowly evaporates, it draws heat away from the inner pot, keeping vegetables and fruit stored inside noticeably fresh and cool for days, with no electricity at all.

Condensation is also what brings evaporated water back down to Earth. Air carrying water vapour rises higher into the sky, for the same reason a gas balloon filled with a lighter gas floats upward: water vapour is lighter than the air around it, so air carrying plenty of it tends to rise. At higher altitudes it becomes steadily cooler; eventually the vapour condenses into countless tiny droplets, usually forming around microscopic dust particles floating in the air. These droplets float together as clouds, and as more and more of them join up into bigger, heavier drops, they eventually become too heavy to stay suspended and fall as rain, or under especially cold conditions, as hail or snow. Water that falls this way eventually flows into rivers, lakes and oceans, evaporates back into the sky, and repeats the entire journey once again; this endless circulation of water between the Earth's surface and its atmosphere is called the water cycle.

Even though the water cycle keeps recycling the same water endlessly, only a small share of all the water on Earth is actually fit for drinking, farming or daily use; most of it sits in the oceans as salty water that cannot be used directly. As more people need water every year, using it wisely and keeping rivers, lakes and groundwater free of pollution genuinely matters. Some modern machines called Atmospheric Water Generators put this same science to direct use: they cool humid air on purpose, so that water vapour in it condenses out as clean, drinkable water, essentially manufacturing drinking water from the same trick a cold glass of lemonade performs by accident.