Place an ice cube on the palm of your hand and you will soon have a pool of water. Leave it for long enough and the water will evaporate and disappear. What makes water appear as ice rather than a vapor? What makes it change between solid, liquid, and gas?
Every substance is made up of atoms or molecules that jiggle around in constant motion. Depending on how hot or cool it is, and how much "squeezing (pressure) a substance receives from its surroundings, its atoms or molecules move quicker or slower, spring together or bounce apart. It's the strange dance of atoms inside a substance that makes it shift between solid, liquid, gas, or plasma - the four main states of matter.
Although the solid, liquid, or gas forms of a substance contain the same number of atoms, they have different inner structures. Ice, water, and water vapor look very different and behave in amazingly different ways - all because of the patterns their atoms take up inside them.
When something is cooled or put under pressure, its atoms or molecules lock tightly together to form the strong bonds of a solid. These powerful links between atoms make solids difficult to bend or reshape. Some materials form orderly solids called crystals, while others form more random (amorphous) solids.
Water in a solid state
Most substances contract (shrink) when they freeze, but water is different. It expands as it freezes, making ice slightly less dense than water. This is why ice floats and why water pipes can burst when they freeze.
If you cool a liquid slowly, it has time to arrange its atoms and molecules into a very regular form called a crystal. Many metals are like this.
Some materials cool and snap together into a more random structure. Glass is like this - a mix between an orderly solid and a chaotic liquid.
Liquids are usually hotter and less compressed than solids, so their atoms and molecules are slightly further apart from one another. The forces between the particles are weaker, so they can move around more freely. This is why liquids have no fixed shape, but spread out to line the container in which they are placed.
Water in a liquid state
Life exists on Earth because there is water. It's a liquid at everyday temperatures and pressures, which means it is easy to transport and recycle, and just as easy for plants and animals to absorb.
The weaker bonds between atoms and molecules in liquids allow liquids to flow as you pour them. A liquid’s viscosity means how slowly or quickly it flows.
In gases, atoms and molecules are not bonded together but move quickly and freely and have enough energy to flow all by themselves. Constantly bumping into one another, they spread out to fill whatever container they are inside.
Water in a gaseous state
Gases are normally hotter than their liquid forms, which is why steam is hotter than water. Gases can also form when the pressure is low. In clouds, water is a cold gas (water vapor), due to the low air pressure high above Earth.
Heat a gas or lower its pressure enough, and the atoms come apart to form a cloud with an electric charge, called a plasma. Plasmas are made of charged particles, called electrons, and ions (atoms missing electrons), so they behave in strange ways when electricity and magnetism are nearby.
The northern lights (Aurora Borealis) happen when plasma from the Sun hits Earth's atmosphere and bends in its magnetic field.
Boil a kettle and you will transform water from a liquid to a gas (steam). Freeze food and you will change the water inside it into a solid (ice). It is easy to make water change states because its solid, liquid, and gas forms can exist at everyday temperatures and pressures. It is harder to change materials like metals into liquids or gases, since much higher temperatures and pressures are needed.
You can change a solid into a gas without making liquid first. The dry ice used to make smoke in concerts is made from frozen lumps of carbon dioxide. Once exposed to the air, it heats up rapidly and forms a cold gas.
Gases can turn directly into solids without first becoming liquids. If water vapor in the air is cooled enough, it condenses and freezes to form snow in clouds, or frost on the ground.
As liquids lose energy, their atoms and molecules move about more slowly and gradually come together. Bonds form between them and they lock in a rigid structure, making a solid.
Solids change to liquids by melting. For instance, ice cream quickly soaks up heat from the atmosphere. Water molecules in the ice gain energy and move apart, becoming liquid.
When you heat a liquid, the atoms and molecules gain much more energy. They collide more often and start to push apart. Some have enough energy to escape from the liquid, forming a gas (vapor) directly above it.
If you cool a gas or lower its pressure, it will turn into a liquid. Water vapor inside your home will often condense on your windows on cold days if the outside temperature falls low enough.