When a gas is cooled, its particles slow down and condensate to form a liquid. This is known as condensation, and it happens at the same temperature as boiling. Evaporation happens when particles in a liquid move straight into the gas state at temperatures lower than the liquid's boiling point. The vapor given off during evaporation can be cloudy because of small particles in the liquid that get suspended in the gas.
Condensed liquids are called "solids" for short. Solids are denser than gases because their particles are closer together. So if you put some condensed water on a table and let go of it, it will fall because it is heavier than air. If you were to try this with normal water, it would become a gas and rise back up because there are no solid particles in it.
The process of changing one thing from a gas to a liquid to another something else again is called a phase change. There are three main ways things phase change: freezing, boiling, and evaporating. These methods use different parts of the temperature scale.
If you drop frozen water onto ice, it will melt it instead because they have the same temperature when they first touch each other. As the frozen water melts, it changes into liquid water which is now warmer than the ice. So the ice melts before the water does!
Condensation is the transformation of a gas into a liquid. For example, if you pour water out of a bottle in cold weather, it will immediately begin to condense into droplets of liquid.
Gas molecules are made up of small positive ions called atoms, which are joined together by strong forces. In a pure gas, there are no bonds between the molecules; they are only attached to each other by inertia. If one molecule moves away from another, there is an increase in entropy (a non-living state of order), so it is unlikely that two molecules will move away from each other without some kind of force being applied to them.
Atoms have a nucleus composed of protons and neutrons inside them, and electrons orbiting the nucleus. In chemistry labs, gases are often prepared by heating a metal such as sodium until it becomes a vapour, then cooling it down quickly so that the sodium does not burn up. The heat causes the electrons to orbit around the nucleus, causing the atom to become a gas. If you keep the sodium and add more heat, it will continue to gasify, until all the metal has been turned into vapour.
The particles in a gas slow down as it cools. Droplets of liquid develop when particles travel slowly enough for their attractions to bring them together. This process, which is the inverse of vaporization, is referred to as condensation. When a gas condenses to a liquid, the thermal energy it absorbed to form a gas is released. Thus, liquids always contain less thermal energy than gases of the same temperature.
Liquids can also be formed by freezing gases. As ice crystals form, they trap any remaining gas bubbles. The resulting solid is called frozen gas. Frozen gases are transparent and brittle. They may appear white or colored depending on the type of gas that was present in the original gas cloud before it froze out.
Gas clouds can also collapse due to gravity. This usually happens only with very light gases such as hydrogen. Before collapsing, a gas cloud forms a thin, nearly spherical shell of material. This shell is called an "ionosphere" because it contains many charged particles from previous nuclear reactions. As gravity pulls ions towards the planet's center, it creates a region of high negative charge density near the surface. This is why the ionosphere affects radio communications and satellites above Earth's atmosphere.
Ionospheres can be measured by instruments on board spacecraft. These include magnetometers, which measure changes in magnetic flux, and electroscopes, which measure electric potential differences between different points on the surface of the planet.
Vaporization occurs when a liquid transforms into a gas. The process can occur as a result of either boiling or evaporation. At this moment, the liquid particles will evaporate and enter the gas phase. When particles on the surface of a liquid shift into the gas phase, this is referred to as evaporation. Parts of the liquid may also boil away if the temperature is high enough.
As a liquid turns to a vapor, some properties of the liquid are lost and other properties of the gas are gained. For example, water turns to steam as it escapes from your tap at a temperature of 100°C (212°F). The heat from the water is what causes the vapor to change phase. As the molecules in the water move more freely, they give off their energy in the form of heat.
At low temperatures, all liquids transform into vapors without passing through a solid phase. But at higher temperatures, some liquids begin to melt before turning into vapors. For example, oil becomes a liquid before melting at 32°C (90°F), but it begins to vaporize at 104°C (220°F).
Some substances are completely impossible to liquefy. These include gases and liquids with an extreme difference in pressure. For example, it would take millions of times the pressure at sea level to compress nitrogen gas into a liquid.
The transformation of a liquid into a vapor is important in chemistry and physics.