Looking for a Gas
Gas is everywhere. There is something called the atmosphere. That's a big layer of gas that surrounds the Earth. Gases are random groups of atoms. In solids, atoms and molecules are compact and close together. Liquids have atoms that are spread out a little more. Gases are really spread out and the atoms and molecules are full of energy. They are bouncing around constantly.
Gases can fill a container of any size or shape. It doesn't even matter how big the container is. The molecules still spread out to fill the whole space equally. That is one of their physical characteristics. Think about a balloon. No matter what shape you make the balloon, it will be evenly filled with the gas molecules. The molecules are spread equally throughout the entire balloon. Liquids can only fill the bottom of the container, while gases can fill it entirely. The shape of liquids is really dependent on the force of gravity, while gases are light enough to have a little more freedom to move.
You might hear the term "vapor." Vapor and gas mean the same thing. The word vapor is used to describe gases that are usually found as liquids. Good examples are water (H2O) or mercury (Hg). Compounds like carbon dioxide (CO2) are usually gases at room temperature, so scientists will rarely talk about carbon dioxide vapor. Water and mercury are liquids at room temperature, so they get the vapor title when they are in a gaseous phase.
Compressing Gases
Gases hold huge amounts of energy, and their molecules are spread out as much as possible. With very little pressure, when compared to liquids and solids, those molecules can be compressed. It happens all of the time. Combinations of pressure and decreasing temperature force gases into tubes that we use every day. You might see compressed air in a spray bottle or feel the carbon dioxide rush out of a can of soda. Those are both examples of gas forced into a smaller space than it would want, and the gas escapes the first chance it gets. The gas molecules move from an area of high pressure to one of low pressure.
Evaporation of Liquids
Sometimes a liquid can be sitting in one place (maybe a puddle) and its molecules will become a gas. That's the process called evaporation. It can happen when liquids are cold or when they are warm. It happens more often with warmer liquids. You probably remember that when matter has a higher temperature, the molecules have a higher energy. When the energy in specific molecules reaches a certain level, they can have a phase change. Evaporation is all about the energy in individual molecules, not about the average energy of a system. The average energy can be low and the evaporation still continues.
You might be wondering how that can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and normal air pressure. Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. Not all of the molecules in a liquid have the same energy. When you have a puddle of water (H2O) on a windy day, the wind can cause an increased rate of evaporation even when it is cold out.
Energy Transfer
The energy you can measure with a thermometer is really the average energy of all the molecules in the system. There are always a few molecules with a lot of energy and some with barely any energy at all. There is a variety, because the molecules in a liquid can move around. The molecules can bump into each other, and when they hit... Blam! A little bit of energy moves from one molecule to another. Since that energy is transferred, one molecule will have a little bit more and the other will have a little bit less. With trillions of molecules bouncing around, sometimes individual molecules gain enough energy to break free. They build up enough power to become a gas once they reach a specific energy level. In a word, when the molecule leaves, it has evaporated.
The rate of evaporation can also increase with a decrease in the gas pressure around a liquid. Molecules like to move from areas of higher pressure to lower pressure. The molecules are basically sucked into the surrounding area to even out the pressure. Once the vapor pressure of the system reaches a specific level, the rate of evaporation will slow down.
