States of Matter: Facts (Science Trek: Idaho Public Television)
For a better understanding, take a look at these animations of the behavior of solids, liquids and gases .
Change of State
Matter can move from one state to another, but can still be the same substance. A change of state, also called a phase change , is a physical change from one state of matter to another, for example, from solid to liquid or from liquid to gas.
How does matter move from one phase to another? If the motion of the atoms is altered by pressure or temperature, the state can change too. By lowering the temperature of water, it can freeze into a solid. By heating water, it can become steam which is a gas. Whether solid, liquid  or gas &dash; water is still water.
Pressure can change matter from one state to another. Deep in the earth solids turn to liquids because the heavy weight of layers and layers of the earth push down on the solids causing them to turn to liquid magma . This is just one example of how pressure can change matter too.
Other matter changes too, but often only exists in two states or requires the help of humans and technology to move through all three phases. Water is the only matter on earth that can be found naturally in all three - solid, liquid and a gas.
More About Atoms
An atom is made up of a nucleus of neutrons and protons &dash; even smaller particles than an atom. They are called subatomic particles . Electrons circle around this nucleus. Atoms tend to have the same number of electrons as the number of protons in the nucleus. This number is very significant. Each kind of atom has a different number of protons. For example: oxygen has 8 protons. That means it also has 8 electrons. The 8 is known as its atomic number. But gold has 79 protons and 79 electrons. If you wanted to know how many protons and electrons are found in any given atom, the Periodic Table of the Elements is the place to find out.
Elements
The table only  tells us about the atoms of the 100 or so elements found on the earth. An element cannot be broken down into smaller components . Gold is not made of anything &dash; except the element gold. The smallest possible piece of gold is just gold. If it were any smaller it would be the subatomic particles &dash; protons, neutrons and electrons.
Of these elements, some are solids like gold or silver. Others are gases like oxygen and hydrogen, and still others can be liquid like mercury. Of course this is based on room temperature. If an element is heated or cooled, or if enough pressure is applied to the element that could change its state of matter.
Compounds
I bet you can't find salt on the Periodic Table of the Elements. That's because salt is actually made of two elements; sodium and chloride. You can find both of those on the table. The sodium and chloride atoms  of salt are linked together much like magnets can link together. We call this a compound . Water is a compound made of oxygen and hydrogen. Sugar &dash; the white stuff you put into cookies &dash; is made of carbon, hydrogen and oxygen. Lots of the substances around us are compounds. From vinegar to household bleach to nail polish remover to baking soda to aspirin &dash; we use compounds all around our homes.
Properties
All matter has qualities about it that describe it scientifically. We call these properties . They can explain the physical or the chemical qualities of a matter. Some of the ways that we describe matter are by its color, its shininess, and its state at room temperature or its odor. Mass, temperature at which it changes states, electric conductivity and flexibility are also  properties of matter. These are different for each type of matter and can help scientists identify a matter.
Plasma
We often talk about the three states of matter; solid, liquid and gas. Most of the matter that we use is in one of those three forms. But there is another that we see and use and that we would have a hard time living without &dash; plasma . It is often called the fourth state of matter. Plasma is electrically charged, does not hold its shape, has a huge amount of energy and is very difficult state to manipulate without a laboratory. Plasma can be found here on the earth in flames, lightning, and the polar auroras. The sun, the stars, and some other space events and objects are also made of plasma matter.
More States of Matter?
Did you know that there are more phases of matter? We aren't as familiar with them nor do we see them every day. Some exist only in theory, others can be reproduced in laboratories, some are so new that scientists are still figuring out the details and others might exist, but have not yet been found in nature. They include: quark-gluon plasmas, Bose-Einstein condensates, fermionic condensates, strange matter, liquid crystals, quantum spin liquid (QSL), superfluids, supersolids and the paramagnetic  and ferromagnetic phases of magnetic materials. New theories and discoveries are happening all of the time, so you'll have to keep studying your physical science to learn more about these states of matter.
What is meant by room temperature?
State of matter of an element or atom is based on its behavior at room temperature. But what exactly is room temperature? Room temperature refers to air temperature not being specifically heated or cooled. Usually between 20 to 25°C (68 to 77°F). While the actual temperature could be hotter or cooler than these figures, it simply means that the matter has not been placed in an oven or a freezer, but was left out in the room to attain the temperature of the rest of the room. If an ice cube was left out in the room for a period of time, it would melt and become liquid water. Water is a liquid at room temperature. A diamond, on the other hand, is a solid at room temperature and will not change state no matter how long it sits there.