All matter exists as solids, liquids, or gases. These are
called the states of matter. Matter can change from one state to another if
heated or cooled. If ice (a solid) is heated it changes to water (a liquid).
This change is called MELTING. If water is heated, it changes to
steam (a gas). This change is called BOILING. The particles of
ice, water, and steam are identical, but arranged differently.
When solid, the particles of a substance are tightly packed
together, making it rigid. A substance can change from a solid state to a
liquid state, and from a liquid state to a solid state.
The particles in a liquid can move past one another. This allows
liquids to flow. A substance in a liquid state can change to a solid state and
also to a gas state.
Particles in a gas are spread out and free to move around. This is
why gases fill all the space around them. A substance that is a gas can change
to a liquid, and a liquid substance can change to a gas.
The glowing corona of the Sun visible during a total eclipse is made
of a fourth state of matter called plasma. Plasma is formed when a lot more
energy is given to a gas, such as by heating the gas or passing electricity
through it. This extra energy splits the particles of the gas into even smaller
pieces so hot that they glow.
When a solid is heated, the particles are given more
energy and start to vibrate faster. At a certain temperature, the particles
vibrate so much that their ordered structure breaks down. At this point the
solid melts into liquid. The temperature at which this change from solid to
liquid happens is called the melting point. Each solid has a set melting point
at normal air pressure. At lower air pressure, such as up a mountain, the
melting point lowers.
Lava is liquid rock, which erupts through a volcano at
temperatures as high as 1,500ºC (2,732ºF) through a volcano.
However, the red-hot lava cools as it meets the Earth’s surface, and
turns back into solid rock again. This change from liquid to solid is called
freezing or solidifying. It is the opposite process to melting.
When a liquid is heated, the particles are given more
energy. They start to move faster and further apart. At a certain temperature,
the particles break free of one another and the liquid turns to gas. This is
the boiling point. The boiling point of a substance is always the same; it does
not vary.
Water boils when it reaches its boiling point of 100ºC
(212ºF). This is the temperature at which water turns to steam. Steam is
an invisible gas. When it reaches the lid it cools back to a liquid.
Even without boiling water in a kettle, some of the liquid water
changes to gas. This is evaporation. It occurs when a liquid turns into a gas
far below its boiling point. There are always some particles in a liquid that
have enough energy to break free from the rest to become a gas.
Dewdrops are often found on a spider’s web early in the
morning after a cold night. Water that is present as a gas in the air cools
down and changes into tiny drops of liquid water on leaves and windows. This
change from gas to liquid is called condensation.
