Changes in States of Matter
The five states of matter we learned about are solids, liquids, gases, plasmas, and Bose-Einstein
condensates (BEC). Each of these states is also known as a phase. Elements and compounds can move
from one phase to another when specific physical conditions are present. One example is temperature
(increase in movement of the molecules). When the temperature of a system goes up, the matter in
the system becomes more excited and active. Scientists say that it moves to a higher energy state.
Generally, as the temperature rises, matter moves to a more active state.
Think about it this way. Remember that glass of water (H2O) we talked about? When the
temperature of the water goes up, the molecules get more excited and bounce a lot more. If you give a
liquid water molecule enough energy, it escapes the liquid phase and becomes a gas. Have you ever
noticed that you can smell a turkey dinner after it starts to heat up? As the energy of the molecules
inside the turkey heat up, they escape as a gas. You are able to smell those volatile gas molecules.
A "phase" describes a physical state of matter. The key word to notice is physical. Things only
move from one phase to another by physical means. If energy is added (like increasing the
temperature) or if energy is taken away (like freezing something), you have created a physical change.
A compound or element can move from one phase to another, but still be the same substance.
You can see water vapor, in the form of steam, over a boiling pot of water. That vapor (or gas) can
condense and become a drop of water. If you put that drop in the freezer, it would become a solid
piece of ice. No matter what phase it was in, it was always 2 hydrogen atoms connected to 1 oxygen
atom (water).
In a solid the molecules usually form some type of crystal or shape, but are still H2O. In a liquid
the molecules have enough energy to slide past each other and in a gas they have so much energy the
molecules zoom around. The molecule is unchanged in shape or size in a phase or state change.
Start from:
Change to:
solid
liquid
liquid
solid
liquid
gas
gas
liquid
solid
gas
(skipping liquid phase)
gas
solid
(skipping liquid phase)
Name
Solid to a Liquid
One change of state happens when you add energy to the substance. This
change of state is called melting. By adding energy to the molecules in a solid the
molecules begin to move quicker and can break away from the other molecules.
This happens slowly, molecule by molecule, as each molecule in the substance
has to get enough energy to move quicker. Once the molecules have enough
energy they begin to slide past the other molecules, and that is what makes a
solid turn into a liquid.
The temperature at which a substance goes from a solid to a liquid is it
melting point. Ice melts into water at 32o F (0o C), whereas gold melts at
1,947.52 °F (1064.18 °C). That is quite a difference. A soft metal called gallium
will melt in your hand because its melting point is 98 °F (30 °C), but the
compound salt will not melt in your hand. It’s melting temperature is 1473 °F
(801 °C).
For a solid to melt, its molecules must overcome some of their attractions
to each other. When a solid melts all of the energy goes into breaking the
attractions that hold the molecules in place. Because you are adding energy melt
is an endothermic change.
Liquid to a Solid
The change of state from a liquid to a solid is called freezing. Freezing
is the reverse process to melting. For a liquid to freeze, the attraction
between the particles must overcome the motion of the molecules sliding
past each other. Think of it like a friend trying to grab your arm to hold you in
place when you are stuck in the mob of students moving down the hallway.
Your friend has to be stronger than the flow of traffic to hold you in place.
When a liquid turns into a solid the molecules slow down and begin
to vibrate in place. To cause freezing energy must be removed. Because you
are removing heat to freeze a substance it is called a exothermic change.
The point at which this happens is called the freezing point. Freezing
points and melting point happen at the same temperature it just depends on
whether you are adding or removing heat.
For a liquid to freeze, its molecules must lose energy so that their
attractions to each other overcome their movement. When a liquid freezes
all of the energy goes into the air. Because you are taking energy, freezing is
an exothermic change.
Liquid to Gas
If you’ve ever filled your bathroom full of steam you’ve experienced this next change of state called
evaporation. Or how about that glass of cool aid you left out on the counter. You come back a few days
later and it has less liquid in it than before….the liquid evaporated. Evaporation occurs at the surface of a
liquid. You can see rapid evaporation in the form of boiling. The difference between boiling and
evaporation is that boiling causes more of the molecules to move faster and thus they escape their
attractions to each other quicker. Evaporation happens on the surface and is a much slower process. Each
molecule must get enough energy to break its attraction to the other molecules.
By adding energy you cause the molecules to move faster and break away from the others. The
temperature that evaporation occurs is called the vaporization point or boiling point. Water boils at 212 °F
(100 °C).
Gas to a Liquid
The change of state from a Gas to a Liquid is called Condensation. If you’ve ever noticed that you mirror
gets foggy in the bathroom or that the grass is wet on a spring morning even though it hasn’t rained, then
you’ve seen condensation. For gas molecules to become a liquid the attraction for each other has to
overcome the extreme movement of the molecules. Again think of the hallway example, but the crowd is
moving much quicker. This again happens when energy is removed or leaves the molecules, thus slowing
movement of each molecule down.
The point at which a substance goes through condensation is called its condensation point. For the quick
moving molecules in a gas to slow down they have energy removed. If you haven’t figured it out yet,
Condensation is evaporation in reverse. 
Remember the molecules do not change chemically in a change of state, they only change in movement.
Solid to Gas
Dry ice is probably the most famous example of something going from a solid
to a gas without going through the liquid phase. During Sublimation the molecules
gain enough energy to break away from the attraction and move throughout the
space. Dry ice is the common name for solid carbon dioxide. At ordinary atmospheric
pressures, carbon dioxide cannot exist as a liquid. So instead of melting, solid carbon
dioxide changes directly into a gas. As it changes state, the carbon dioxide absorbs
thermal (heat) energy.
Another example of sublimation is snow and ice, although this process happens more slowly. Think
about really cold days some of the snow seems to disappear, even though its not melting into a liquid.
Some of the molecules are gaining enough energy to break away from the attraction and thus moving from
the solid ice phase to the water vapor phase without going through the liquid phase.
Gas to Solid
Deposition, also known as desublimation, is a phase change in which gas transforms into solid.
The reverse of deposition is sublimation.
One example of deposition is when water vapor changes directly to ice without becoming a liquid.
This is how snow forms in clouds, and frost on the ground. Another example is when frost forms on a leaf.
For deposition to occur, thermal (heat) energy must be removed from the molecules in a gas. When the
leaf becomes cold enough, water vapor in the air surrounding the leaf loses enough thermal energy to
change into a solid. When the supercooled water vapor touches the cold leaf it immediately begins to
condensate, but by this point is already past the freezing point. This causes the water vapor to change
directly into a solid.