Chemistry
Ms. Boon
Name: ___________________________________________
Period: _______ Date: ___________________
Changing States of Matter
All matter can move from one state to another. It may require extreme temperatures or extreme pressures, but it can
be done. Sometimes a substance doesn't want to change states. You have to use all of your tricks when that happens.
To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. Some of you
know about liquid nitrogen (N2). It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay
a liquid. What if you wanted to turn it into a solid but couldn't make it cold enough? You could increase the pressure
to push those molecules together. The opposite works too. If you have a liquid at room temperature and you wanted
a gas you could use a combination of high temperatures and low pressures to solve your problem.
CHEMISTRY TERM
Fusion (melting)
Freezing
Vaporization (boiling)
Condensation
Sublimation
Deposition
PHASE CHANGE
Solid to Liquid
Liquid to Solid
Liquid to Gas
Gas to Liquid
Solid to Gas
Gas to Solid
Phase changes happen when certain points are reached. Sometimes a
liquid wants to become a solid. Scientists use something called a freezing
point to measure the temperature at which a liquid turns into a solid. As
a substance changes from a liquid to a solid, energy is released and the
kinetic energy of the molecules decreases. Therefore, freezing is an
exothermic process. There are physical effects that can change the
freezing point. Pressure is one of those effects. When the pressure
surrounding a substance goes up, the freezing point and other special
points also go up. That means it's easier to keep things solid at higher
pressures. Just remember that there are some exceptions. Water (H2O) is
special on many levels. It has more space between its molecules when it
is frozen. There's a whole expanding effect when the molecules organize
into a solid state. Generally, when temperatures get colder, solids shrink
in size. They become more dense because the molecules in the solid state are closer together than in the liquid state.
Solid to Liquid and Back to Solid
Imagine that you are a solid. You're a cube of ice sitting on a counter. You dream of
becoming liquid water. You need some energy. The atoms in a liquid have more
energy than the atoms in a solid. The easiest energy to find is probably heat.
Because energy must be absorbed to change from a solid to a liquid, melting is an
endothermic process. There is a special temperature for every substance called the
melting point. When a solid reaches the temperature of its melting point, it can
become a liquid. For water, the temperature has to be a little over zero degrees
o
Celsius (0 C). If you were salt, sugar, or wood, your melting point would be higher than that of water. How do you
know that? If their melting points were lower, they would be liquids at room temperature. The reverse of the melting
process is called freezing. Liquid water freezes and becomes solid ice when the molecules lose a lot of energy.
Solid to Gas and Back to Solid
You are used to solids melting and becoming liquids. Some of you may have also seen a solid become a gas. It's a
process called sublimation. The easiest example of sublimation might be dry ice. Dry ice is solid carbon dioxide (CO2).
Amazingly, when you leave dry ice out, it just turns into a gas. Have you ever heard of liquid carbon dioxide? It can be
made, but not in normal situations. Can you go from a gas to a solid? Sure. It's called deposition when a gas becomes
a solid without going through the liquid state of matter. Those of you who live near the equator may not have seen it,
but closer to the poles we see frost on winter mornings. Those little frost crystals on plants build up when water vapor
Chemistry
Ms. Boon
Name: ___________________________________________
Period: _______ Date: ___________________
becomes a solid.
Liquid to Gas and Back to Liquid
When you are a liquid and want to become a gas, you need to find a lot of energy. Once you can start to pump that
energy into your molecules, they will start to vibrate. If they vibrate enough, they can escape the limitations of the liquid
environment and become a gas. Because energy is required to change a liquid into a gas, evaporation or vaporization is
an endothermic process. When you reach your boiling point, the molecules in your system have enough energy to
become a gas.
The reverse is true if you are a gas. You need to lose some energy from your very excited gas atoms. The easy answer is
to lower the surrounding temperature. When the temperature drops, energy will be transferred out of your gas atoms
into the colder environment. This release of energy makes condensation an exothermic process. When you reach the
temperature of the condensation point, you become a liquid. If you were steam over a boiling pot of water and you hit a
wall, the wall would be so cool that you would quickly become a liquid. The wall absorbed some of your extra energy.
Practice:
1. (a) Label the five steps shown in the water phase change diagram. (b) Indicate whether temperature is changing
during each step. (c) Draw what the molecules are doing at each step. (1 is completed as an example.)
(1) ice; temperature is increasing
(2)
(3)
(4)
(5)
2. Why is the temperature not rising when the water
is a mixture of ice and water?
3. According to the diagram, what is the boiling point, melting point, and freezing point of water?
4. What happens to the kinetic energy of water molecules as water boils? Is boiling an endothermic or exothermic
process?
5. What happens to the kinetic energy of water molecules as water freezes? Is freezing an endothermic or exothermic
process?