Chemistry
States of Matter
Key Ideas/Takeaways:
1. Characteristics of the three states of matter: solid, liquid, and gas
2. How a substance can change between the three states – by pressure or temperature
Duration of Lesson: 60 minutes
Materials:
-Access to a large screen to show an online video
-Alternative:
-Empty, clear 2 liter soda bottle with the labels removed
-A few tablespoons of rubbing alcohol
-Bike pump
-Rubber stopper with a hole in the middle for the bike pump/something to create
a tight seal around the bike pump and the soda bottle that can be removed quickly
-Enough of each of the following for each student:
-Two Dixie cups
-Two strips of paper towel
-Few strips of tape
-A pencil
-Large cup
-Four ice cubes
-String
-A few pinches of table salt
-Bottle of rubbing alcohol
-Bag of cotton balls
-Water
-Role of tape
Introduction: (15 minutes)
Begin by asking the students if they have heard of states of matter before and if they can name the
three states: solid, liquid, and gas. Explain that the states of matter are three distinct, physical forms that
matter takes on. Another word used to describe a state of matter is a phase. Matter is anything that takes
up space, even if you can’t see how small it is. So, everything in the universe is made out of matter! Ask
the students to list characteristics of each state and write them up on the board. Then, ask students to
name examples of substances that they know change phases, such as water. Follow up by asking the
students how they make these substances change phases. For example, for water, to change from a liquid
to a solid, we freeze it. To change from a liquid to a gas, we boil it. Explain that one way to change the
state of matter is by changing the substance’s temperature. Each substance has its own melting and
boiling point. The melting point is the temperature at which a solid changes into a liquid or vice-versa.
The boiling point is the temperature at which a liquid changes into a gas, and the other way around.
Explain that when a substance’s temperature is raised, the substance has more energy. So when a solid
changes to a liquid, the liquid has enough energy to break some of the bonds between the molecules so
they can move more freely. Therefore, a gas has more energy than a liquid which has more energy than a
solid. Heating up a substance adds energy whereas freezing a substance removes it.
Now, have four or five student volunteers come up to the front of the classroom to help
demonstrate the differences in the three phases. Start as a solid, with everyone huddled really closely
together. Explain that the molecules that make up a solid objects are tightly packed together and can’t
move – that’s why a solid feels rigid and it doesn’t change shape. Next, tell the students that they are
going to warm up into a liquid. Have the volunteers grab hands. Even though the molecules are still
connected and close to each other, they can move freely around. This is like a liquid which can moved
and take the shape of the container it’s in, but it doesn’t completely spread out. Finally, have the students
gain even more energy and change into a gas. Everyone let’s go of each other’s hands and they can spin
and run around the class. Gas molecules have a lot of energy and thus, don’t like to be near each other,
otherwise they’ll run into each other.
The main part of the vocabulary for this lesson comes from the diagram below. This is a great
visual aid for students to understand and relate the three phases of matter to one another. It’s important to
walk through the diagram and explain that a solid doesn’t always have to change to a liquid before it
changes to a gas. One example of a solid changing to a gas is dry ice which students should be familiar
with. Dry ice is frozen carbon dioxide which has a melting point of -109.3 °F! To give the students
perspective on how cold this is, explain that water freezes at 32 °F and boils at 212 °F. Because the
environment surrounding dry ice is so warm (room temperature is around 70 °F), the carbon dioxide
sublimes or changes straight from a solid to a gas.
Cloud in the Bottle Demonstration Video: (5-10 minutes)
Ask the students if they know the other way to change a substance’s state of matter. While they’re
thinking about it, this is a good time to get the video set up to show:
https://www.youtube.com/watch?v=msSVQ903T8k
Alternatively, you can do this demo yourself. Take a two liter soda bottle, pour a few tablespoons of
rubbing alcohol in it, then turn the bottle sideways and let the rubbing alcohol cover the inside of the
bottle, all the way up to the opening. Then, place the bike pump in the opening of the bottle and create a
tight seal so no air escapes. With one hand on the seal and another on the walls of the bottle, have a
second person pump up the amount of air in the bottle. When the walls are very rigid and don’t bend
under your hands, stop the pumping and quickly release the seal. A cloud should form in the bottle like in
the video. While the cloud is still visible, you can also repump up the pressure in the bottle to force the
alcohol vapor into its liquid form again. When you let go, the cloud will reappear.
If they’re having trouble thinking of the answer (pressure), ask them if they’re ever watched the
weather report on the news with their parents. Ask them what the weatherman talks about with sunny
days versus cloudy days – he mentions changes in atmospheric pressure. So, the other way to change
phases is by increasing or decreasing the pressure on the object. Pressure is another word for force. So,
when there is a lot of pressure, a lot of force is being applied on the object. Now, explain the cloud in the
bottle, telling students that you will show them how to create their own cloud using pressure, just like in
the atmosphere. Pause the video after the bottle is pumped full of air and ask the students to predict what
they think will happen when the scientists releases the seal. Then, show that portion, and pause again after
the cloud forms to discuss what is happening. When the scientists pumps air into the bottle, all of the air
molecules are being forced into a certain space, so they have to get really close together to fit, and as a
result, there is a lot of pressure built up in the bottle. When the seal is released, the pressure is released,
and as a result, the liquid rubbing alcohol turns into a gas. This is because the molecules are no longer
being forced to be really close together in the liquid state and can instead, spread out into a gas. Now,
show the class when the pump is used to push the cloud back into the liquid state, again, pausing the
video to ask the students what will happen now that pressure is being applied to the gas-phase rubbing
alcohol. So the takeaway message that students should get is that to change from a solid to a liquid to a
gas, pressure has to be decreased so the molecules can spread out. In contrast, pressure has to be increased
for a gas to change to a liquid and then to a solid.
Rubbing Alcohol Evaporation Experiment: (15 minutes)
Pass out the Dixie cups and strips of tape to students. They should already have pencils, but if
not, pass those out as well. Then, show students how to construct their “drying line.” To do so, flip the
Dixie cups upside-down on the table and lay a pencil on top of the two cups, making a bridge. Tape down
the two pencil ends so it doesn’t role off. Now, have the students make theirs. Next, pass out two Dixie
cups to each table group, one containing rubbing alcohol and one water. Also pass out two paper towel
strips to each student. Have them label the strips so they know which one is for water and which one is for
rubbing alcohol. Now, ask students whether different substances change phases at the same rate. After
they’re done discussing, have them predict whether they think the water or the rubbing alcohol will
evaporate faster. The students will then dip the water strip in the water cup and the alcohol strip in the
alcohol cup and lay them out to dry over the pencil. Have the students watch the strips to observe how
quickly the strips dry, noting which one is changing faster.
Come back as a class to discuss their observations and discuss why the rubbing alcohol
evaporated more quickly. This is because the rubbing alcohol is made up of different molecules than
water, and these molecules don’t bond really well with each other. As a result, it requires less energy for
them to break apart and change from a liquid to a gas. In comparison, the water molecules really like each
other and bond very strongly, so it’s harder to break them apart and it takes longer to do so.
Now, pass out cotton balls to each student. Either have them dip the cotton balls in the rubbing
alcohol or walk around the classroom and use the original bottle. Have student wipe the tops of their
hands with the rubbing alcohol and then make observations. They should notice how cool the rubbing
alcohol feels and how quickly it evaporates, even faster than when it was hanging on the pencil. Some
students won’t want to do this activity, but reassure them it’s perfectly safe and won’t hurt.
Reconvene as a class to discuss their observations. Ask them if it felt cool or warm while it
evaporated. A change that feels cool is endothermic. Endothermic reactions feel cool because they
require energy in the form of heat. Another way to think of this is that if you have to share a bed with a
sibling and your sibling steals the blanket (acting as energy in this scenario), then you’re going to be left
without the energy and feel cold as a result. That’s what’s happening on your skin – the rubbing alcohol is
using your warm body temperature to help it evaporate more quickly than it did on the pencil.
Fishing with String Experiment: (15 minutes)
Now, students will be trying to fish for ice cubes using string. Pass out the larger cups filled with
cold water and four ice cubes dropped in each for each student. They can also do this activity in pairs.
Also pass out the string and a Dixie cup or two containing table salt for each table. Then, have students
try to use the string to “fish” for an ice cube. The rules of the game are that they can’t use their hands,
they can only dip the string in the cup. The students won’t be able to pick up an ice cube. Now, have the
students hold the string so that one end is resting on the surface of an ice cube. Now, have them pour salt
over both the ice cube and the string, counting to 15 seconds after they’re done pouring. Now, have the
students try fishing again. They should find that initially the salt melted the ice, but then the ice refroze to
the string and now they can pull the ice cube out.
Come back together as a class to discuss why the salt made fishing for ice cubes possible. We use
salt on roads in the winter time to prevent them from freezing, but if it’s cold enough, then the water on
the road will refreeze just like they did in the cup. As the ice cube refroze, the string got stuck in the ice
which is why students were able to pull it out.
Conclusion: (5 minutes)
Go over the characteristics of the three states of matter and the two ways those states can be
changed (temperature and pressure). Then, finish up by going over the vocabulary from the triangle
diagram above.