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Understanding the main phases of matter (Solids, Liquids, and Gasses) and their phase transitions is a fundamental skill for any young student. Since it can be difficult to understand at first, we developed this experiment to help students visualize what’s going on!

Age Range: This concept is usually taught in the third and fourth grades, but can be introduced to students who are much younger.

Key Concepts: States of Matter, Phase Changes, Density, Energy

Materials (photos link to amazon, in case you don’t have one of the necessary materials):

1 flask (or a glass bottle with a narrow opening. A balloon needs to be able to fit over it).

1 sharpie or dry erase marker (only if you aren’t using a glass with measure marks on the side)

1 Balloon (or more, if the one you uses rips)

1 Food Scale (or any scale that can measure somewhat small weights (between a gram and a few pounds))

1 pot or pan, which can hold the flask and an inch of water.

1 pair of oven gloves, or something you can use to carry the hot flask. 

And finally, access to a freezer and a stove top, either gas or electric will work.

What’s going on:

Heat is just energy, and when we add energy to something, the particles move faster. When we take energy away, the particles slow down.

How fast particles are moving is visible to us as different phases of matter: in a solid, particles aren’t moving very much at all. In a liquid, they are moving a moderate amount, and in a gas, they are moving around a lot.

Imagine a party, with a large room full of people. In our example, they go through three situations:

1) First, they are standing fairly still, talking to one another. The room is cold, so they stand close together.

2) Next, they will move around a little bit, to meet and talk to new people. The room warms up a little, as the people are exercising a bit more than they were before.

3) Finally, the music turns on and people start dancing. Because they are moving a lot, they spread out so they don’t hit each other. And the room warms up quite a bit because they are moving so much.

This is a good analogy for particle motion.

When particles are cool, they cluster closer together and don’t move very much. The particles in a solid are so close together, they often stick together in chemical bonds (kind of like holding hands. You need to be close to hold hands). The reasons that solids have a definite shape is because the particles are frozen, and locked together.

An example of a solid crystalline molecular structure, taken from Wikipedia.

 

As they heat up, they move around a bit more, breaking the chemical bonds that held them together. It’s hard to hold hands with people when it’s hot and you’re moving around a room in different directions. They flow past one another, but are still fairly close. This is the liquid phase. In a liquid, the particles are confined to the container you put them in, whether that’s a bottle, a lake, or a puddle on the ground. They are drawn to the bottom of the container, because the force of gravity is pulling them down. If you pour the liquid into a new container, the particles will flow to take the shape of that new container, again collecting at the bottom. But the particles aren’t moving so much that they escape the forces of gravity pulling them down, until more energy is added.

When you add even more energy, the particles are moving so quickly that they break free of the liquid, and overcome gravity’s pull. They run and fly all around the room, and bounce off the walls. You can trap a gas in a container like a balloon or a bottle, but the second you open the container, the gas will diffuse and fill the room. If I open a container of a gas outside, the particles spread out to fill the atmosphere.

Molecular states of Solids, Liquids, Gasses, and Plasma. From Encyclopedia Britannica.

Experiment:

In this experiment, we will demonstrate the three main phases of water- solid, liquid, and gas.

Part 1:

We will measure a specific amount of water to put into our glass, and weigh the entire system on our food scale. You can subtract the weight of the glass itself to find the weight of the water you added. Make a line on the side of the glass to mark the water level.

It’s good practice to record our data in charts and graphs, so you can make a chart like this to record the weight of the system with your child. I filled it out with the values I recorded in my experiment. Yours will be different, depending on which glass you use and how much water you add.

Before Freezing Hypothesis: Will the Weight Change After Freezing? After Freezing
Weight of Glass (g)  78g  No
Weight of Glass+ Water (g)  178g  No
Weight of Water (g)  178 – 78 = 100g  No

 

We will then freeze our flask. This can take a while, depending on how cold your freezer is. Notice how the water expands as it freezes into ice! Use your pen to mark the side of the glass, at the level of the ice. Ask your children if they think there is more water or the same amount as there was before? Challenge them to guess the weight of the flask now (if there is more water, it would be heavier). Fill in the chart with their hypothesis. After they make a guess, weigh it.

Once you weigh the glass, you should notice that the weight is the same. Record the data in your chart, and fill out the rest of the values. Explain to your children that even though the water takes up more of the glass, it’s still the same amount of water we started with. This is because as water freezes, the molecules arrange themselves in a crystal form, where they happen to be more spaced apart than they were as a liquid.

Before Freezing Hypothesis: Will the Weight Change After Freezing? After Freezing
Weight of Glass (g)  78g  No  78g
Weight of Glass+ Water (g)  178g  No  178g
Weight of Water (g)  178 – 78 = 100g  No  100g

You’ll notice that in the chart, I wrote “We can assume Xg” in the after freezing column. The reason is because the only thing we weigh after freezing is the glass and water combo. So we can’t really say that the glass or water didn’t change in weight right off the bat. What if the water gained a gram, and the glass lost a gram? We would still get the same combined weight.

However, using what we know of physics, we can reason that weight is dependent on how much stuff there is. The glass isn’t going to get heavier if we freeze it, because there isn’t anywhere to get more glass particles from. It’s also not going to get lighter, as the glass won’t evaporate or sublimate. Therefore, its weight will remain the same. To be 100% sure, you can also test this in a separate experiment, where you weigh the dry glass by itself, and then freeze it without adding any water. You should see that the weight is the same. If you do this step, you can just fill in the real, measured weight instead of the assumed weight.

But in our shorter experiment, we’re just making the assumption that the glass won’t change weight, so we can subtract the weight of the glass from the combo to get the water weight (178g-78g=100g).

Now, let’s return to our analogy of the people huddling for warmth in a cold room. Why is it that our water didn’t shrink, the way the people in the room got closer together? As it happens, almost all other materials will shrink when frozen. However, water (and a few other things) are special. The way the molecules arrange themselves is most stable when they are slightly farther apart than in liquid water. As if the people in the room found themselves more comfortable when standing in a pattern of stepping away from each other and holding hands, instead of huddled in a cluster. But only for water, silicon, and a few other things! For other materials, the people do like to cluster close together.

This unique property of water is actually one of the things that allows life to exist on earth! Because ice is less dense (particles are more spaced apart) than water, it floats. This might seem like a minor thing, but imagine if ice in the ocean would sink. Then the water on top would freeze again, sink to the bottom again, and the cycle would repeat until our entire planet was frozen. We wouldn’t be able to live like that for very long, as our warm liquid oceans make most of the oxygen we breathe through the algae living in it!

Part 2:

After you are finished weighing the glass, stretch the balloon over the top of it so that it forms a seal. Be careful not to rip the balloon!

Ask your child to observe the balloon. Is it flat, or does it have air trapped in it? It should be relatively flat.

Let the ice melt, either by leaving it at room temperature for a while, or by placing it in room temperature water. Be careful not to try and speed up the process by using hot water, as this will likely cause the glass to crack.

Once the system is liquid again, place a small amount of water in your pot or pan, and place your flask in the center of it. Make sure it sits solidly in the bottom- we don’t want it to tip over or float. If it doesn’t sit solidly, take some water out of the pan.

Once it sits solidly, turn the heat on the stove, and begin to heat the water in the pan. It should soon begin to simmer. Let it simmer without letting it all evaporate away. The glass should always be in a little bit of water. And be careful about letting it reach a high boil- the force of the bubbles might knock over our flask!

As for why we place it in water, we want the heat to be evenly distributed around your glass so it doesn’t crack. We didn’t put it directly on the fire, because that can unevenly heat it. The heat from the fire would also melt the balloon.

After about 15 minutes of the water simmering, look at the balloon. Is it still flat, or is there something inflating it? Does it continue to grow?

Direct your child to observe the steam rising from the pan. That is the water molecules evaporating, leaving the pan. They were heated so much they were more comfortable running all over the room, in gas form. They are still water, but now are adding to the air humidity instead of being something we can drink. If we want to drink that water again, we have to cool the room down so much that it condenses. It would condense first on the coldest parts of the room, like the windows.

You can demonstrate condensation by taking a glass of ice water and placing it on the counter. Within a few minutes, you should see beads of water forming on the outside. Explain to your child that this is the water in the air, but when it bumped into the cold glass, it cooled down enough to become liquid again! We’ll also see condensation in our little system once we turn off the heat.

This is basically what happens with the water cycle. The ocean is full of warm water. When the sun heats it, some of it evaporates into steam. This condenses into clouds in the upper atmosphere, which then precipitates as rain when it cools down enough. Some of that rain might freeze and become snow if it’s really cold. The water eventually rejoins the ocean, and the cycle continues.

A graphic of the water cycle, from wikipedia.

In our balloon, we are preventing the steam from fully escaping. Instead, it is trapped inside the balloon. Notice how the water level is a little lower on the side of our glass than it was before. That is because some of the water molecules are gas instead of liquid, and they are pushing on the balloon as they try to escape.

After you turn the stove off, ask your child what will happen to the water vapor in the balloon. We stopped adding heat, so the balloon should deflate as the water vapor looses energy and becomes liquid water again.

Sure enough, you should start to see drops condensing on the side of the glass, and the water level rising again.

We aren’t using the scale for this part of our experiment, as it’s unlikely that you lost enough water as vapor to see a difference in the weight.

But as a thought experiment, you can challenge your child to guess if while the balloon is inflated, the weight of the system should change at all.

While the same amount of matter is present in the system (the water hasn’t gone anywhere), some of it is no longer pushing down on the bottom of the glass. Instead, it’s rising into the balloon. This water vapor can’t really be weighed, as it’s in gas form. However, if you compare the molecular weight of water to the molecular weight of the common gasses in air, it weighs slightly less. That balloon is pulling the glass up very slightly. Not nearly as much as a helium balloon would, but very slightly. So if we evaporated enough water, we could theoretically see a lower weight on the scale than from when we started. However, we aren’t applying enough heat to evaporate that much water here, so it would be difficult to measure that effect.

Follow-Through:

You can use this chart to visualize the phase changes. There are two more main phase changes we didn’t discuss here, sublimation and deposition. Sublimation is where a solid enters the gas phase without melting a first, such as we see in dry ice.

Deposition is the opposite, where air humidity freezes, such as we see in frost. There are other phases of matter too, such as plasma, Bose-Einstein Condensates, and many others. But we don’t really discuss those until higher level chemistry and physics courses.

Reinforcing science concepts in every day life is very important, and luckily, there are many opportunities to talk about phase changes with your kids.

When you see a water bottle with condensation on the inside, it’s because the warmth in the room is enough to make some water molecules escape into the air above the water line. They are trapped in the bottle, though, and the air can only fit so many. So they eventually bump into the sides of the bottle, lose energy, and condense into a liquid. If you heat the bottle slightly, the balance will shift and the condensation will re-enter the gas phase. If you cool it slightly, you’ll see the condensation re-form.

However, if we took the cap off and left the bottle for a very long time, it would all eventually evaporate. Even when we aren’t boiling a liquid, particles evaporate from the surface all the time. It’s a good thing they do, otherwise nothing would ever dry. Your wet hair would never dry after a shower, and puddles on the sidewalk would be there forever.

When water condenses on the outside of a cold glass, where does it come from?

The air we breathe has humidity in it. When the water molecules in the air hit the side of the cold glass, they slow down enough to become water again. This is what causes the water droplets on the outside of the glass. Not the water from inside the glass escaping through it.

When you fit a lot of people into a car, the windows sometimes fog up-especially when it’s cold outside! This is for the same reason as the water glass condensation: we exhale a bit of water when we breathe. This adds to the air humidity, and all of that can condense on a cold surface like a car window.

Another important thing to reinforce is that not all things have the same melting and boiling points.

Melting/Freezing Point (solid to liquid) Boiling Point (liquid to gas)
Water 32 F 212 F
Oxygen -361F -297 F
Gold 1948F 4892 F

 

As we live most frequently in the temperature range of 32-100 F, we can see why water is a liquid, gold is a solid, and oxygen is a gas in everyday temperatures. But on cold days, we might pass the freezing point of water, and end up with solid water instead of a liquid.