Main->Readings->4th Grade Readings->Matter- >Part 5: Changes in Matter

Part 5: Changes in Matter

You know that in a solid, the molecules wiggle in place. In a liquid, they move more and slide over each other. In a gas, the molecules bounce around and move very fast. The difference between the phases is a difference in how the molecules move.

Therefore, if you want to make matter change phase, you have to make its molecules move differently.

How do you make a molecule move faster? It is simple. Remember that heat is the energy of moving molecules. To make a molecule move faster you have to heat it.

How do you make a molecule move more slowly? To make a molecule move more slowly, you have to take heat away.

Changing Phase

Water (H₂O) is a very common kind of matter on Earth. The water molecule is made of two atoms of hydrogen and one atom of oxygen. Water is solid, a liquid, and a gas at some very ordinary temperatures. Water is a good way to talk about the phases of matter and how they change.

The solid phase of water is called ice. In ice, the molecules are wiggling in place. They keep their position as long as there is not too much heat. On a cold winter day or in a freezer, an ice cube keeps its shape. It is stiff and hard.

Suppose you heat the ice cube. You take it out of the freezer and put it in a pan in your kitchen. The air in your kitchen is moving very fast. It bounces against the ice cube and bumps into its molecules. The molecules start wiggling faster. Some of them start sliding. Because of gravity, they slide downwards to the bottom of the pan. Your ice cube is turning into a liquid called water. Eventually it will be a puddle on the bottom of the pan.

The ice has changed phase. Now it is water. It is still the same kind of matter. Its molecules are still H₂O. It is only the phase which is different.

You can make the water change phase too. If you put the pan on the stove, you can make the molecules move faster. If they move fast enough, they will bounce right out of the liquid and into the air, spreading out in every direction. This is the gas phase of water, which is called steam. The water has changed phase. It is still the same kind of matter. Its molecules are still H₂O. It is only the phase which is different.

You cannot see steam. When you see little wispy clouds above a boiling pan, you are seeing condensation. Water molecules in the steam hit the cool air, lost some of their heat, and turned back into water again. Condensation is the change of phase from gas to liquid.

Likewise, you can change water to ice. Put the water in the freezer. It will lose some heat, its molecules will slow down, and it will freeze.

Change Points

The temperature at which water must change phase from solid to liquid (ice to water) is 0° Celsius. That temperature is the melting point of water. The temperature at which it must change phase from liquid to gas (water to steam) is 100° Celsius. That is the boiling point of water. A melting point is the temperature at which a solid will change to a liquid. A boiling point is the temperature at which a liquid will change to a gas.

Suppose you heat a piece of solid metal, like an electric stove burner. To begin with, though its molecules vibrate faster, they still hold their position. You heat the burner more, and the molecules vibrate even more, but they still hold their position. In fact, you can make its molecules move so fast they start giving off light, but the burner is still a solid.

The melting point of the stove burner is much hotter than the melting point of ice, isn't it? Remember that every kind of matter has its own properties, such as color, hardness, shininess, and density. Every kind of matter also changes phase at its own special temperatures.

Below is a chart showing the melting points and boiling points of some common kinds of matter. A minus number is a number below zero. The larger a minus number is on the temperature scale, the colder it is.

You can see that nitrogen goes from a solid to a liquid and a liquid to a gas at very low temperatures. Iron, on the other hand, stays a solid until it gets very hot, and does not boil until the heat is almost incredible! The molecules of nitrogen move apart from each other very easily. The molecules of iron do not.

In this unit, you have found out that everything around you is made of matter. People call reality "solid matter," but now you know matter does not have to be solid. It can be a liquid, a gas, or even a plasma. Yet it is still matter.

Homework

Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:

1. If you want to make a piece of matter change phase from solid to liquid, what must you do to its molecules?
2. What is the difference between a melting point and a boiling point?
3. Look at the chart above. What phase of matter is mercury most likely to be when the temperature is normal? (Hint: look at the freezing point of water.)
4. A chemist trying to identify a piece of matter might heat it. Why do you think the chemist would do this?

Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.

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This page last modified August 15, 2002

Copyright ©2000 Delia Marshall Turner. All rights reserved.

Questions? Send me a note at dturner@haverford.org