States of Matter and Chemical Versus Physical Changes to Matter

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  • 0:06 Changes
  • 0:34 Phase Changes
  • 3:31 Physical Changes
  • 4:07 Chemical Changes
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

The world around us is constantly changing. Chemists put those changes into two main categories: physical changes and chemical changes. This lesson will define and provide examples of each.


Recall from a previous lesson that everything around you is matter and that matter can have physical properties and chemical properties. Just like matter can have physical and chemical properties, it can also undergo physical and chemical changes. A lot of times in chemistry we show a change by using an arrow. The stuff we started out with goes on the left side of the arrow, and the stuff we created is shown on the right side of the arrow.

Phase Changes

When you boil water, what do the bubbles contain? Hot air? Oxygen? Water vapor? Don't feel bad if you don't know the answer; this is a question that often gets answered incorrectly. To answer it, we would first need to determine what is happening to the particles in the water. When you add energy to the particles of a substance by heating it on a stovetop, it causes the temperature to increase, which means that the particles are moving faster. If they get moving fast enough, the temperature will eventually stop increasing, and the bubbles will start forming. You have just reached the boiling point. If the temperature stops increasing, then where is all that extra energy going? It is being used to turn the liquid water into steam - or water in the gas phase. What you're seeing inside of those bubbles is really just water vapor. This transition from a liquid to a gas is called a phase change.

Boiling water changing into steam is an example of a phase change.
Water Vapor in Boiling Bubbles

A phase change is the transition from one state of matter to another. You may already know the three common states of matter: solids, liquids and gases. Solids have a definite shape and volume, meaning that, for the most part, they keep their shapes, and they can't be compressed. If you were to add energy to a substance in its solid state, it would turn into a liquid. This is called melting, and the reverse process is called freezing. Liquids have a definite volume but not a definite shape, which means that they will take the shape of their container but, like solids, they're not compressible. Hydraulics and car brakes work because liquids are incompressible, but they are still able to flow. Now, if we were to heat this liquid enough, it would turn into a gas. This is called vaporization, and the reverse process is called condensation. You may notice on a warm day water droplets form on the outside of a cold drink. This is what happens when water vapor in the air cools down after coming in contact with your cold beverage. Gases do not have a definite shape or a definite volume. They will take the shape of their container, and they can be compressed.

So, back to the bubbles in the boiling water: we now know that they are filled with water vapor, but why do they rise to the surface? Gases are a lot less dense than liquids, so they rise to the surface of a liquid. All of these transitions I just discussed are phase changes, meaning that the particles still have the same composition (ice, water and steam are all H2 O), but they are in a different state.

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