This neat row of cola bottles represents matter in three different states—solid, liquid, and gas. The bottles and caps are solids, the cola is a liquid, and carbon dioxide dissolved in the cola is a gas. It gives cola its fizz. Solids, liquids, and gases such as these have different properties. Solids have a fixed shape and a fixed volume. Liquids also have a fixed volume but can change their shape. Gases have neither a fixed shape nor a fixed volume. What explains these differences in states of matter? The answer has to do with energy.
Energy is the ability to cause changes in matter. For example, your body uses chemical energy when you lift your arm or take a step. In both cases, energy is used to move matter—you. Any matter that is moving has energy just because it’s moving. The energy of moving matter is called kinetic energy. Scientists think that the particles of all matter are in constant motion. In other words, the particles of matter have kinetic energy. The theory that all matter consists of constantly moving particles is called the kinetic theory of matter.
Kinetic Energy and States of Matter
Differences in kinetic energy explain why matter exists in different states. Particles of matter are attracted to each other, so they tend to pull together. The particles can move apart only if they have enough kinetic energy to overcome this force of attraction. It’s like a tug of war between opposing sides, with the force of attraction between particles on one side and the kinetic energy of individual particles on the other side. The outcome of the “war” determines the state of matter.
- If particles do not have enough kinetic energy to overcome the force of attraction between them, matter exists as a solid. The particles are packed closely together and held rigidly in place. All they can do is vibrate. This explains why solids have a fixed volume and a fixed shape.
- If particles have enough kinetic energy to partly overcome the force of attraction between them, matter exists as a liquid. The particles can slide past one another but not pull apart completely. This explains why liquids can change shape but have a fixed volume.
- If particles have enough kinetic energy to completely overcome the force of attraction between them, matter exists as a gas. The particles can pull apart and spread out. This explains why gases have neither a fixed volume nor a fixed shape.
Look at the Figure below. It sums up visually the relationship between kinetic energy and state of matter.
Q: How could you use a bottle of cola to demonstrate these relationships between kinetic energy and state of matter?
A: You could shake a bottle of cola and then open it. Shaking causes carbon dioxide to come out of the cola solution and change to a gas. The gas fizzes out of the bottle and spreads into the surrounding air, showing that its particles have enough kinetic energy to spread apart. Then you could tilt the open bottle and pour out a small amount of the cola on a table, where it will form a puddle. This shows that particles of the liquid have enough kinetic energy to slide over each other but not enough to pull apart completely. If you do nothing to the solid glass of the cola bottle, it will remain the same size and shape. Its particles do not have enough energy to move apart or even to slide over each other.
- According to the kinetic theory, particles of matter are in constant motion. The energy of motion is called kinetic energy.
- The kinetic energy of particles of matter determines the state of matter. Particles of solids have the least kinetic energy and particles of gases have the most.
- State the kinetic theory of matter.
- Explain the relationship between kinetic energy and state of matter.
Watch the video below and then answer the questions that follow.
- Describe the motion of particles in ice, liquid water, and water vapor.
- Apply the kinetic theory of matter to explain the differences in your answer to question 1.