States of Matter and Phase Changes

Particles in Motion

In Lessons 1 and 2, you learned that all matter is made of atoms and that different substances have different properties. Now it is time to look at matter from a new angle: how do the tiny particles that make up matter actually behave?

The answer depends on the state of matter you are looking at. Matter exists in three common states: solid, liquid, and gas. The difference between these states comes down to one thing: how the particles are arranged and how much they move.

The Kinetic Molecular Theory

The kinetic molecular theory is the scientific idea that all matter is made of tiny particles (atoms or molecules) that are in constant motion. These particles never stop moving entirely. Even in the hardest rock or the coldest ice, the particles are vibrating. The amount of motion varies depending on how much energy the particles have.

Temperature is a direct measure of this motion. Specifically, temperature measures the average kinetic energy (energy of motion) of the particles in a substance. The faster the particles move, the higher the temperature. The slower they move, the lower the temperature.

Solids: Locked in Place

In a solid, particles are tightly packed in a fixed, organized arrangement. Think of a neatly organized grid. The particles do not wander from their positions. Instead, they vibrate in place, jiggling back and forth in tiny movements. Because the particles are locked together, a solid has a definite shape and a definite volume. A wooden block stays the same shape whether you put it on a table or in a box.

Imagine a classroom where every student is sitting in an assigned seat. The students can fidget and shift in their chairs (vibrate), but they stay in their spots. That is a solid.

Liquids: Sliding Around

In a liquid, particles are still close together, but they are not locked into fixed positions. They can slide past each other and move around freely within the substance. This gives a liquid a definite volume (it does not expand to fill extra space), but no definite shape. A liquid takes the shape of whatever container holds it. Pour water from a tall glass into a wide bowl, and the water changes shape to match the bowl.

Think of students walking through a crowded hallway between classes. They are close together and bumping shoulders, but they are moving around, not stuck in one spot. That is a liquid.

Gases: Free and Fast

In a gas, particles are far apart and moving rapidly in all directions. They bounce off each other and off the walls of their container. Because the particles are so spread out and energetic, a gas has no definite shape and no definite volume. A gas expands to fill any container completely. Open a bottle of perfume in one corner of a room, and eventually the gas molecules will spread to fill the entire room.

Imagine students running freely on a large open field. They spread out in all directions, change direction when they bump into each other, and fill all the available space. That is a gas.

Summary Comparison

Phase Changes: Adding and Removing Energy

Now you know how particles behave in each state of matter. The next question is: how does matter change from one state to another? The answer is energy.

A phase change (also called a change of state) occurs when matter transforms from one state to another. Every phase change is caused by adding or removing thermal energy (heat). When you add energy to particles, they speed up and can break free from their neighbors. When you remove energy, particles slow down and can lock together more tightly.

There are six phase changes in total. Three involve adding energy, and three involve removing energy.

Phase Changes That ADD Energy

Melting (solid → liquid): When you add energy to a solid, the particles vibrate faster and faster until they break free of their fixed positions and begin to slide past each other. The solid becomes a liquid. Example: an ice cube melting in a warm room.

Evaporation/Boiling (liquid → gas): When you add more energy to a liquid, the particles move faster and faster until some have enough energy to escape the liquid entirely and become gas. Evaporation happens slowly at the surface of a liquid at any temperature. Boiling happens rapidly throughout the liquid when it reaches its boiling point. Example: a pot of water boiling on the stove.

Sublimation (solid → gas directly): In rare cases, a solid can change directly into a gas without becoming a liquid first. This happens when particles at the surface of the solid gain enough energy to escape directly into the gas phase. Example: dry ice (solid carbon dioxide) produces a white fog as it turns directly into CO2 gas. Freeze-dried food also uses sublimation.

Phase Changes That REMOVE Energy

Freezing (liquid → solid): When you remove energy from a liquid, the particles slow down until they lock into fixed positions, forming a solid. Example: water freezing into ice in a freezer.

Condensation (gas → liquid): When you remove energy from a gas, the particles slow down and come closer together, forming a liquid. Example: water droplets forming on the outside of a cold glass on a humid day.

Deposition (gas → solid directly): The reverse of sublimation. A gas changes directly into a solid without becoming a liquid first. Example: frost forming on a cold window on a winter morning. The water vapor in the air touches the freezing-cold glass and becomes solid ice crystals instantly.

Phase Change Summary

The Temperature Plateau: A Surprising Fact

Here is something that surprises many students: during a phase change, the temperature of the substance does not change, even though energy is being added or removed.

When ice is melting, the temperature stays at exactly 0°C the entire time the ice is turning into water. When water is boiling, the temperature stays at exactly 100°C the entire time the water is turning into steam. Where does the energy go? Instead of making the particles move faster (which would raise the temperature), the energy goes into breaking the bonds between particles, allowing them to change from one arrangement to another.

This creates a distinctive pattern on a heating curve, which is a graph of temperature versus energy added.

Phase Changes in Everyday Life

Phase changes are not just a topic for science class. They happen all around you, every single day.

At Home

Every time you take an ice cube out of the freezer and put it in a drink, you watch melting in action. When you boil water on the stove for pasta, you are adding enough thermal energy to cause evaporation (boiling). If you leave a cold glass of lemonade on the table on a humid summer day, you will see water droplets form on the outside of the glass. That is condensation: water vapor in the warm air hits the cold glass, loses energy, and changes from gas to liquid.

Outside

In the morning, you may notice dew on the grass. That is condensation: overnight, the air temperature dropped enough that water vapor in the air turned into liquid droplets on cool surfaces. On a winter morning, you might see frost on car windows. Frost is an example of deposition: water vapor changed directly from gas to solid ice crystals without becoming liquid first.

Puddles on the sidewalk after a rainstorm slowly disappear. Where does the water go? It evaporates: energy from the sun gives liquid water molecules enough energy to escape into the air as gas.

The Water Cycle

The entire water cycle is driven by phase changes. Water evaporates from oceans, lakes, and rivers. The water vapor rises, cools, and condenses into tiny droplets that form clouds. When the droplets combine and grow heavy enough, they fall as precipitation (rain, snow, sleet, or hail). Snow and ice on the ground eventually melt and flow back to bodies of water, and the cycle starts again.

Why Sweating Cools You Down

Here is a real-world application: when your body gets hot, you sweat. The sweat (liquid water) sits on your skin. As it evaporates, it absorbs thermal energy from your skin. This is why you feel cooler when sweat evaporates. Evaporation is an energy-absorbing phase change, and it draws heat away from your body.