Boiling is when a liquid gets hot enough to turn into a gas, forming bubbles that rise and burst at the surface. Condensation is the opposite — when a gas cools down and turns back into a liquid. Water boils at 100 degrees Celsius (212 degrees Fahrenheit) and condenses when steam hits a cool surface. These two changes are reversible, just like melting and freezing, but they happen between the liquid and gas states instead.
Boil water in a clear pot so students can watch bubbles form and steam rise. Hold a cold metal plate above the steam so students can see water droplets form on it — that is condensation. Point out condensation on cold drink glasses in a warm room.
You know about melting and freezing — the changes between solid and liquid. Now meet the changes between liquid and gas: boiling and condensation. These work the same way, just at higher temperatures. Add heat to a liquid and it becomes a gas; take heat away from a gas and it becomes a liquid again.
When you heat water on the stove, the temperature rises steadily until it hits 100 degrees Celsius. At that point, something dramatic happens — bubbles start forming inside the water, rising to the surface and bursting. Those bubbles are not air. They are water vapor, which is water in the gas state. The liquid water at the bottom of the pot is gaining so much heat that it turns into gas right there, forming bubbles of vapor that float upward. This is boiling.
Here is a surprising fact: once water starts boiling, the temperature stops going up. You can turn the burner to maximum and the water will still be exactly 100 degrees Celsius. All the extra heat goes into changing liquid water into gas instead of making the temperature rise. This is just like how melting ice stays at zero degrees — changing state uses energy that would otherwise raise the temperature.
Condensation is the reverse trip. When water vapor (gas) touches something cold, it loses heat and turns back into liquid. You see this every day. After a hot shower, the bathroom mirror fogs up — that is warm water vapor hitting the cool mirror and condensing into tiny droplets. A cold glass of lemonade gets "sweaty" on a summer day because warm, humid air condenses on the cold glass. The water on the outside of the glass came from the air, not from inside the glass.
A common confusion is about the white cloud you see above a boiling kettle. People call it "steam," but real steam — water vapor — is actually invisible. That white cloud is tiny liquid water droplets that formed when the invisible vapor hit the cooler air and condensed almost immediately. So you are actually seeing condensation right above the spout, not the gas itself. The truly invisible zone right at the mouth of the kettle is where the real water vapor lives.