Ice, water, and steam are the same substance. One thing decides which one you get, and you can control it with a stove or a freezer.
You leave an ice cube on the counter. An hour later there is a puddle. Three hours later the counter is dry.
Most students say the water "disappeared" or "dried up." But last week you learned that matter is made of atoms, and atoms do not just stop existing.
Where are those water molecules right now? Point to where they went. Be specific.
They are in the air of this room, mixed in with the oxygen and nitrogen you are breathing. The molecules picked up enough energy to break away from the puddle and fly off one at a time as a gas. Nothing was destroyed. Some of those molecules are inside your lungs right now.
The temperature around you changes constantly — day to night, summer to winter, and even between two objects in the same room. Walk into a sunbeam and you feel heat on your skin. Open the refrigerator and you feel cold pour out.
In hot matter, the particles are vibrating and flying around quickly. They carry a lot of energy of motion. That is all "hot" means at the particle level.
In cold matter, the particles have lost energy and slowed down. "Cold" is not a thing that gets added — it is energy being removed.
Your weather app uses Fahrenheit. Science labs worldwide use Celsius. Both are correct — they are just different rulers, like inches and centimetres.
| What is happening | °C | °F |
|---|---|---|
| Water boils | 100 | 212 |
| Hot summer day in Virginia | 35 | 95 |
| Comfortable classroom | 21 | 70 |
| Refrigerator inside | 4 | 39 |
| Water freezes | 0 | 32 |
| Home freezer | -18 | 0 |
Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F). Celsius was designed around water, which is why those numbers are so tidy.
Think about water. When it is really cold it is hard and frozen into a solid. At room temperature it is a liquid. Heat it until it boils and it becomes a steamy gas called water vapor. Whether matter is a solid, liquid, or gas is called its phase, and temperature is what sets it.
Particles have so little energy that attractions lock them into a rigid pattern. Ice.
Enough energy to break out of the pattern and slide, but not enough to escape entirely. Liquid water.
Particles move fast enough to fly free of each other completely. Water vapor.
Every one of these has a proper name. Step through them and say each name out loud.
We start with ice at −20 °C and add heat at a steady rate. The graph draws itself. Watch for the two flat parts. Something surprising happens there.
During melting and boiling, you keep adding heat but the temperature stops rising. All that energy goes into breaking the particles apart from each other instead of speeding them up. The thermometer only starts climbing again once every particle has changed phase.
Put a thermometer in a glass of ice water and stir. It reads about 0 °C. Leave it in the warm room for ten minutes and stir again — still about 0 °C, as long as ice remains. The temperature will not budge until the last ice cube is gone.
D melts first (warm water touches the whole surface and transfers heat fast), then B (light energy warms it), then A, and C is last by a wide margin.
The towel surprise: most students predict the towel keeps the ice warm and melts it fastest. The towel is an insulator — it slows heat from the room getting in. That is the same reason a towel keeps your coffee hot: insulation works both directions, because it slows heat flow either way.
The outside is wet, and the glass never leaked. That water came out of the air. There is always invisible water vapor in the room. When those fast-moving gas molecules touch the cold glass, they lose energy, slow down, and clump back into liquid drops. That is condensation.
This is the exact process that makes dew on morning grass and clouds in the sky — and it is the reverse of the vanishing puddle you explained on slide 2.
For each everyday event, decide which phase change is happening. Tap your answer.
Heat does not only cause phase changes. Even while matter stays a solid, warming it makes the particles vibrate harder and push each other slightly farther apart. The object expands. Cool it down and it contracts.
Almost every substance contracts as it freezes. Water expands. Its molecules lock into an open six-sided pattern that takes up more room than the liquid.
That is why ice floats, why a forgotten soda can bursts in the freezer, and why a lake freezes on top instead of from the bottom — which is what lets fish survive the winter.
Concrete absorbs sunlight energy and holds it. Grass loses energy through the water it releases from its leaves, which cools it — the same cooling you feel when you step out of a pool.
You sweep away the warm, moist air sitting on the surface, so more fast molecules can escape. Every molecule that leaves carries energy away with it.
Sweat evaporating off your skin takes energy with it. Your body uses the same physics as the vanishing puddle to keep from overheating.
They are the same temperature. Metal pulls heat out of your legs far faster, and "cold" is really the sensation of losing heat quickly.
Cover the definition, say it in your own words, then flip to compare.
Gravity. So far everything has been about what matter is. Next week we look at a force that acts on all of it: the pull that brings you back down every time you jump, and that keeps every planet circling the Sun.
Question for tonight: if you dropped a bowling ball and a tennis ball from the roof at the same moment, which one hits the ground first?