What's Happening?
But have you ever watched someone crack an egg into a hot pan? The egg changes from gooey and clear to white and firm. Can you put that cooked egg back the way it was? No way!
Big Ideas
When we heat or cool things, they change. But here is the important part: some changes can be undone, and some changes cannot!
Reversible Changes
Not Reversible Changes
Heating and Cooling Cause Changes
Let's Try It!
You can try this at home or in class!
Ice Cube Test
What you need: An ice cube, a plate, a small cup, a freezer.
What you will see: The ice melts into water (heating). The water freezes back into ice (cooling). This change is reversible!
What We Found Out
When we did the ice cube test, we saw that ice can melt into water and then freeze back into ice. The water did not change into something new. It just changed its form (how it looks and feels).
But when cookie dough is baked, something different happens. The heat changes the dough in a big way. It becomes something new — a cookie! Cooling it down does not turn it back into dough.
Here is the big idea: If heating or cooling just changes the form of something (like solid to liquid), the change is usually reversible. If heating changes something into a whole new thing, the change is usually not reversible.
| Change | Reversible? | Why? |
|---|---|---|
| Ice melting 🧊→💧 | Yes ✓ | Just changes form |
| Water freezing 💧→🧊 | Yes ✓ | Just changes form |
| Chocolate melting 🍫→💧 | Yes ✓ | Just changes form |
| Cooking an egg 🥚→🍳 | No ✗ | Becomes something new |
| Baking bread 🫗→🍞 | No ✗ | Becomes something new |
Do You See the Pattern?
Scientists love to look for patterns. A pattern is when the same thing happens again and again. Let's look for a pattern with heating and cooling changes!
| Example | What Happens |
|---|---|
| Butter melts, then gets hard in the fridge | Reversible — same stuff, different form |
| Crayon melts, then gets hard when it cools | Reversible — same stuff, different form |
| Popcorn pops in the microwave | Not reversible — becomes something new |
| Cake batter bakes into a cake | Not reversible — becomes something new |
The pattern: When heating or cooling just makes something go from solid to liquid (or liquid to solid), you can usually change it back. When heating changes something into a brand new thing, you usually cannot change it back.
Science in the Real World
People use what they know about heating and cooling every day!
🍦 Making Ice Cream
🏗️ Building with Metal
Sometimes we want a reversible change (like melting and re-freezing ice). Sometimes we want a change that cannot be undone (like baking a strong brick). Knowing which changes are reversible helps people make good choices!
Words to Know
- Heating — Making something hotter. This can change how it looks or feels.
- Cooling — Making something colder. This can change how it looks or feels.
- Reversible change — A change that can be undone. You can change the thing back to how it was before.
- Not reversible change — A change that cannot be undone. The thing stays different forever.
- Observe — To look closely and carefully at something to learn about it.