Rocket warms his hands over a mug of hot water a grown-up poured. "This is exothermic," he says. "The water is giving me heat."
"Careful," Raven says. "The water is cooling, not reacting. Heat is moving, but is anything changing into something new?"
Rocket frowns. "No. It is just warm water losing heat to the cold air."
Nova hovers near the window, where frost edges the glass. "Heat moving and matter changing are two different ideas," she says. "Find where they meet."
"A campfire," Rocket says. "Wood changes into new stuff, and that change releases heat. That is where they meet."
Raven smiles and opens the Bench Log. "Now list five more."
A campfire is everyday thermochemistry. A chemical reaction converts wood into carbon dioxide and water, among other things, and releases heat as it goes.
That heat can cook food or warm a cold camper. The energy was stored in the wood and is released by the change.
The gas stove in some homes burns methane. That reaction is also exothermic and raises the temperature of its surroundings.
Only grown-ups work with fire or a stove. At the Bench, the only reactions you run are the fizzing kind.
Heat is measured in joules or calories. One calorie is the heat needed to raise 1 gram of water by 1 degree Celsius.
One calorie equals 4.184 joules. Raising 100 grams of water by 2 degrees takes 200 calories.
Food labels use a bigger unit, the Calorie with a capital C. One food Calorie is 1,000 small calories, or 1 kilocalorie.
When a label says a snack has 85 Calories, it means your body releases about 85,000 small calories of energy from it.
| Unit | How much | Where you see it |
|---|---|---|
| Joule | The standard unit of energy | Science texts |
| Calorie (small c) | Warms 1 gram of water by 1 degree Celsius | Chemistry labs |
| Calorie (capital C) | 1,000 small calories | Food labels |
| Week review | True or false? |
|---|---|
| Energy is neither created nor destroyed in a chemical or physical change. | ? |
| An exothermic change makes its surroundings cooler. | ? |
| Instant cold packs work because a solid dissolves and absorbs heat. | ? |
| A disposable hand warmer heats up because iron reacts with oxygen. | ? |
| Two nested foam cups with a lid help keep heat from leaking during a measurement. | ? |
Great week, chemist. Tomorrow is Bench Day: a family hunt for warm and cool changes around your home.