Raven brings a cloth bag of ice to the Bench. "Sports teams use instant cold packs," she says. "No freezer needed."
"How?" Rocket asks. "Nothing is cold until you squeeze it. Then it is freezing."
Nova dims her light. "Think about Monday," she says. "If the pack gets cold, which direction did the heat move?"
"Into the system!" Rocket says. "The stuff inside soaked up heat from the water. Endothermic!"
Raven nods. "And a hand warmer does the opposite. What do you notice about both? Neither one is plugged in."
"So the energy change comes from the matter itself," Rocket says, already reaching for the baking soda.
"Measure first," Raven says. "Guess second."
When a solid dissolves in water, energy moves too. Some solids release heat as they dissolve. Others absorb heat.
Many instant cold packs hold a bag of water and a separate pouch of solid ammonium nitrate. Squeezing bursts the inner pouch.
As the ammonium nitrate dissolves, it absorbs heat from the water, so the pack turns cold. Pressed on a sore knee, it then draws heat from the body.
Many hot packs use calcium chloride instead. Calcium chloride releases heat as it dissolves, so the pack warms up.
The amount of heat absorbed or released by dissolving one measured amount of a substance is called its heat of solution.
A common disposable hand warmer holds iron powder, salt, water and a few helper materials in a sealed pouch.
When you rip open the outer wrapper, oxygen in the air reaches the iron. The iron and oxygen react to form iron oxide, which is rust.
This reaction is exothermic. The salt speeds it up, so heat comes faster. Fillers spread the heat evenly through the pouch.
A reusable hand warmer works another way. It holds a solution of sodium acetate and a small metal disc.
Bending the disc makes the sodium acetate crystallize, and crystallizing releases heat. Warming the pack later dissolves the crystals again.
| Device | What happens inside | Heat moves | Kind of change |
|---|---|---|---|
| Instant cold pack | Ammonium nitrate dissolves in water | Into the system | Endothermic |
| Hot pack | Calcium chloride dissolves in water | Out to the surroundings | Exothermic |
| Disposable hand warmer | Iron reacts with oxygen to form iron oxide | Out to the surroundings | Exothermic |
| Reusable hand warmer | Sodium acetate crystallizes | Out to the surroundings | Exothermic |
| Statement | True or false? |
|---|---|
| A hot pack and a cold pack can both work by dissolving a solid in water. | ? |
| Rust forming is an endothermic change. | ? |
| A reusable hand warmer releases heat when sodium acetate crystallizes. | ? |
| The water in a cold pack gets warmer as the solid dissolves. | ? |
Strong thinking. Tomorrow is Bench Lab: you will build a two-cup measuring tool and test a fizzing reaction with a thermometer.