Hot vs Cold Diffusion Lab

1. Drag the Hot beaker slider to set its temperature. You can also tap − or + to change it one degree at a time.
2. Set the Cold beaker the same way. It must stay at least 5 °C colder than the hot beaker, so the slider stops before the temperatures cross.
3. Press Drop red dye to test the hot beaker or Drop blue dye to test the cold beaker. You can test one beaker at a time. Press Drop both to race them at the same moment.
4. Watch the dye spread. The Mixed number shows how evenly the dye has spread. The Time counter stops when a beaker reaches 90% mixed.
5. Look at the round zoom windows. They show the water molecules moving. Compare how fast they move in each beaker.
6. Check the graph and the data table. Each beaker adds a row when it is mixed, so you can compare results.
7. To try a new temperature, move a slider or press Drop again. Reset empties both beakers. Use Speed 2× or 4× if you don't want to wait.

Water is made of tiny particles called molecules. They are always moving, even when the water looks still.

Temperature is a measure of how fast the molecules move on average. Hot water molecules move faster. Cold water molecules move slower.

When you add dye, water molecules crash into the dye molecules. In hot water the crashes are faster and happen more often. This pushes the dye around more, so it spreads out sooner.

This spreading is called diffusion. Diffusion means particles move from a crowded place to a less crowded place until they are spread out evenly.

• Molecule: the smallest piece of a substance, like one tiny bit of water.
• Kinetic energy: the energy something has because it is moving. Faster molecules have more of it.
• Temperature: a measure of the average kinetic energy of the molecules in a substance.
• Diffusion: when particles spread from where there are many to where there are few.
• Evenly mixed: when the dye is spread the same amount everywhere in the beaker.

Each beaker holds 10,000 dye particles. When the drop hits the water, it sinks and curls into mushroom shapes. These first swirls come from the falling drop, so they are the same in both beakers.

After that, two things spread the dye. First, each dye particle takes tiny random steps as water molecules bump into it. Second, the water slowly churns and folds the dye into thin streaks. Both happen faster in warm water, because its molecules move faster and the water is thinner (less viscous).

The speed-up comes from a real science rule that uses the water's temperature and thickness. By this rule, dye spreads about 4 times faster at 80 °C than at 10 °C. The sim uses that number to set how fast each beaker mixes.

The Speed number is the real average speed of water molecules at that temperature. It is found with √(3RT/M). The zoom windows show this motion much slower, and the difference is made bigger so you can see it.

The timer counts sim seconds. Real food coloring takes longer to mix, so the sim is sped up.