Solubility.

Step 1 of 5

Saturated

Add salt to water spoonful by spoonful, and at some point the crystals stop vanishing. The water holds all the salt it can, and the solution is saturated. The most solute that dissolves in a given amount of solvent, at a given temperature, is the solubility. It is usually given in grams per 100 g of water: for table salt, about 36 g at 20 °C. The Scene works out 36.4 g, from a straight line that fits the measured values between 0 °C and 100 °C.

The glass now holds 50 g of salt.

Step 2 of 5

Stirring

In a saturated solution the motion does not stop: every second some ions leave the crystals and others come back and stick on, at the same rate. This is a dynamic equilibrium, which is why the pile does not change. Stir is now off; guess what will happen when you turn it on.

Step 3 of 5

Hot water

For most solids the solubility rises with temperature: in hot water the molecules hit the crystal harder and pull ions off more easily. But by how much depends on the substance. Water temperature is now fixed at 293 K, 20 °C; guess how much will dissolve at 100 °C.

Step 4 of 5

Heat until it all dissolves

The glass holds 38 g: a little more than dissolves at 20 °C. The Scene works out the solubility of table salt as a straight line, $35.7 + 0.035\,(T - 273)$⁠ grams, a good fit to the measured values. Salt added is fixed, and Water temperature is free.

Step 5 of 5

Gases

Gases dissolve in water too: fish breathe dissolved oxygen, and a fizzy drink has carbon dioxide dissolved in it under pressure. For gases the trend is the other way: the warmer, the less dissolves. In warm water the gas molecules move fast enough to escape from the liquid.