Dissolving at Home: A Science Activity for Grade 5 (5-PS1-2)

The question

Where does the salt go when it disappears in water? A kitchen scale gives a surprising answer.

A 10-minute version

You need a kitchen scale that reads in grams, a clear jar, salt, sand, water and a stirring spoon.

1. Pour about 100 mL (a little less than half a cup) of water into the jar.
2. Put the jar, the spoon and a small cup holding 10 g of salt on the scale together. Write down the reading.
3. Pour in the salt and stir until the water is clear. Put the jar, the spoon and the empty cup back on the scale. Read it again.
4. Repeat with 10 g of sand in a fresh jar.

What to notice

The salt water looks clear, but the scale reads the same as before, within about a gram. The sand sinks and stays put, and the scale reads the same too.

The science idea

Salt dissolves: the grains break into particles too small to see, and these spread evenly between the water particles. Dissolved salt is still there, so the total mass does not change. Dissolving is not melting. Salt would have to be hotter than about 800 degrees Celsius to melt. There is a limit too: at room temperature roughly 36 g of salt will dissolve in 100 mL of water.

A common mix-up

Clear water does not mean nothing is dissolved in it, and cloudy water does not mean something has dissolved. Flour makes water cloudy because its grains float, not because it dissolved.

Safety

Do not taste any of the powders. Wipe spills so nobody slips.

Go further

Test sugar and flour too. The full lesson, aligned in part with NGSS 5-PS1-2, is here: Where does the salt go when it disappears in water?