Mixtures I: Rocks and Salt


Before the class, I filled a cup of water for each student about 34 full and marked the water level. I put 5 rocks and a small cup of salt on a paper towel in front of each student. I asked them to tell me what they observed about the objects in front of them. My son shouts out “it’s salt!” I tell them to tell me what they observe, not what they think they know. (This is a difficult lesson that we keep going over.) I ask them if they hadn’t seen the salt container on the counter, what other substance it could be. They seem to really get it when they realize it could be sugar also. I start a list on the board with two columns (one for the salt and one for the rocks) about their observations. It’s neat when they start coming up with good adjectives and comparisons (like snow, but not cold).

The students had a worksheet where they wrote down their best guess (hypothesis) of what would happen when they put the rocks in the water and what would happen when they put the salt in the water. Some wrote the things would get wet and quite a few guessed that the rocks would sink and the salt would float. Note: there are no wrong answers to this part!

Before they conducted the experiment, I asked them what was different about this cup of water. They noticed the line and I asked them to really notice whether the water was just above or just below or just touching the line. When they put the rocks in the water, they all noticed that the water level went up. One girl noticed that it went up even more when she put her hand in the water!

Next they added the salt to the water. They observed that the water was cloudy. That you could still see though it. I introduced to word: DISSOLVE. They wanted extra salt so they could see what would happen when they added more. They noticed that when you keep adding salt some of it sinks to the bottom.

Toward the end of class, I overheard a table of four girls saying “I love science!” and “science is fun!” That was the best reward there could possibly be.

- 20 cups
- 100 small rocks (from a pile of gravel in my back yard)
- 20 small cups of salt (1-2 tsp)

worksheet (word doc)

inspired by UTexas First Grade Lab