What Happens When Solids Mix with Water?
Students mix salt and sand with water, observe that salt dissolves while sand remains visible, and record the results.
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Ask What Mixing Might Do
What do you think will happen when a solid is stirred into water? A solid may stay visible, sink, float, or seem to disappear. When a solid spreads through water so evenly that we cannot see its pieces, we say it dissolves. First, look at a cup of water, a spoonful of salt, and a spoonful of sand. Make a prediction before mixing. For example, you might predict, “The salt will dissolve, but the sand will stay visible.” Tell a partner why you think so. You can use what you see, a classroom book, and your teacher’s directions as sources of information. Do not taste any material during the investigation.
Observe Salt and Sand
Before mixing, use your eyes to observe the two solids. Salt is made of small, light-colored grains. Sand is made of tiny rock pieces that may be tan, brown, black, white, or other colors. Both materials are solids because they keep their own material form instead of flowing like water. Compare their observable properties. Are the grains the same color? Do they look like the same size? For example, you may notice that the salt grains look white and similar to one another, while the sand grains show several colors. Share one observation with your group. Use only safe observations: look closely, but do not taste the salt or touch your face.
Mix Each Solid with Water
Set up two clear cups with the same amount of water. Add one spoonful of salt to the first cup. Add one spoonful of sand to the second cup. Use a different spoon for each cup, and stir each mixture the same number of times. Keeping the amounts and stirring alike makes the comparison fair. Watch carefully while you stir. The salt grains spread through the water and dissolve. The water may still look clear even though the salt is still in it. The sand moves through the water while you stir, but it does not dissolve. After stirring stops, much of the sand settles at the bottom. Follow the teacher’s safety directions and never drink the mixtures.
Compare What Remains Visible
Let both cups rest after stirring. Look from the side and from above without moving the cups. In the salt cup, you can no longer see separate salt grains because the salt dissolved in the water. The salt did not stop existing; it is spread throughout the water. In the sand cup, grains remain visible, and many settle on the bottom. This means the sand did not dissolve. For example, if you point to grains at the bottom of one cup, you can identify it as the sand mixture. Compare your observations with a partner. Use evidence from what you can see: “Salt dissolved because its grains are no longer visible. Sand did not dissolve because I can still see it.”
Record and Share Results
Make a class chart to record the results. Create one row for salt and one row for sand. Add each material to a category: “dissolved” or “did not dissolve.” Place one mark in the dissolved category for salt and one mark in the did not dissolve category for sand. Count the marks in each category and compare the totals. In this investigation, one material dissolved and one did not. Then share a complete sentence with the class, such as, “We observed that salt dissolved in water, but sand remained visible.” Listen while classmates speak, take turns, and ask questions about their evidence. Your observation chart, classmates, teacher, and classroom science book can all help answer questions about the investigation.
