Have you ever wondered why some substances dissolve in water while others don’t? The answer: solubility.
Solubility is the ability of a solid, liquid, or gaseous chemical substance (or solute) to dissolve in a solvent (usually a liquid) and form a homogenous solution. There are three factors that affect solubility:
- Solvent: To determine whether a solute will dissolve in a solvent, remember this saying: “Like dissolves like.”
- Temperature: This factor affects the solubility of both solids and gases, with solubility increasing with the temperature.
- Pressure: This factor affects the solubility of gases, with solubility also increasing with pressure.
The Science Behind Solubility
Put simply, a substance is considered to be soluble if it can be dissolved, most commonly in water. When a solute, such as table salt, is added to a solvent, such as water, the salt’s molecular bonds are broken before combining with the water, causing the salt to dissolve.
However, for the salt to remain soluble and dissolve completely, there must be a larger concentration of water than salt in the solution. A solution becomes saturated when the solvent can dissolve no more solute. But adding heat or pressure can help to increase the solubility of the solute, depending on its state.
Check out Chemistry Rocks! 3 Simple Science Experiments To Teach Students Chemistry for more activity ideas!
Testing the Solubility of Substances
For this experiment, your students will explore basic chemistry concepts by testing the solubility of different substances in water. From the example above, we know that table salt is highly soluble in water. What other substances can dissolve in water?
- Clear containers, such as cups, beakers, or bowls
- Materials to test, such as sugar, sand, chalk, baking soda and Epsom salts
- Stirring rods
- Measuring spoon
- STEM journals (optional)
- Begin by discussing the science of solubility, and have students write down their predictions about which materials are soluble or insoluble. Students can also document the scientific process in their STEM journals.
- Fill each container with lukewarm tap water.
- Add a specific amount—for example, 1 tablespoon—of a test material to a container using the measuring spoon. Repeat, adding an equal amount of a different material to each container of water.
- Use the stirring rods to mix the contents in each container.
- Observe which materials dissolved in the water and which did not. Did students make the right predictions?
Once the experiment is complete, use the following questions to deepen students’ understanding of solubility and how it works:
- What are the qualities of the soluble materials versus those of the insoluble materials? For example, the soluble materials are likely powdery and dry, while insoluble materials may have a hard, grainy texture.
- For the materials that dissolved in the water, what do you think will happen if you keep adding more to the water?
- What are other examples of soluble substances?
- What are other examples of insoluble substances?