Have you ever added a spoon of sugar to your tea and wondered why it disappeared? Where did it go? The sugar did not actually disappear—it changed from its solid form into a dissolved form in a process called chemical dissolution. The result is a tea-sugar mixture in which individual sugar molecules become uniformly distributed in the tea. But what happens if you increase the amount of sugar that you add to your tea? Does it still dissolve? In this science activity, you will find out how much of a compound is too much to dissolve.
Time: 20-30 minutes
- Distilled water, found in the bottled water section of grocery stores.
Note: You can also use tap water. However, as tap water contains additional ions that have been removed in distilled water, your solubility values may not match the published solubility values.
- Glasses or cups, 8 oz. (8)
- Spoons (4)
- Measuring spoon (1 teaspoon)
- Epsom salt (150 g)
- Table salt (50 g)
- Table sugar (cane sugar) (250 g)
- Baking soda (20 g)
- Optional: thermometer
- Using the marker, label two cups with each compound: “table salt,” “table sugar,” “baking soda,” and “Epsom salt.”
- Into one “baking soda” cup, measure 20 grams of baking soda.
- Into one “table salt” cup, measure 50 grams of salt.
- Into one “table sugar” cup, measure 250 grams of sugar.
- Into one “Epsom salt” cup, measure 150 grams of Epsom salt.
- Weigh each cup and write down their masses for each one.
- Add 100 mL of distilled water to each of the remaining cups. Use the measuring cup to make sure each cup has the same amount of water. The water should be at room temperature and the same for all cups. You can use a thermometer to verify that.
- Take both of the cups you labeled with “baking soda.” With the measuring spoon, carefully add one teaspoon of baking soda to the 100 mL of distilled water.
- Stir with a clean spoon until all the baking soda has dissolved. Question: What did you notice when you stir the solution with baking soda?
- Keep adding one teaspoon of baking soda to the water and stirring each time, until the baking soda does not dissolve anymore. Question: How does the solution look when the baking soda does not dissolve anymore?
- Repeat steps 1–3 with both cups labeled “Epsom salt.” Question: At what point does the Epsom salt solution become saturated?
- Repeat steps 1–3 with the table salt. Question: How many teaspoons of table salt can you dissolve in 100 milliliters of water?
- Repeat steps 1–3 with the sugar. Question: Could you add more or less sugar compared to the other compounds?
- Put each of the cups containing the remaining solids onto the scale and write down the mass of each one.
- Subtract the measured mass from your initial mass (see Preparation) for each compound. Question: What does the difference in mass tell you about the solubilities of each of the compounds? Which compound is the most or least soluble in distilled water?
Did all of your tested compounds dissolve in distilled water? They should have—but to different extents. Water, in general, is a very good solvent and is able to dissolve lots of different compounds. This is because it can interact with a lot of different molecules. You should have noticed that sugar had the highest solubility of all your tested compounds (sucrose: about 200 grams per 100 mL of water), followed by Epsom salt (Magnesium sulfate heptahydrate: 113 grams/100 mL), table salt (NaCl: 35.17 g/100 mL), and baking soda (NaHCO3: 9.6 g/100 mL).
This is because each of these compounds has different chemical and physical properties based on their different molecular structures. They are all made of different chemical elements and have been formed by different types of bonding between these. Depending on this structure, it is more or less difficult for the water molecules to break these bonds and form new bonds with the solute molecules to dissolve them.
Svenja Lohner, PhD, Science Buddies