Reading Selection, Lesson 13

Solubility and Saturated Solutions

The sugar solution in the hummingbird feeder is colorless and clear. Some solutions are colored but all are clear.

At room temperature, a solvent (such as water) can dissolve only a certain amount of solute. For example, in Inquiry 13.1, after adding a few lab scoops of sodium chloride to the water, you could see a white solid (undissolved sodium chloride) at the bottom of the tube. The white solid indicated that the water could not dissolve any more sodium chloride. When this happens, the solution is called saturated. The mass of solute dissolved in a given volume or mass of a solvent is its solubility. Solubility is usually measured in grams of solute per unit volume of solvent (for example, grams per liter) or in grams per 100 g of solvent.

The solubility of a solute changes with changing temperature. For example, sodium nitrate becomes more soluble as the temperature rises. It is about twice as soluble at 80 ° C as it is at 1 ° C. There are some substances that become less soluble as the temperature rises. When you heated water in Lesson 7, you may have noticed that bubbles appeared, even though the water was well below the boiling point. These were bubbles of gases, such as oxygen and nitrogen, that were dissolved in the water. The gases became less soluble as the water was heated, and they were released from solution.

QUESTION

Why is it that when you put sugar in iced tea, the sugar tends to sink to the bottom as crystals, but when added to hot tea, it tends to dissolve readily?