# Solubility - Activity

## Introduction

Water is an excellent solvent and many substances dissolve in it. A substance that dissolves in a solvent is called a solute. The resulting mixture is called a solution. When so much of a solute has dissolved in a certain volume of a solvent that no more can dissolve, we say that a saturated solution has formed. Different solutes have different solubility. One solute might be slightly soluble - that is, not much of it will dissolve in a given volume of water. Another might be very soluble - a lot of it will dissolve in the same volume of water.

In this activity, we compare the solubility in water of four compounds and draw conclusions based on the comparisons.

Model 1

1. Open Yenka file Model 1. The display shows four jars of chemicals, each containing 10 g of chemical. Beside each jar stands a beaker. At the left of the display there is a bottle containing 25 cm3 of water. The graph above the chemicals will show you the mass of compound dissolved as time passes. The beakers have labels on them to show you the colour of the corresponding trace on the graph. Now, add 25 cm3 of water to each beaker and the contents of the jar on its left. Allow the model to run for 90 seconds by clicking the pause button and then pause it again. Explain which compound dissolves fastest at the very beginning.
The potassium nitrate dissolves fastest at the beginning. We can see this because the red plot is the steepest at the start.
2. Explain which compound dissolves fastest overall.
The ammonium nitrate (purple trace) dissolves fastest overall. This is because it is the first (and only) compound to dissolve completely - the mass falls to zero.
3. (a) How long does it take for equal volumes of potassium nitrate and ammonium nitrate to dissolve?
(b) At this point, how much of each has dissolved?
(a) About 20 seconds. We can tell this, because the lines cross at this point.
(b) About 5 g of each compound has dissolved by this time.
4. (a) How long does it take for equal volumes of potassium nitrate and sodium chloride to dissolve?
(b) At this point, how much of each has dissolved?
(b) Just over 5 g.
5. Write down the order of solubility of the compounds, beginning with the least soluble and ending with the most soluble.
Sodium hydrogencarbonate (NaHCO3), potassium nitrate (KNO3), sodium chloride (NaCl), ammonium nitrate (NH4NO3).
6. Explain whether the initial rate of dissolving lets us predict how much will eventually dissolve.
It does not let us predict the final solubility. At the start, potassium nitrate dissolves faster than all the others, but eventually two of the other compounds are more soluble.
7. How can we tell that the solubility of ammonium nitrate is greater than 10 g in 25 cm3?