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.
- 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.
AnswerThe potassium nitrate dissolves fastest at the beginning. We can see this because the red plot is the steepest at the start.
- Explain which compound dissolves fastest overall.
AnswerThe 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.
- (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?Answer(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.
- (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?Answer(a) About 34 seconds.
(b) Just over 5 g.
- Write down the order of solubility of the compounds, beginning with the least soluble and ending with the most soluble.
AnswerSodium hydrogencarbonate (NaHCO3), potassium nitrate (KNO3), sodium chloride (NaCl), ammonium nitrate (NH4NO3).
- Explain whether the initial rate of dissolving lets us predict how much will eventually dissolve.
AnswerIt 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.
- How can we tell that the solubility of ammonium nitrate is greater than 10 g in 25 cm3?
AnswerThe mass remaining falls straight to zero without showing any sign of levelling off.
- Calculate the solubility of sodium hydrogencarbonate in g / litre.
AnswerThe graph shows us that about 2.5 g have dissolved in 25 cm3 of water. This means that 10 g would dissolve in 100 cm3 of water, so that 100 g would dissolve in a litre.
When we ask how soluble a compound is, we must be clear about what we mean. Do we mean 'How quickly does the compound dissolve?' or do we mean 'How much of the compound will dissolve?' This difference is seen clearly from the graphs in the model, especially in the comparison of potassium nitrate and ammonium nitrate. Potassium nitrate dissolves faster than ammonium nitrate, but, in the end, more ammonium nitrate dissolves.
- This activity is intended primarily as a Problem Solving item, although the key vocabulary of solubility is summarised in the introduction. The earlier questions are more suitable for early GCSE students, while advanced GCSE and A-level students should be able to tackle all the questions confidently. If this activity is used for a whole class (e.g. projected on screen) the activity can be built up in complexity by starting perhaps with just one or two compounds and comparing their behaviour.