Solubility curves
5 marks · HardThe table below shows the solubility of sodium sulfate at different temperatures.
| Temperature /°C | Solubility / g per 100 g of water |
|---|---|
| 10 | 12.5 |
| 30 | 19.6 |
| 50 | 29.4 |
| 70 | 40.8 |
| 90 | 53.1 |
On graph paper, plot a solubility curve for sodium sulfate using the data in the table. Label the axes appropriately.
Estimate the solubility of sodium sulfate at 60°C using your curve.
Describe the trend shown by the solubility curve as temperature increases.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) — · must show Points plotted at correct coordinates (all five points from data), Axes labelled: x – Temperature (°C), y – Solubility (g per 100 g of water), Line or smooth curve joining points
(b) Approximately 34 to 35 g per 100 g of water g per 100 g · must show Reading interpolated value correctly from curve
(c) Solubility increases as temperature rises, with the rate of increase greatest at higher temperatures. · must show States solubility increases, Mentions increase is not linear or becomes more rapid
Must show: Accurate plot of points, Smoothly joined curve/line, Correctly labelled axes
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · All points plotted accurately (within half a small square on graph paper)
B2 (a) · 1 mark · Axes labelled with quantities and units; smooth curve or line drawn accurately through points
B3 (b) · 1 mark · Correctly reads/interpolates solubility at 60°C (allow 34–35 g per 100 g, accept any value between 33–36 g) (follow-through)
B4 (c) · 1 mark · States solubility increases as temperature rises
B5 (c) · 1 mark · Mentions the increase is not uniform/linear or that the rate increases at higher temperature
Worked solution
a. Plot points: (10,12.5), (30,19.6), (50,29.4), (70,40.8), (90,53.1). Label x-axis as 'Temperature (°C)', y-axis as 'Solubility (g per 100 g of water)'. Join points with a smooth curve.
b. At 60°C, reading from the curve between 50°C (29.4) and 70°C (40.8), estimate solubility to be about 34–35 g per 100 g.
c. The solubility of sodium sulfate increases as temperature increases. The increase becomes more marked at higher temperatures.