Quick answer: Solubility is the maximum mass of a solute that dissolves in 100 g of water at a given temperature. This grade 7 science lab on solubility gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How much salt can a given amount of water hold, and does hot water dissolve sugar faster? In this lab you add salt in small portions until a solution is saturated and then time sugar dissolving at four temperatures. You use your results to separate how much dissolves from how fast it dissolves.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS1-2
Aim
To estimate the solubility of common salt in water at room temperature, and to see how water temperature changes the time sugar takes to dissolve.
Equipment
- Beakers (250 mL) and a measuring cylinder
- Balance reading to 0.1 g
- Glass rod or spoon
- Table salt and granulated sugar
- Thermometer
- Kettle, ice, stopwatch
Safety
Hot water scalds, so an adult should pour water above 40 °C. Do not taste anything made in the lab. Wipe up spills at once because wet floors are slippery.
Method
Part A: how much salt dissolves.
- Pour 50 mL of water at room temperature (about 20 °C) into a beaker.
- Weigh 3.0 g of salt, add it, and stir for 2 minutes.
- If it all dissolves, add another 3.0 g portion and stir again. Keep a running total.
- Stop when some salt stays at the bottom after 2 minutes of stirring.
Part B: how fast sugar dissolves.
- Prepare 100 mL of water at 10 °C, 30 °C, 45 °C and 60 °C.
- Add 5.0 g of sugar and stir at a steady speed, starting the timer.
- Stop the timer when no crystals can be seen.
Results
Part A, results from a class in Accra (50 mL of water at 20 °C):
| Total salt added (g) | 3.0 | 6.0 | 9.0 | 12.0 | 15.0 | 18.0 | 21.0 |
|---|---|---|---|---|---|---|---|
| Observation | clear | clear | clear | clear | clear | a few grains left | much salt left |
Part B, time for 5.0 g of sugar to dissolve:
| Water temperature (°C) | 10 | 30 | 45 | 60 |
|---|---|---|---|---|
| Time (s) | 90 | 55 | 34 | 24 |
Analysis questions
Question 1 Saturation range
Use the Part A table to state the range of masses between which 50 mL of water became saturated.Question 2 Convert to 100 mL
Convert the range to grams per 100 mL of water.Question 3 Estimate and compare
Take the middle of the range as your estimate. The accepted value is 36 g per 100 g of water. Calculate the percentage difference.Question 4 Pattern in Part B
Describe the relationship between water temperature and dissolving time.Question 5 Dissolving rate
Calculate the average dissolving rate (g/s) at 10 °C and 60 °C, and how many times faster the second is.Question 6 Explain with particles
Explain the Part B results using the particle model.Question 7 Speed or amount
Salt solubility hardly changes with temperature, yet salt dissolves faster in hot water. Explain.Question 8 Control variables
Name three variables kept constant in Part B and one possible source of error.
