Quick answer: A pure substance contains only one kind of particle, while a mixture contains two or more substances that are not chemically joined. This grade 7 science lab on pure substances gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How can you show that some clear liquids contain dissolved solids? In this lab you evaporate 20 mL of four different waters and measure the residue left on a watch glass. You then compare your results with a label and decide which sample is closest to a pure substance.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS1-2
Aim
To compare how much solid is left behind when four different waters are evaporated, and to decide which are closer to pure substances.
Equipment
- 4 clean watch glasses (or shallow dishes)
- Beaker and hot plate or kettle
- Balance reading to 0.001 g
- 20 mL measuring cylinder
- Distilled water, tap water, bottled mineral water, salt solution (35 g of salt per litre)
- Heat-proof gloves
Safety
Hot water and steam can scald. Wear heat-proof gloves and eye protection, keep the beaker away from the table edge and let the glasses cool before you handle or weigh them.
Method
- Weigh each watch glass when it is clean and dry and record its mass.
- Measure 20 mL of one sample into each watch glass.
- Place each watch glass on top of a beaker of gently boiling water, one at a time or in separate beakers, until the liquid has gone.
- Let each glass cool and weigh it again.
- Calculate the mass of residue = final mass – starting mass.
Results
A class in Mombasa obtained these results:
| Sample | Volume (mL) | Residue (g) |
|---|---|---|
| 1 Distilled water | 20 | 0.000 |
| 2 Tap water | 20 | 0.012 |
| 3 Mineral water | 20 | 0.007 |
| 4 Salt solution | 20 | 0.700 |
Analysis questions
Question 1 Rank the samples
Rank the four samples from least residue to most residue.Question 2 Residue per litre
Calculate the mass of residue per litre (g/L) for tap water and for mineral water.Question 3 Check the salt solution
The salt solution was made with 35 g of salt per litre. Calculate the mass of salt in 20 mL and compare it with the measured residue.Question 4 Compare with the label
The mineral water label says 340 mg/L. Convert your result for mineral water to mg/L and compare.Question 5 Which is purest?
State which sample is closest to a pure substance and explain how the data show this.Question 6 Improve the method
Suggest two ways to make the results more reliable.Question 7 Explain with particles
Explain, using the particle model, why the residue stays behind when the water evaporates.
