The method and the results are on the pure substances lab. Do the analysis yourself first, then check the answers below.
Quick answer: A pure substance contains only one kind of particle, while a mixture contains two or more substances that are not chemically joined. These answers go with the grade 7 science lab on pure substances: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
These model answers use the sample class results from the table. Your measurements will differ a little, but the order of the samples should be the same. Check your calculations and conclusions against the steps below.
Distilled 0.000 g, mineral 0.007 g, tap 0.012 g, salt solution 0.700 g. Distilled, mineral, tap, salt solution.
Question 2 Residue per litre
20 mL = 0.020 L. Tap: 0.012 ÷ 0.020 = 0.60 g/L. Mineral: 0.007 ÷ 0.020 = 0.35 g/L. 0.60 g/L and 0.35 g/L.
Question 3 Check the salt solution
35 g/L × 0.020 L = 0.70 g, which equals the residue 0.700 g. The prediction of 0.70 g agrees with the measurement.
Question 4 Compare with the label
0.35 g/L = 350 mg/L. The label value is 340 mg/L, a difference of 10 mg/L, about 3 %. The measurement agrees with the label within 3 %.
Question 5 Which is purest?
Distilled water left no residue, so it contains almost no dissolved solids. Distilled water.
Question 6 Improve the method
Repeat each sample three times and take the mean; use a larger volume such as 100 mL so the residue is easier to measure; dry the glasses fully in the same way before the final weighing. Repeat and use a larger volume.
Question 7 Explain with particles
Water particles gain enough energy to leave as vapour, but the dissolved particles of salt are held together in the solid and do not evaporate at this temperature. Only the water particles escape.
