The method and the results are on the conservation of mass lab. Do the analysis yourself first, then check the answers below.
Quick answer: The law of conservation of mass says that the total mass of all substances in a closed system stays the same during any change. These answers go with the grade 7 science lab on conservation of mass: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers based on the sample data. The three parts show a clear pattern: mass is unchanged in closed jars and falls in the open cup because a gas leaves. Your own readings will differ slightly, but they should show the same pattern.
A: (0.0 + (−0.1)) ÷ 2 = −0.05 g (about zero). B: (0.0 + 0.0) ÷ 2 = 0.0 g. C: ((−1.1) + (−1.2)) ÷ 2 = −1.15 g.
Question 2 Dissolving and melting
The salt particles spread among the water particles and the ice particles stay the same particles. None leave the sealed jar, so the total mass is unchanged: mass is conserved.
Question 3 The open cup
The tablet reacts with water and makes carbon dioxide gas, which escapes from the open cup and carries its mass away. Also, the gas left the system, so the balance reading fell.
Question 4 Balance uncertainty
The balance only reads to 0.1 g, so a change of 0.1 g is within its uncertainty. Water on the outside of the jar or a small reading error could also cause it. In addition, it is not a real loss.
Question 5 Percentage loss
Mean starting mass = (158.2 + 159.0) ÷ 2 = 158.6 g. Percentage loss = 1.15 ÷ 158.6 × 100 = 0.73 %.
Question 6 Drying the jar
Water vapour in the air condenses on a cold jar and adds a small mass of water on the outside. Moreover, this would make the jar seem to gain mass. Drying removes this error.
Question 7 Showing mass is conserved
Put the tablet in a large closed container so the gas stays inside. A problem is that gas builds up pressure and the container may burst, so only a very small amount of reactants and a strong container should be used. Keep the gas in the weighed system.
Question 8 Conclusion
Mass stays the same when a substance dissolves or melts in a closed container, and falls only when a gas escapes from an open container, so the total mass is conserved.
