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. This grade 7 science lab on conservation of mass 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.
Is mass really conserved when substances dissolve, change state or react? In this lab you weigh three changes before and after, some in closed jars and one in an open cup, and use the balance readings to decide where mass can seem to disappear. It needs only a kitchen-style balance and common materials.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS1-5
Aim
To find out whether the total mass changes when salt dissolves, when ice melts and when a tablet reacts with water, and to explain any change in terms of closed and open systems.
Equipment
- Digital balance reading to 0.1 g
- Two glass jars with screw lids and one plastic cup
- Table salt, about 10 g
- Ice cubes and tap water
- Plain effervescent tablets (two)
- Spoon, paper towel and a measuring cylinder
Safety
Wear safety goggles. Never eat or drink anything in the lab. Use only an open cup for the tablet, never a closed container, because the gas can build up pressure. Wipe up spills at once and wash your hands afterwards.
Method
- Part A: put 100 cm³ of water and a spoon of salt (about 5 g) into a jar. Do not mix yet. Screw the lid on tightly, dry the outside and record the mass.
- Shake the jar until the salt has dissolved. Dry the outside again and record the mass.
- Part B: put three ice cubes in a second jar, screw on the lid, dry the outside and record the mass. Leave the jar until the ice has melted, dry the outside and record the mass again.
- Part C: put 100 cm³ of water in the cup and stand it on the balance with one tablet beside it. Record the mass of everything, then drop the tablet in. When the fizzing stops, record the mass again.
- Repeat each part once more with fresh materials.
Results
Sample measurements from one group:
| Part | Trial | Mass before (g) | Mass after (g) | Change (g) |
|---|---|---|---|---|
| A Salt dissolving (lid on) | 1 | 286.4 | 286.4 | 0.0 |
| A Salt dissolving (lid on) | 2 | 301.2 | 301.1 | −0.1 |
| B Ice melting (lid on) | 1 | 214.8 | 214.8 | 0.0 |
| B Ice melting (lid on) | 2 | 220.3 | 220.3 | 0.0 |
| C Tablet fizzing (open cup) | 1 | 158.2 | 157.1 | −1.1 |
| C Tablet fizzing (open cup) | 2 | 159.0 | 157.8 | −1.2 |
Analysis questions
Question 1 Mean changes
Calculate the mean change in mass for parts A, B and C.Question 2 Dissolving and melting
Use the particle idea to explain why the mass hardly changed in parts A and B.Question 3 The open cup
Explain why the mass fell in part C.Question 4 Balance uncertainty
The change in part A trial 2 was −0.1 g. Suggest why this does not mean that mass was lost.Question 5 Percentage loss
Calculate the mean percentage loss of mass in part C, using the mean starting mass.Question 6 Drying the jar
Explain why the outside of the jar in part B must be dried before weighing.Question 7 Showing mass is conserved
Suggest how part C could be changed so that the total mass would not fall, and state one problem with your idea.Question 8 Conclusion
Write a one-sentence conclusion that links the results of parts A, B and C.
