Start here
This guide addresses separating final mass from mass loss in before-and-after evidence. Ask: What was each starting mass, and how much did each sample change before reaching its final mass? The aim is a decision model that survives unfamiliar wording and diagrams.
Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Mechanism, contrasting cases, misconception repair, independent practice and delayed transfer will make that model usable under examination pressure.
Use the complete Learner’s Guide volume directory, the original PSLE Learning Guide, and the Science hub. Current format information comes from the official SEAB PSLE formats examined in 2026.
Mechanism before method
Mechanism 1
A final reading states where a sample ended. A change states the difference between start and finish. These answer different questions and must not be substituted for one another.
Mechanism 2
Calculate mass loss as starting mass minus final mass. Write both measurements with the same unit before subtracting and keep the result attached to the correct sample.
Mechanism 3
If two samples finish at 40 g but began at 70 g and 55 g, their losses are 30 g and 15 g. Equal endpoints can hide different changes.
Mechanism 4
When starting masses are equal, the sample with the lower final mass has lost more. When final masses are equal, the sample with the higher starting mass has lost more.
Mechanism 5
A mass decrease alone does not identify a mechanism. Evaporation, escaping gas, removed material or measurement problems require context and evidence. Separate amount of change from explanation.
Use four moves: name the exact job, select controlling evidence, apply the relationship, and reject the nearest alternative with a reason. This makes the method visible and correctable.
Worked decision laboratory
Case 1: Sample A changes from 30 g to 20 g. Sample B changes from 26 g to 20 g. Compare their mass losses.
Answer: A loses 10 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 2: Sample A changes from 37 g to 25 g. Sample B changes from 34 g to 25 g. Compare their mass losses.
Answer: A loses 12 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 3: Sample A changes from 44 g to 30 g. Sample B changes from 42 g to 30 g. Compare their mass losses.
Answer: A loses 14 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 4: Sample A changes from 51 g to 35 g. Sample B changes from 50 g to 35 g. Compare their mass losses.
Answer: A loses 16 g; B loses 15 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 5: Sample A changes from 58 g to 40 g. Sample B changes from 58 g to 40 g. Compare their mass losses.
Answer: A loses 18 g; B loses 18 g; the losses are equal. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 6: Sample A changes from 65 g to 45 g. Sample B changes from 66 g to 45 g. Compare their mass losses.
Answer: A loses 20 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 7: Sample A changes from 72 g to 50 g. Sample B changes from 56 g to 50 g. Compare their mass losses.
Answer: A loses 22 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 8: Sample A changes from 65 g to 55 g. Sample B changes from 64 g to 55 g. Compare their mass losses.
Answer: A loses 10 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 9: Sample A changes from 32 g to 20 g. Sample B changes from 32 g to 20 g. Compare their mass losses.
Answer: A loses 12 g; B loses 12 g; the losses are equal. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 10: Sample A changes from 39 g to 25 g. Sample B changes from 40 g to 25 g. Compare their mass losses.
Answer: A loses 14 g; B loses 15 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 11: Sample A changes from 46 g to 30 g. Sample B changes from 48 g to 30 g. Compare their mass losses.
Answer: A loses 16 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 12: Sample A changes from 53 g to 35 g. Sample B changes from 56 g to 35 g. Compare their mass losses.
Answer: A loses 18 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 13: Sample A changes from 60 g to 40 g. Sample B changes from 46 g to 40 g. Compare their mass losses.
Answer: A loses 20 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 14: Sample A changes from 67 g to 45 g. Sample B changes from 54 g to 45 g. Compare their mass losses.
Answer: A loses 22 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 15: Sample A changes from 60 g to 50 g. Sample B changes from 62 g to 50 g. Compare their mass losses.
Answer: A loses 10 g; B loses 12 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 16: Sample A changes from 67 g to 55 g. Sample B changes from 70 g to 55 g. Compare their mass losses.
Answer: A loses 12 g; B loses 15 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 17: Sample A changes from 34 g to 20 g. Sample B changes from 38 g to 20 g. Compare their mass losses.
Answer: A loses 14 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 18: Sample A changes from 41 g to 25 g. Sample B changes from 46 g to 25 g. Compare their mass losses.
Answer: A loses 16 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 19: Sample A changes from 48 g to 30 g. Sample B changes from 36 g to 30 g. Compare their mass losses.
Answer: A loses 18 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 20: Sample A changes from 55 g to 35 g. Sample B changes from 44 g to 35 g. Compare their mass losses.
Answer: A loses 20 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 21: Sample A changes from 62 g to 40 g. Sample B changes from 52 g to 40 g. Compare their mass losses.
Answer: A loses 22 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 22: Sample A changes from 55 g to 45 g. Sample B changes from 60 g to 45 g. Compare their mass losses.
Answer: A loses 10 g; B loses 15 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 23: Sample A changes from 62 g to 50 g. Sample B changes from 68 g to 50 g. Compare their mass losses.
Answer: A loses 12 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 24: Sample A changes from 69 g to 55 g. Sample B changes from 76 g to 55 g. Compare their mass losses.
Answer: A loses 14 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 25: Sample A changes from 36 g to 20 g. Sample B changes from 26 g to 20 g. Compare their mass losses.
Answer: A loses 16 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 26: Sample A changes from 43 g to 25 g. Sample B changes from 34 g to 25 g. Compare their mass losses.
Answer: A loses 18 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 27: Sample A changes from 50 g to 30 g. Sample B changes from 42 g to 30 g. Compare their mass losses.
Answer: A loses 20 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 28: Sample A changes from 57 g to 35 g. Sample B changes from 50 g to 35 g. Compare their mass losses.
Answer: A loses 22 g; B loses 15 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 29: Sample A changes from 50 g to 40 g. Sample B changes from 58 g to 40 g. Compare their mass losses.
Answer: A loses 10 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 30: Sample A changes from 57 g to 45 g. Sample B changes from 66 g to 45 g. Compare their mass losses.
Answer: A loses 12 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 31: Sample A changes from 64 g to 50 g. Sample B changes from 56 g to 50 g. Compare their mass losses.
Answer: A loses 14 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 32: Sample A changes from 71 g to 55 g. Sample B changes from 64 g to 55 g. Compare their mass losses.
Answer: A loses 16 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 33: Sample A changes from 38 g to 20 g. Sample B changes from 32 g to 20 g. Compare their mass losses.
Answer: A loses 18 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 34: Sample A changes from 45 g to 25 g. Sample B changes from 40 g to 25 g. Compare their mass losses.
Answer: A loses 20 g; B loses 15 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 35: Sample A changes from 52 g to 30 g. Sample B changes from 48 g to 30 g. Compare their mass losses.
Answer: A loses 22 g; B loses 18 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 36: Sample A changes from 45 g to 35 g. Sample B changes from 56 g to 35 g. Compare their mass losses.
Answer: A loses 10 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 37: Sample A changes from 52 g to 40 g. Sample B changes from 46 g to 40 g. Compare their mass losses.
Answer: A loses 12 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 38: Sample A changes from 59 g to 45 g. Sample B changes from 54 g to 45 g. Compare their mass losses.
Answer: A loses 14 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 39: Sample A changes from 66 g to 50 g. Sample B changes from 62 g to 50 g. Compare their mass losses.
Answer: A loses 16 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Case 40: Sample A changes from 73 g to 55 g. Sample B changes from 70 g to 55 g. Compare their mass losses.
Answer: A loses 18 g; B loses 15 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Apply the model: Mass loss equals starting mass minus final mass; equal final masses imply equal loss only when starting masses are equal. Then name one changed condition that would permit a different conclusion.
Common misconceptions and targeted repair
1. Equal final masses mean equal losses.
Change also depends on the starting mass. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
2. The lowest final mass always lost most.
Compare start minus finish for each sample. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
3. Subtract starting mass from final mass for a loss.
Use starting minus final to state a positive amount lost. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
4. A mass decrease proves evaporation.
Other mechanisms or measurement issues may explain it. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
5. Different units can be subtracted directly.
Convert to a common unit first. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
6. Percentage loss and mass loss are identical.
Percentage also depends on the starting mass. Repair the habit by naming the tense, rotation match, baseline measurement, operands or local key and citing one visible cue.
Independent practice with explained answers
1. Sample A changes from 30 g to 20 g. Sample B changes from 26 g to 20 g. Compare their mass losses.
A loses 10 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
2. Sample A changes from 37 g to 25 g. Sample B changes from 34 g to 25 g. Compare their mass losses.
A loses 12 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
3. Sample A changes from 44 g to 30 g. Sample B changes from 42 g to 30 g. Compare their mass losses.
A loses 14 g; B loses 12 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
4. Sample A changes from 51 g to 35 g. Sample B changes from 50 g to 35 g. Compare their mass losses.
A loses 16 g; B loses 15 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
5. Sample A changes from 58 g to 40 g. Sample B changes from 58 g to 40 g. Compare their mass losses.
A loses 18 g; B loses 18 g; the losses are equal. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
6. Sample A changes from 65 g to 45 g. Sample B changes from 66 g to 45 g. Compare their mass losses.
A loses 20 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
7. Sample A changes from 72 g to 50 g. Sample B changes from 56 g to 50 g. Compare their mass losses.
A loses 22 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
8. Sample A changes from 65 g to 55 g. Sample B changes from 64 g to 55 g. Compare their mass losses.
A loses 10 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
9. Sample A changes from 32 g to 20 g. Sample B changes from 32 g to 20 g. Compare their mass losses.
A loses 12 g; B loses 12 g; the losses are equal. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
10. Sample A changes from 39 g to 25 g. Sample B changes from 40 g to 25 g. Compare their mass losses.
A loses 14 g; B loses 15 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
11. Sample A changes from 46 g to 30 g. Sample B changes from 48 g to 30 g. Compare their mass losses.
A loses 16 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
12. Sample A changes from 53 g to 35 g. Sample B changes from 56 g to 35 g. Compare their mass losses.
A loses 18 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
13. Sample A changes from 60 g to 40 g. Sample B changes from 46 g to 40 g. Compare their mass losses.
A loses 20 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 40 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
14. Sample A changes from 67 g to 45 g. Sample B changes from 54 g to 45 g. Compare their mass losses.
A loses 22 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 45 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
15. Sample A changes from 60 g to 50 g. Sample B changes from 62 g to 50 g. Compare their mass losses.
A loses 10 g; B loses 12 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 50 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
16. Sample A changes from 67 g to 55 g. Sample B changes from 70 g to 55 g. Compare their mass losses.
A loses 12 g; B loses 15 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 55 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
17. Sample A changes from 34 g to 20 g. Sample B changes from 38 g to 20 g. Compare their mass losses.
A loses 14 g; B loses 18 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 20 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
18. Sample A changes from 41 g to 25 g. Sample B changes from 46 g to 25 g. Compare their mass losses.
A loses 16 g; B loses 21 g; B loses more. Subtract each final mass from its own starting mass. The common final reading 25 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
19. Sample A changes from 48 g to 30 g. Sample B changes from 36 g to 30 g. Compare their mass losses.
A loses 18 g; B loses 6 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 30 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
20. Sample A changes from 55 g to 35 g. Sample B changes from 44 g to 35 g. Compare their mass losses.
A loses 20 g; B loses 9 g; A loses more. Subtract each final mass from its own starting mass. The common final reading 35 g does not make the changes equal because the starting readings differ. Attempt before opening. If wrong, classify the failure as reading, evidence selection, relationship, execution or checking.
Three-round independence ladder
Round 1: visible model
Solve five items with the mechanism visible and add a short evidence tag beside each answer.
Round 2: hidden model
After twenty minutes, reconstruct the rule and solve five different items before checking explanations.
Round 3: mixed transfer
Mix four new items with unrelated topics and include a boundary case where a condition changes.
Delayed transfer
The next day, retrieve the controlling question, principle and two contrasting examples. Invent and solve a fresh case.
Three days later, change one condition and predict the effect before solving. A working model can explain what changes and what remains stable.
Use a correction record with cue, first decision, missed evidence and repaired rule.
Examination control
- Name the exact job.
- Mark controlling evidence.
- State the relationship.
- Execute the transformation, count, subtraction or interpretation.
- Check tense, matches, units, baselines, labels and keys.
When uncertain, compare the two most plausible readings against explicit evidence and reject the one that contradicts the structure.
Target known failure points rather than repeating the first attempt unchanged.
For parents and tutors
Ask what evidence controlled the answer before correcting. This separates missing knowledge from context selection and execution errors.
Replace “be careful” with an observable action matched to the guide’s mechanism.
Frequently asked questions
Must I memorise every case?
No; learn the relationship.
Why explain?
It separates reasoning from guessing.
Can a changed condition reverse the answer?
Yes; identify the dependency.
Does appearance settle meaning?
No; use structure, labels and conditions.
How fast?
Accuracy first, then timed retrieval.
What is mastery?
Delayed retrieval, new-case solution and boundary recognition.
Next routes
Continue through the complete directory, the original guide, and the Science hub.