The method and the results are on the chemical change lab. Do the analysis yourself first, then check the answers below.
Quick answer: A physical change alters the shape, size or state of a material without making a new substance, while a chemical change makes new substances with different properties. These answers go with the grade 7 science lab on chemical change: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers using the sample data. One pattern should stand out: gas bubbles and a colour change are strong clues, while a small temperature change is a weak clue. If your own readings differ, check that your reasoning still follows the evidence.
A: physical: the salt dissolves with no new substance and no gas. B: chemical: strong fizzing is a new gas. C: chemical: bubbles of gas form on the chalk. D: chemical: the colour change shows a new substance (the dye changes form with the acid).
Question 2 Temperature changes
A: 20.8 °C − 21.0 °C = −0.2 °C. B: 17.5 °C − 21.0 °C = −3.5 °C.
Question 3 Is temperature enough?
Cup A (dissolving, physical) and cup B (reaction) both cooled, so cooling does not separate the types. Use the other clues: B made a gas, A did not. Temperature alone is not proof.
Question 4 The gas in B and C
The gas is carbon dioxide. Bubble it through limewater with a delivery tube. Limewater turns milky if carbon dioxide is present.
A change of 0.2 °C or 0.3 °C is as small as the uncertainty of reading the thermometer and the effect of room temperature. Repeat each test three times, use the same volumes and calculate a mean. Repeat and average.
Question 7 A fair test
Any three of: the volume of liquid, the starting temperature, the mass of solid used, the way the mixture is stirred, the time of the final reading. Control these.
Question 8 Conclusion
Cup A is a physical change; B, C and D are chemical changes because gas bubbles and a colour change show that new substances were made.
