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. This grade 7 science lab on chemical change 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.
Which everyday mixtures are really chemical changes? In this lab you mix four common household materials and look for the clues of a chemical change: bubbles, colour change and temperature change. You will see that some clues are strong and others can mislead, so you must use several clues together before you decide.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS1-2
Aim
To decide, from observations and temperature readings, which of four common mixtures involve a chemical change.
Equipment
- Four clear plastic cups labelled A to D
- Thermometer reading to 0.1 °C
- White vinegar (about 150 cm³) and tap water
- Table salt, baking soda and two small pieces of blackboard chalk
- Red cabbage juice (cooked cabbage leaves in hot water, cooled)
- Measuring cylinder, teaspoon and stopwatch
Safety
Wear safety goggles. Do not taste anything. Vinegar can sting the eyes, so rinse with water if you splash it. Stand cups on a tray in case of overflow, and wash your hands at the end.
Method
- Cup A: put 50 cm³ of water in the cup. Record its temperature, then add one teaspoon of salt and stir.
- Cup B: put 50 cm³ of vinegar in the cup. Record its temperature, then add one teaspoon of baking soda.
- Cup C: put 50 cm³ of vinegar in the cup. Record its temperature, then add a piece of chalk.
- Cup D: put 20 cm³ of red cabbage juice in the cup. Record its temperature and colour, then add 20 cm³ of vinegar.
- Watch each cup for one minute, noting bubbles, colour change and smell. Read the thermometer again and record the lowest or highest value.
Results
Sample measurements from one group:
| Cup | Start temperature (°C) | End temperature (°C) | Change (°C) | Observations |
|---|---|---|---|---|
| A | 21.0 | 20.8 | −0.2 | Salt vanishes, clear liquid, no bubbles |
| B | 21.0 | 17.5 | −3.5 | Strong fizzing, then clear liquid |
| C | 21.0 | 21.3 | +0.3 | Slow, small bubbles form on the chalk |
| D | 21.0 | 21.0 | 0.0 | Purple turns pink at once, no bubbles |
Analysis questions
Question 1 Classify the cups
Classify the change in each cup as physical or chemical and give the evidence.Question 2 Temperature changes
Check the temperature change shown in the table for cups A and B by calculation.Question 3 Is temperature enough?
Cup A and cup B both got colder. Explain why the temperature alone cannot decide which one is chemical.Question 4 The gas in B and C
Name the gas in cups B and C and describe a test that would prove it.Question 5 Word equation for cup C
Write the word equation for chalk (calcium carbonate) reacting with ethanoic acid in vinegar.Question 6 Reliability
The changes in A and C were small. Explain why they may be within measurement error and suggest how to improve reliability.Question 7 A fair test
State three variables that should be kept the same in cups A to C.Question 8 Conclusion
Write a conclusion that links the evidence to the type of change in each cup.
