Year 7 Science · Unit 2 · Lesson 11
Challenge Worksheet
Learning Goals
What if…?
Scenario
A chemist at the CSIRO in Canberra is testing an unknown white powder mixed with water. After adding a few drops of a clear acid, she observes only one thing: the mixture turns pale yellow. No bubbles form. No temperature change is detected. The mixture does not go cloudy. She wonders whether a chemical change has occurred.
The chemist knows that mixing coloured food dye into water also changes the colour of a mixture — but that is definitely not a chemical change. She is unsure whether this colour change is meaningful evidence.
1. The chemist observes only a colour change. Using what you know about the five signs, explain whether this one observation is enough to conclude that a chemical change has occurred. What additional evidence would strengthen her conclusion?
2. Suggest TWO specific tests or observations the chemist could carry out to gather more evidence. For each, describe what a positive result would look like and what it would indicate.
1. A scientist says: "In science, we use the word 'evidence' rather than 'proof' when we describe signs of chemical change." Explain why this distinction is important, using at least one example of a sign that can also occur in a physical change.
2. Rank the following combinations of observations from WEAKEST to STRONGEST evidence for a chemical change, and justify your ranking: (i) a colour change alone, (ii) bubbles + temperature rise + colour change, (iii) a temperature drop alone.
Wrap Up
In one sentence, what was the main idea of this lesson?