Year 10 Science · Unit 2 · Lesson 3
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Learning Goals
Read the graph
The chart below shows the colours produced by red cabbage juice (anthocyanin indicator) at five different pH values. Study the chart and answer the questions.
Data: Anthocyanin colour transitions, adapted from Hale & Marken, Journal of Chemical Education, 2020
(a) A student tests vinegar with red cabbage juice and gets a pink-red colour. What is the approximate pH of vinegar and is it an acid or a base?
(b) A solution turns the indicator blue. Compare this to a solution that turns it yellow-green. Which is more basic, and what does this tell you about their OH⁻ ion concentrations?
(c) A student wants to use red cabbage as a field test kit during a river clean-up in the Blue Mountains. What are one advantage and one limitation of using red cabbage juice instead of a digital pH meter for this purpose?
Real-world context
Red cabbage contains anthocyanins, natural pigments that change colour depending on pH. Scientists and science educators across Australia use red cabbage as an accessible, low-cost indicator for demonstrating acid-base chemistry. In 2022, a CSIRO citizen science initiative invited Year 9–10 students to test the pH of local waterways using natural indicators.
(a) You plan to investigate: "Does the pH of common household substances affect the colour of red cabbage indicator?" Identify the independent variable and the dependent variable for this investigation.
(b) List three variables you would need to control (keep the same) to make this a fair test, and explain why each one matters.
(c) Describe one safety precaution specific to this investigation (not a general lab rule) and explain why it is necessary.
1. Aboriginal Australians traditionally used certain plant products medicinally, including naturally acidic or basic plant extracts. Suggest one reason why an awareness of plant chemistry (such as acidity) would have been important for traditional medicine, without needing formal pH measurement tools.
Wrap Up
In one sentence, what was the main idea of this lesson?