All lessons › Reef in hot water › Lesson 3
Australian primary science · Year 6 · Lesson 3 of 3
Muddy water, less light
Inquiry question: How does soil in water change how much light gets through?
Check practical materials
Fair testing
What students learn
Soil carried into water makes it cloudy (turbid), which blocks light. Less light reaching corals and seagrass means less food made by their algae, affecting growth and survival.
Success criteria
- I can plan a fair test changing only the amount of soil.
- I can measure clarity using a reading card.
- I can explain how my results connect to runoff and the Reef.
Preparation and materials
Before this lesson: Students know corals need light because of their algae, and that rivers can carry soil to the Reef (lesson 1).
- Clear plastic cups, all the same size — 52 (4 per pair)
- Water — About 1 L per pair
- Fine garden soil, sieved (collected by the teacher; not potting mix) — 1 small bucket
- Teaspoons — 13
- Measuring jug, or a marker to draw the same fill line on every cup — 13 or 1
- Reading cards: six lines of text getting smaller (handwritten or printed) — 13
- Torch (optional) — 1–13
- Paper towel — 1 roll
Teacher setup
- Make reading cards: write six lines on a strip of paper, from very large (line 1) to very small (line 6), or print them.
- Sieve the soil to remove sticks and stones.
- Draw a fill line on each cup (for example, 3 cm from the top) so every cup holds the same amount of water.
- Print one investigation record per pair.
Safety and adaptations
- Use garden soil collected by the teacher, not potting mix or compost.
- Wash hands with soap after handling soil.
- Wipe up spills straight away so no one slips.
Review your school policies and curriculum requirements before teaching.
How to investigate
The lesson sequence should leave pupils with evidence to interpret, not only an activity to finish.
- Pairs write their fair test plan (change, measure, keep the same).
- Fill four cups to the line with water.
- Add 0, ½, 1 and 2 teaspoons of soil to cups A to D. Stir each for 10 seconds.
- After 30 seconds, place each cup on the reading card and look straight down through the water. Record the smallest line you can read (0 to 6).
- Leave cup D still for 10 minutes, then read it again: did the soil settle (deposition)?
- Compare results with another pair.
Teacher modelling prompt
Hold up a cup of clear water over the reading card. Think aloud: “I can read all six lines, so clarity is 6. I’m changing only the amount of soil. Same cup, same water level, same card, same distance, same light. I’ll stir, wait 30 seconds, then read down through the water.”
Record and explain evidence
Recording question: How does soil in water change how much light gets through?
| Cup | Teaspoons of soil | Smallest line we could read (0–6) | How it looked (clear, cloudy, very cloudy) | After 10 minutes still (cup D) |
|---|---|---|---|---|
| ________________ | ________________ | ________________ | ________________ | ________________ |
Example recording headings; record actual observations, including inconclusive results.
Discussion prompt
Which claim fits our class results?
Check understanding
What should pupils know? What does turbid mean, and how does it affect light?
Explain with evidence: Use your results to explain how a flood could affect corals near the coast.
Apply the idea: Suggest one action a farmer or community near the Reef could take, and explain how it helps corals.
Teacher answer guidance and saved classroom work are available through the main lesson experience where supported.