Year 8 Science · Unit 1 · Lesson 7
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s summary about a sunflower
A sunflower moves water even though it has no heart or blood. Its roots take in pure water only; the minerals it needs come from its leaves. Water enters through the leaves and travels down the stem to the roots. The stem connects the roots and the leaves. Once water reaches the leaves, all of it is used by the leaf cells, so none escapes into the air.
1. Write the three corrections.
Weighing a thirsty plant
A class watered a potted capsicum plant, then sealed the pot and soil inside a plastic bag so that water could only leave through the plant’s leaves. They weighed the whole plant and pot three times on a warm, sunny day. The plant has 40 leaves. 1 g of water has a volume of 1 mL.
| Time | 8:00 am | 12:00 noon | 4:00 pm |
|---|---|---|---|
| Mass (g) | 2450 | 2370 | 2210 |
Source: sample class results.
2. Calculate the volume of water lost from 8:00 am to 4:00 pm, and the average loss per hour.
Average loss mL per hour
3. Calculate the average volume of water each leaf lost over the 8 hours.
Answer mL per leaf
4. Compare the loss in the morning (8 am to noon) with the loss in the afternoon (noon to 4 pm). Suggest a reason for the difference, and explain why sealing the pot in a bag was needed for a fair measurement.
Plan an investigation
Plan a test of this question, using the weighing method above: “Does moving air (wind) increase the amount of water a plant loses from its leaves?” You have several similar potted plants and a desk fan with low and high settings.
Prediction
If the wind speed is increased, then the water lost will …, because …
Variables
What result would show your prediction is wrong?
Evaluate the claim
A gardener’s tip
“When a plant wilts, spray water on its leaves. The leaves are where a plant drinks, so that is the quickest way to rescue it.”
5. Evaluate the gardener’s tip. State what is wrong with the reasoning and give better advice, explaining why it works.
6. Spinifex grass grows in the dry Australian outback. Its leaves are very narrow, tightly rolled and covered in a waxy coating. Explain how these leaf features help spinifex survive with little water in the soil. Then identify one disadvantage of having such small leaves.
Find the errors
1. (i) Roots take in water and dissolved minerals from the soil; minerals do not come from the leaves. (ii) Water enters through the roots and travels up the stem to the leaves. (iii) Only some water is used by leaf cells; some evaporates from the leaves into the air. [3 marks]
Your Turn
2. Mass lost = 2450 − 2210 = 240 g = 240 mL of water. Time = 8 hours. Average = 240 ÷ 8 = 30 mL per hour. [3 marks]
3. 240 ÷ 40 = 6 mL per leaf. [1 mark]
4. Morning 80 mL; afternoon 160 mL, twice as much. The afternoon was hotter, so water evaporated from the leaves faster. The bag stopped water evaporating from the soil, so the mass lost came only from the leaves. [3 marks: comparison with values; reason; purpose of bag]
Plan an investigation
Prediction: more wind, more water lost, because moving air carries water vapour away from the leaves so more evaporates. Independent: wind speed (no fan, low, high). Dependent: mass lost in a set time (for example 2 hours), by weighing the sealed pot before and after. Controlled (any 3): same kind and size of plant; same watering; same temperature and light; same time; same distance from the fan; pot sealed each time. Wrong if: mass lost is about the same or less at the higher setting.
Evaluate the claim
5. Wrong: plants take in water through their roots; leaves are mainly where water is lost. Better: water the soil (and move the plant out of hot sun or wind). The roots take in the water, the stem carries it up, and the leaves refill. [3 marks]
Show What You Know
6. Marking guide: 1 leaves are where most water is lost; 1 narrow leaves (less surface, less loss); 1 rolled leaves (surface tucked inside, away from hot, dry, moving air); 1 waxy coating (waterproof layer slows evaporation); 1 balance (little uptake from dry soil, so loss must stay low); 1 disadvantage (less surface to catch light, so slower food-making and growth). [6 marks]
Common error to watch for: saying the wax “stores water”. The wax is a waterproof layer that slows water loss; it does not hold water.