Year 7 Science · Unit 2 · Lesson 18
Apply Worksheet
Learning Goals
Real-world context
In 2023, Australian long-distance hikers on the Larapinta Trail in the Northern Territory carried electrolyte powder sachets to replace salts lost through sweating. At the end of a hot day's hike, some hikers had warm water bottles (around 35°C) while others had access to cold water from a creek (around 12°C). Both groups tried to dissolve the same 5 g electrolyte sachet into 500 mL of water.
(a) Predict which group — warm water or cold water — would dissolve the electrolyte powder faster. Explain your prediction using particle ideas from the lesson.
(b) A hiker suggests that shaking the bottle vigorously while adding the powder would also help. Explain why this is correct, and name the dissolving factor involved.
Read the graph — dissolving salt under different conditions
The bar chart shows the time taken for 5 g of table salt to fully dissolve in 200 mL of water under four conditions. Study the chart, then answer the questions.
Data: simulated experiment, based on standard dissolving rate principles
(a) Which condition resulted in the fastest dissolving? Which was the slowest?
(b) By how many seconds did stirring reduce the dissolving time when using cold water? Show your calculation.
(c) Using particle ideas, explain why warm + stirred water dissolved the salt so much faster than cold + still water.
1. A student argues: "The warm + stirred water will dissolve more salt than the cold + still water in total." Is this correct? Explain your answer carefully.
2. An experiment only tests two conditions at a time (e.g. cold vs warm). Name ONE variable that must be controlled (kept the same) in this experiment, and explain why controlling it matters for a fair test.
Wrap Up
In one sentence, what was the main idea of this lesson?