Year 7 Science · Unit 2 · Lesson 10
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Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Stirring increases the maximum mass of sugar that can dissolve in a cup of water.
Correct it:
2. A powder dissolves faster than a lump of the same mass because more of its surface touches the water.
Correct it:
3. Temperature can change both how fast a solid dissolves and how much of it can dissolve.
Correct it:
4. Hot water dissolves more of every solute, including gases.
Correct it:
5. Crushing a tablet into powder means more of it will dissolve in the end.
Correct it:
1. What does the word solubility mean?
Cause and effect: a river in a heatwave
Fill in the two empty boxes. Each box is caused by the one before it. Read across the top row, then across the bottom row.
Use a solubility limit
Scenario: a salt substitute
Potassium chloride is sold as a salt substitute. At 20 °C, at most about 34 g of it dissolves in 100 g of water. At 60 °C, at most about 46 g dissolves. Priya adds 40 g of potassium chloride to 100 g of water at 20 °C and stirs well.
2. How many grams of potassium chloride will not dissolve? Show your working.
Answer g
3. Priya heats the mixture to 60 °C. Will the leftover solid dissolve? Then work out how much more potassium chloride she could add at 60 °C before some stays on the bottom.
Answer g
Read the data: dissolving salt four ways
Scenario: cold or warm, still or stirred
A class recorded the seconds needed for 5 g of table salt to vanish in 200 mL of water: cold or warm, stirred or still. Every cup ended with all 5 g dissolved.
| Water | Stirred? | Time to dissolve (s) |
|---|---|---|
| Cold (15 °C) | No | 210 |
| Cold (15 °C) | Yes | 130 |
| Warm (40 °C) | No | 90 |
| Warm (40 °C) | Yes | 45 |
Source: sample class results for this worksheet.
4. How much quicker, in seconds, was the stirred cold cup than the unstirred cold cup?
Answer s
5. Start from cold, unstirred water. Which change made the bigger difference to the time: warming the water or stirring it? Use numbers from Table 1.
6. A student says: “Table 1 proves that warm, stirred water dissolves more salt in total than cold, still water.” Evaluate this claim.
7. A café makes a very sweet sugar syrup every morning. The owner wants to dissolve as much sugar as possible in 1 L of water, as quickly as possible, before opening. Recommend a method that uses all three factors. For each factor, say whether it lets more sugar dissolve or only makes it dissolve faster.
True or false
1. False: stirring only speeds up dissolving; the maximum stays the same. 2. True. 3. True. 4. False: gases dissolve less in hot water. 5. False: the same amount dissolves; the powder just dissolves faster.
Question 1
How much of a solute can dissolve in a solvent at a given temperature (1). [1 mark]
Cause and effect: a river in a heatwave
Box 1: the water particles move faster and jiggle more (1). Box 2: dissolved oxygen is shaken out from between the water particles, so less oxygen stays dissolved (1). [2 marks]
Use a solubility limit
2. Only 34 g dissolves at 20 °C (1); 40 − 34 = 6 g is left (1). [2 marks]
3. Yes: at 60 °C up to 46 g can dissolve, more than her 40 g (1). 46 − 40 = 6 g more could dissolve (1). [2 marks]
Read the data: dissolving salt four ways
4. 210 − 130 = 80 s (1). [1 mark]
5. Warming (1): it cut 210 − 90 = 120 s; stirring cut only 80 s (1). [2 marks]
6. Not supported. All 5 g dissolved in every cup, and the table measures time (rate), not total amount (1). Stirring never changes the maximum (1). Warm water may hold a little more salt because of temperature, but testing that needs salt added until some stays undissolved (1). [3 marks]
Question 7
Marking guide: 1 clear method (hot water, caster sugar, stir throughout); 1 hot water: faster particles knock sugar loose more quickly; 1 hot water also raises the maximum; 1 caster sugar: more surface attacked at once; 1 stirring: brings fresh water to the grains; 1 caster sugar and stirring change only the rate. [6 marks]
Model answer: Heat the water, add caster sugar and stir the whole time. Hot water particles move faster and knock sugar loose more quickly, and hot water can hold more sugar, so heating is the only step that lets more dissolve. Caster sugar has far more surface touching the water, so more is attacked at once. Stirring brings fresh water to the grains. Caster sugar and stirring only make it faster; they do not raise the maximum.
Common error: saying stirring or caster sugar lets more sugar dissolve; they change only the rate.