Year 7 Science · Unit 2 · Lesson 10
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word from the word bank. Each word is used at most once. Two words are not needed: cross them out.
Dissolving can be sped up in three ways. Crushing a solid into a powder gives it a bigger , so water particles can attack more of it at the same time, and the powder takes a time to dissolve. sweeps away the water that is already full of dissolved solute and brings fresh water to the grains. Crushing and stirring change only the of dissolving, not the amount that can dissolve. Temperature is the only one of the three factors that changes how much can dissolve: for most solids, hot water dissolves than cold water. Gases are the opposite, so a warm fizzy drink goes flat quickly as escapes from the water.
Odd one out
In each row, three items fit the rule and one does not. Write the odd one out and give a reason.
| Rule | Group | Odd one out, and why |
|---|---|---|
| 1. A factor that affects dissolving | temperature · stirring · surface area · colour of the cup | |
| 2. Has a large surface area for its mass | icing sugar · crushed tablet · fine salt · a large sugar cube | |
| 3. Changes only the rate, not the maximum amount | stirring · crushing to a powder · heating the water · using finer grains |
Sort it
Each change is made to a cup of water with a solid in it. Write each change in the correct box. The boxes do not hold the same number of items.
Dissolves faster
Dissolves more slowly
No effect on the rate
1. A student crushes a sugar cube into a fine powder and stirs it into water. Name two factors that are now helping the sugar dissolve faster. For each one, explain why it speeds up dissolving.
Read the data: making fertiliser
Scenario: a strong plant food
Potassium nitrate is a salt used in plant fertiliser. A nursery wants to dissolve as much of it as possible in water. Table 1 compares it with table salt. Each number is the most solid that will dissolve before extra grains stay on the bottom.
| Water temperature (°C) | Table salt (g) | Potassium nitrate (g) |
|---|---|---|
| 0 | 36 | 13 |
| 20 | 36 | 32 |
| 40 | 37 | 64 |
| 60 | 37 | 110 |
| 80 | 38 | 169 |
Source: published solubility tables, rounded to whole grams.
2. The nursery wants to dissolve 80 g of potassium nitrate in 100 g of water. What is the lowest temperature in Table 1 that will work? Give a reason.
3. Compare how temperature affects the two solids. What is the same, and what is different? Use numbers from Table 1.
4. Jake adds 45 g of table salt to 100 mL of water at 20 °C. He stirs for five minutes, but about 9 g of salt still sits on the bottom. Jake says: “I just need to stir harder. Stirring makes more salt dissolve.” Explain whether Jake is right. Then use Table 1 to say whether heating the water to 80 °C would dissolve all 45 g.
Fill the gap
surface area; shorter; stirring; rate; maximum; more; carbon dioxide. Not used: longer, colour.
Odd one out
1. Colour of the cup: it has no effect on dissolving (1). 2. A large sugar cube: it has a small surface area for its mass; the others are fine powders or grains (1). 3. Heating the water: it changes both the rate and the maximum amount; the other three change only the rate (1). [3 marks]
Sort it
Dissolves faster: stir the whole time; hot water instead of cold; grind the salt finer; caster sugar instead of sugar lumps. Dissolves more slowly: a sugar cube instead of the same mass of powder; chill the water in the fridge first; stop stirring the cup. No effect on the rate: a red cup instead of a white one; switch the lights off.
Question 1
Surface area: the powder has more surface touching the water, so more is attacked at once (1). Stirring: it sweeps away sugar-filled water and brings fresh water to the grains (1). [2 marks]
Read the data: making fertiliser
2. 60 °C (1): at 60 °C up to 110 g can dissolve, which is more than 80 g, but at 40 °C only 64 g can (1). [2 marks]
3. Same: for both solids, more dissolves as the temperature rises (1). Different: salt hardly changes, only 36 g to 38 g from 0 °C to 80 °C (1), but potassium nitrate rises a lot, from 13 g to 169 g (1). [3 marks]
Question 4
Marking guide: 1 clear position (Jake is wrong); 1 stirring only speeds up dissolving by bringing fresh water to the grains; 1 it cannot change the maximum amount, so a saturated solution stays saturated however hard he stirs; 1 Table 1: at 20 °C at most 36 g dissolves in 100 g of water, so 45 − 36 = 9 g is left; 1 heating to 80 °C would not work: at most 38 g dissolves, so about 7 g would still be left. [5 marks]
Model answer: Jake is wrong. Stirring only makes salt dissolve faster by bringing fresh water to the grains; it cannot change the most salt the water can hold. At 20 °C only 36 g dissolves in 100 g of water, so 45 − 36 = 9 g stays on the bottom. Heating to 80 °C would not fix it: only 38 g dissolves, so about 7 g is still left.
Common error: thinking stirring or crushing lets more dissolve; they change only the rate.