Year 7 Science · Unit 2 · Lesson 9
Challenge Worksheet
Learning Goals
1. Name the solute and the solvent in each mixture. One of the mixtures is not a solution: for that one, write “not a solution” and give the reason.
| Mixture | Solute | Solvent |
|---|---|---|
| Air | ||
| Brass | ||
| Soda water | ||
| Sand in water |
2. Priya tests whether temperature changes how much sugar can dissolve. She stirs sugar, one spoonful at a time, into 100 mL of water at 20 °C until no more dissolves. She then does the same with 50 mL of water at 80 °C, but this time she does not stir. She tests each temperature once. Identify three problems with Priya’s method and say how to fix each one.
Evaluate the explanations
Three students explain why sugar dissolves faster in hot tea than in iced tea
Ari: “Hot water has more room between its particles, so the sugar fits in more easily.”
Bea: “The heat melts the sugar, so it turns into a liquid and mixes in.”
Cy: “Hot water particles move faster, so they hit the sugar grains harder and more often, and carry the loose sugar particles away sooner.”
3. Whose explanation is correct? Explain what is wrong with each of the other two.
Read the data: how much sugar will dissolve?
Solubility of sugar
The most solute that can dissolve in a solvent is its solubility. The table shows how the solubility of sugar in water changes as the water gets hotter.
| Temperature (°C) | 0 | 20 | 40 | 60 | 80 | 100 |
|---|---|---|---|---|---|---|
| Most sugar that dissolves in 100 mL of water (g) | 179 | 204 | 238 | 287 | 362 | 487 |
Source: published solubility figures for sugar (sucrose), rounded, as in the lesson.
4. Plot the data from Table 1 on the grid. Label both axes with units and draw a smooth curve through the points.
5. A cook wants to dissolve 300 g of sugar in 100 mL of water to make a thick syrup. Use your graph to find the lowest temperature at which all the sugar will dissolve.
Answer °C
6. Describe the trend in Table 1. Does the solubility rise by the same amount for every 20 °C step? Use numbers from the table.
7. A student says: “If I keep adding sugar to my hot coffee, it will keep dissolving forever.” Evaluate this claim. Use the words saturated and temperature, use a figure from Table 1, and describe what would happen if the coffee then cooled.
Question 1
Air: oxygen and the other gases; nitrogen. Brass: zinc; copper. Soda water: carbon dioxide; water. Sand in water: not a solution, because the sand does not dissolve; it settles out. [4 marks: 1 per row]
Question 2
Any three, each with its fix (1 each): different volumes of water (use 100 mL both times); stirred at 20 °C but not at 80 °C (stir both the same way); each temperature tested only once (repeat each three times and average); spoonfuls are not equal amounts (weigh the sugar added). [3 marks]
Evaluate the explanations
3. Cy is correct (1). Ari: warming water barely changes the space between its particles (under 3% from 20 °C to 80 °C), so extra room cannot explain it; the speed comes from faster collisions (1). Bea: the sugar does not melt at tea temperature; it dissolves, its particles spreading between the water particles (1). [3 marks]
Read the data: how much sugar will dissolve?
4. Axes: temperature (°C) across, sugar dissolved (g) up, with even scales (1); all six points plotted correctly (1); smooth curve through the points (1). [3 marks]
5. Reads 300 g across to the curve and down (1): about 64 °C (accept 62–67 °C) (1). [2 marks]
6. The hotter the water, the more sugar dissolves (1). No: the rise gets bigger at higher temperatures, for example 25 g from 0 to 20 °C but 125 g from 80 to 100 °C (1). [2 marks]
Question 7
Marking guide: 1 the claim is wrong; 1 at any temperature the water can hold only a certain amount of sugar; when it reaches that limit it is saturated; 1 extra sugar then sits undissolved at the bottom; 1 hotter water holds more (for example about 362 g per 100 mL at 80 °C against 204 g at 20 °C), but the limit is still there; 1 as the coffee cools, the limit falls; 1 so the sugar above the new limit comes back out as crystals. [6 marks]
Model answer: The claim is wrong. At each temperature water dissolves only so much sugar; at that limit the coffee is saturated and extra sugar sits on the bottom. Hot coffee has a higher limit (about 362 g per 100 mL at 80 °C, 204 g at 20 °C), so a saturated hot coffee that cools cannot hold all its sugar and the extra comes back out as crystals.
Common error: mixing up “dissolves faster” with “dissolves more”. Heating does both; stirring only makes it faster.