Year 7 Science · Unit 2 · Lesson 9
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Learning Goals
Order the steps: sugar dissolving
Number the events from 1 (first) to 5 (last).
| Order (1–5) | Step |
|---|---|
| The collisions knock sugar particles loose from the surface of the grain, one at a time. | |
| Every sugar particle ends up surrounded by water particles, spread evenly through the liquid, and the solution looks clear. | |
| A sugar grain sits in the water, with its particles packed tightly together. | |
| The freed sugar particles slip into the gaps between the water particles. | |
| Water particles bump into the grain from all sides. |
1. Water is nicknamed the “universal solvent”. What does this nickname mean, and what name is given to any solution in which water is the solvent?
2. Lara keeps adding salt to a glass of water and stirring. After a while, salt crystals stay on the bottom however long she stirs. What has happened? Use the word saturated.
Cause and effect: growing sugar crystals
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
3. A sports drink has sugar and salts dissolved in water. You drink half the bottle, then fill it back up with tap water. Is the drink now more dilute or more concentrated? Explain.
Read the data: drinks on the canteen shelf
Scenario: how sugary is each drink?
The canteen sells three drinks in different-sized bottles. To compare them fairly, the class works out the sugar in every 100 mL of each drink.
| Drink | Volume (mL) | Sugar dissolved (g) | Sugar in every 100 mL (g) |
|---|---|---|---|
| Iced tea | 500 | 30 | |
| Cola | 300 | 33 | |
| Sports drink | 600 | 36 |
Source: made-up drinks for this worksheet, close to typical labels.
4. Complete the last column of Table 1. Show your working for each drink.
5. Which drink has the most sugar in total, and which drink is the most concentrated? Explain why these are not the same drink.
6. Ben says: “Concentrated and saturated mean the same thing. The cola must be saturated, because it is the most concentrated drink.” At 20 °C, 100 mL of water can dissolve about 204 g of sugar. Evaluate Ben’s claim.
Order the steps: sugar dissolving
Reading down the table as printed, the correct step numbers are 3, 5, 1, 4, 2. In order: grain packed together; water particles bump it; sugar particles knocked loose; they slip into the gaps; all spread evenly and the solution looks clear.
Question 1
Water dissolves very many substances (1). Aqueous (1). [2 marks]
Question 2
Saturated (1): the water holds the most salt it can at that temperature, so the extra stays undissolved (1). [2 marks]
Cause and effect: growing sugar crystals
Box 1: as the water cools, the most sugar it can hold goes down (1). Box 2: the water now has more sugar than it can keep dissolved, so the extra comes out of solution (1). [2 marks]
Question 3
More dilute (1). The solute left in the bottle is spread through twice as much water, so each mL holds less (1). [2 marks]
Read the data: drinks on the canteen shelf
4. Iced tea 30 ÷ 500 × 100 = 6 g (1); cola 33 ÷ 300 × 100 = 11 g (1); sports drink 36 ÷ 600 × 100 = 6 g (1). [3 marks]
5. Most sugar in total: sports drink, 36 g (1). Most concentrated: cola, 11 g in every 100 mL (1). Concentration is how much solute is in a given volume; the sports drink’s sugar is spread through much more water (1). [3 marks]
Question 6
Marking guide: 1 Ben is wrong; 1 concentrated = a lot of solute dissolved; 1 saturated = the most solute the solvent can hold at that temperature; 1 cola’s 11 g per 100 mL is far below the 204 g limit, so not saturated; 1 test: stir in more sugar: if it dissolves, not saturated; if it stays on the bottom, saturated. [5 marks]
Model answer: Ben is wrong. Concentrated means a lot of solute is dissolved; saturated means the solvent holds the most it can at that temperature. The cola’s 11 g per 100 mL is far below the 204 g limit at 20 °C, so more sugar stirred in would dissolve. Saturation means reaching the solubility limit; it does not always mean concentrated, because a sparingly soluble substance can form a saturated but dilute solution. To test it, stir in more sugar: if it dissolves, the cola is not saturated; if it stays on the bottom, it is saturated.
Common error: taking the drink with the most sugar in total as the most concentrated.