Materials Around Us · Lesson 5 of 7
Soluble and Insoluble Materials
“Investigate what dissolves in water, what remains separate, and why solubility matters in everyday life and living systems.”
• Explain what it means for a material to dissolve in water. • Distinguish soluble and insoluble materials using observations after stirring and waiting. • Use prediction and observation to investigate sugar, salt, chalk powder, sand and sawdust in water. • Explain that liquids and gases can also differ in how they mix with or dissolve in water. • Connect solubility with everyday examples such as lemonade, oral rehydration solution and dissolved oxygen in water.
What Happens When We Mix a Material with Water?
When sugar is stirred into water, the crystals seem to disappear. The sugar has not stopped existing; it has spread through the water so finely that the separate crystals are no longer visible. Salt behaves similarly. Other materials, such as sand, remain visible even after long stirring.
A material is soluble in water if it dissolves in water under the conditions of the observation.
A material is insoluble in water if it does not dissolve in water under the conditions of the observation.
The words soluble and insoluble describe behaviour with a particular liquid. In this lesson the main liquid is water. A material that does not dissolve in water might behave differently in another liquid, so it is good scientific practice to state the liquid when needed.
Predict, Stir, Wait, Observe
A simple investigation uses separate tumblers of water and small amounts of sugar, salt, chalk powder, sand and sawdust. Before stirring, make a prediction for each material. Then stir well, wait for a short time and record what is actually observed. Comparing prediction with observation is an important part of learning from evidence.
| Material | Likely observation after stirring and waiting | Classification in water |
|---|---|---|
| Sugar | The separate crystals are no longer visible | Soluble |
| Salt | The separate crystals are no longer visible | Soluble |
| Chalk powder | Solid particles remain or make the water cloudy | Insoluble |
| Sand | Grains remain visible and may settle | Insoluble |
| Sawdust | Pieces remain visible; some may float | Insoluble |
From Lemonade to ORS
Everyday drinks show solubility in action. In lemonade, sugar and salt can dissolve in water while lemon juice mixes with the liquid. The ability of water to dissolve many substances is important in daily life and in the functioning of living bodies.
Oral rehydration solution is another practical example. A home-prepared solution can be made by dissolving six level teaspoons of sugar and half a teaspoon of common salt in one litre of boiled and cooled water when an appropriate packaged solution is not available. The quantities matter because the solution is meant for rehydration, not simply for taste.
For treating dehydration, use a properly prepared ORS solution and follow health guidance. The textbook's home mixture uses six level teaspoons of sugar and half a teaspoon of common salt in one litre of boiled and cooled water; changing the amounts can make the mixture unsuitable.
Liquids Do Not All Behave the Same Way
Some liquids mix completely with water and form one uniform-looking liquid. Other liquids do not mix completely and may form a separate layer after the container is left undisturbed. Oil is a familiar example of a liquid that can remain separate from water.
This means solubility is not only a property discussed for solids. Liquids can also be compared by whether they mix with water. Exploring vinegar, honey or oil in water gives a broader picture of how different materials behave.
Problem
A small amount of mustard oil is shaken with water. After the mixture is left undisturbed, a separate layer appears. What does this tell you?
- 1.The shaking temporarily disperses the oil through the water.
- 2.After standing, the oil gathers into a separate layer rather than remaining completely mixed.
- 3.This observation shows that the oil does not dissolve completely in the water under these conditions.
Gases Can Dissolve Too
Gases can also dissolve in water. Oxygen from the air dissolves in water, and this dissolved oxygen is essential for many animals and plants living in ponds, rivers, lakes and oceans. We do not usually see the dissolved gas, but its presence has an important biological effect.
The key chapter idea is broader than 'some powders disappear'. Different kinds of materials—solids, liquids and gases—can show different degrees of mixing or dissolving in water.
When a soluble material 'disappears' in water, it has not vanished. It is still present in the mixture even though its separate particles or crystals are no longer visible.
Quiz
Which material is soluble in water under ordinary classroom conditions?
What is the best evidence that a material is insoluble in water in this activity?
Why is it useful to record a prediction before carrying out the mixing test?
What does a separate oil layer after standing suggest?
Why is dissolved oxygen in water important?
Practice Problems
- Predict and then classify sugar, salt, chalk powder, sand and sawdust as soluble or insoluble in water.
- Explain why saying 'the sugar vanished' is less accurate than saying 'the sugar dissolved'.
- Describe what you would observe if a liquid did not mix completely with water.
- Compare the behaviour of a soluble solid and an insoluble solid after stirring and waiting.
- Explain why dissolved oxygen matters to living things in water.
- A student mixes two materials and sees a clear uniform liquid after stirring. What further observation could help decide whether the mixing remains complete after standing?
- Explain how lemonade and oral rehydration solution both depend on the ability of substances to dissolve in water.
Key Takeaways
• Soluble materials dissolve in water; insoluble materials remain separate under the conditions of the test. • Sugar and salt are typical soluble examples, while sand, chalk powder and sawdust are typical insoluble examples. • Prediction, stirring, waiting and observation form a useful investigation sequence. • Liquids may mix completely with water or form separate layers. • Gases can dissolve in water; dissolved oxygen is important for aquatic life. • Dissolving does not mean that matter has vanished.