1. Dissolving and the Particle Theory
Grade 7 Science: Pure Substance and Mixtures
Ms. Willis

2. Learning goals / success criteria
I can use the particle theory to explain how some substances become solutions (homogeneous mixtures) and others become mechanical mixtures (heterogeneous mixtures) when mixed.
Vocabulary: soluble, insoluble, saturated

3. How can we use the particle theory to explain the behaviour of different solutions?
Last day, you saw a demonstration of mixing sugar and pebbles in water. We started with 100 ml of water each time. We mixed the following:
150ml of water + 50ml of marbles = 180ml of mixture
150ml of water + 50ml of sugar = 190ml of solution
The volumes of the mixture/solution were less than we first hypothesized. Why do you think there is a difference?

4. How can we use the particle theory to explain the behaviour of different solutions?
Recall the 5 characteristics of the particle theory. One of the characteristics states that there are spaces between all particles.
When particles are attracted to each other, the particles move closer to each other. In the case of water and sugar, the smaller water particles fit between the larger water particles.

5. How can we use the particle theory to explain the behaviour of different solutions?
What happens when we mix sugar and oil?
Is the solution homogeneous or heterogeneous?
Can you use the particle theory to explain why?

6. How can we use the particle theory to explain the behaviour of different solutions?
When particles are not attracted to each other, they do not fill in the spaces.
Study the following example of coffee beans with water and oil solutes:

7. How can we use the particle theory to explain the behaviour of different solutions?
If a solute dissolves easily into a solvent, then we can say the particles of those two substances are attracted to each other and vice versa.
This explains why our sugar did not dissolve into the oil or why the pebbles did not dissolve in the water!

8. Vocabulary
Soluble: able to dissolve in a specified solvent (e.g. sugar is soluble in water)
Insoluble: unable to dissolve in a specified solvent (pebbles are insoluble in water)