Lesson focus: dissolvingWhen the particles of a solid (the solute) spread out between the particles of a liquid (the solvent) to make a solution, like sugar in tea. distributes soluteThe substance that dissolves, such as sugar in tea. particles among solventThe liquid that a substance dissolves in, such as water. particles without making them disappear.
Define solute, solvent and solutionA clear mixture made when a solute dissolves in a solvent, like salty water. and explain mass conservation in dissolving.
Build on earlier learning
MixturesExplain the solution formed during mixture separation.
You will be successful when you can…
- Define solute, solvent and solution.
- Explain dissolving with particles.
- Conserve mass in a closed system.
- Explain filter-paper failure.
Words that carry the science
- solute
- Substance that dissolves.
- solvent
- Liquid that dissolves a solute.
- solution
- Uniform mixture of dissolved solute in solvent.
- dissolving
- Process in which solute particles separate and mix among solvent particles.
- concentrated
- Containing a relatively large amount of solute per volume.
Track solute particles after they become invisible
01Three roles, one mixture
In salt water, salt is soluteThe substance that dissolves, such as sugar in tea., water is solventThe liquid that a substance dissolves in, such as water. and salt water is solutionA clear mixture made when a solute dissolves in a solvent, like salty water.. The terms describe roles in a particular mixture.
02Invisible is not absent
Solute particles separate and become distributed among solvent particles. In a closed system, total mass equals solute plus solvent mass. When salt dissolvingWhen the particles of a solid (the solute) spread out between the particles of a liquid (the solvent) to make a solution, like sugar in tea., it breaks up into particles that are too small to see. The water particles surround them and spread them out through the water. The salt is still there: you can taste it.
03Small dissolved particles pass filters
Ordinary filter pores retain insoluble particles but solution particles travel with solvent, so filtration does not separate salt water. Filter paper has tiny holes. Sand grains are too big to go through, but dissolved salt particles are so small that they pass through the holes with the water.
Dissolving sugar in water
The solute (sugar) breaks into particles that spread out between the particles of the solvent (water), making a solution. No mass is lost.
Water Sugar
Controls
Times are illustrative. Smaller pieces have more surface for the water particles to reach.
Predict solution mass
12 g salt dissolves in 88 g water in a sealed flask.
- Identify system as sealed.
- No matter enters or leaves.
- Add masses: 12 + 88.
- Solution mass = 100 g.
How to make the reasoning visible: Loss of visible crystals is not loss of matter.
Solution evidence
Track solute, solvent and mass.
| Mixture | Solute | Solvent | Expected result |
|---|---|---|---|
| 5 g salt + 95 g water | Salt | Water | 100 g solution |
| Sugar + tea | Sugar | Water-rich tea | Uniform sweet solution |
| Sand + water | None dissolved | Water | Suspension; sand settles |
| Salt water filtered | Salt | Water | Salt remains in filtrate |
Why does dissolved salt pass through filter paper?
How could salt be recovered?
Use the table. 5 g of salt dissolves in 95 g of water. What is the mass of the solution? Why?
Check your understanding
Answer from memory, then check against the suggested answer.
What is solute?
What is solvent?
What is solution?
What does dissolving mean?
What is a concentrated solution?
A learner says sugar vanished. What evidence refutes this?
Why can a transparent solution contain much solute?
A student says “the sugar has disappeared, so it is gone”. How could you show that the sugar is still there?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Salt dissolves in water. Answer the parts to explain what happens, and why filtering does not get the salt back.
What happens to the salt when it dissolves?
(1)What do the water particles do to the salt particles?
(1)Is the salt still there? How do you know?
(1)Why do the salt particles go through the filter paper?
(1)Total for this question: 4 marks
Command word: Keep track of the salt after you can no longer see it.