All lessons › Challenge: Particle puzzles › Lesson 1
Australian primary science · Years 5–6 (challenge) · Lesson 1 of 3
The case of the missing millilitres
Inquiry question: How can 50 mL plus 50 mL make less than 100 mL?
Marked no materials needed
Using a model
What students learn
All matter is made of tiny particles with spaces between them. Particles attract each other, so different particles can pack together more closely when they mix. The mass stays the same even when the volume shrinks.
Success criteria
- I can describe what the particle model says about spaces and attraction between particles.
- I can use the data to show that mass stayed the same but volume did not.
- I can explain the missing millilitres using the particle model.
Preparation and materials
Before this lesson: Students can describe solids, liquids and gases using the particle model (Solids, liquids and gases, Year 5).
- iPads with Bottlebrush Education open (nothing else needed) — 13 (one per pair)
- Projector or big screen — 1
Teacher setup
- Open Materials & chemistry › Lab › Has the salt vanished? on your screen to demonstrate.
- Check the iPads are charged and set to Your year › Years 5–6.
Safety and adaptations
Follow school science and equipment risk assessment procedures for the materials used.
Review your school policies and curriculum requirements before teaching.
How to investigate
The lesson sequence should leave pupils with evidence to interpret, not only an activity to finish.
- Pairs study the data table and circle the result that doesn’t fit the ‘100 mL’ pattern.
- Compare the mass columns: did any matter disappear?
- Open Has the salt vanished? and look at all four views, including the particle model.
- Draw two particle pictures on the iPad or paper: 50 mL water and 50 mL alcohol before mixing, then after mixing.
- Write a claim, evidence and reasoning explanation of the missing millilitres.
Teacher modelling prompt
Show the supplied data on the big screen. Think aloud: “Water plus water gives exactly 100 mL. So mixing itself doesn’t lose liquid. But water plus alcohol gives about 96 mL. The mass is still the same before and after, so no stuff has gone. What could have changed? Maybe how tightly the particles are packed.” Then open Has the salt vanished? and show the particle view: the salt particles sit in among the water particles.
Record and explain evidence
Recording question: How can 50 mL plus 50 mL make less than 100 mL?
| Mix | Volume before | Volume after | Did mass change? | Particle explanation |
|---|---|---|---|---|
| ________________ | ________________ | ________________ | ________________ | ________________ |
Example recording headings; record actual observations, including inconclusive results.
Discussion prompt
Which claim fits all of the data, including the mass?
Provided example or reference data
Supplied data: a teacher’s mixing demonstration (rounded)
| Mix | Volume before mixing | Volume after mixing | Mass before | Mass after |
|---|---|---|---|---|
| 50 mL water + 50 mL water | 100 mL | 100 mL | 100 g | 100 g |
| 50 mL water + 50 mL alcohol (ethanol) | 100 mL | about 96 mL | about 90 g | about 90 g |
| Model: 50 mL dried peas + 50 mL rice | 100 mL | about 85–90 mL | not measured | not measured |
Check the data source in the teacher lesson. Illustrative or fictional data are not measurements collected by your class.
Check understanding
What should pupils know? What does the particle model say about the spaces between particles?
Explain with evidence: If 4 mL of liquid had evaporated, what would the mass column show? Use this to explain why evaporation is not the answer.
Apply the idea: Salt dissolves in a glass of water and the water level rises less than you would expect from the salt you added. Use the particle model to explain why.
Teacher answer guidance and saved classroom work are available through the main lesson experience where supported.