A mixed question describes something that happens and asks you to explain or predict it. The skill is to name the model that explains it before you write anything.
Choose the model first, then the words. This lesson sits in mixed original practice.
How do I match a question to a model?
Look at what changes in the description, then ask which model explains that kind of change. The table below is a starter, built from invented descriptions.
| What the question describes | Model to consider |
|---|---|
| Something spreads or mixes without stirring | Particle model, diffusion |
| Reaction speeds up with heat, powder or concentration | Collision theory |
| Heat moves along a solid | Particle model of conduction |
| A cell swells or shrinks in a solution | Osmosis, water movement |
| A device gets warm while it works | Energy transfer and dissipation |
| A substance only works at a certain temperature or pH | Enzyme shape and active site |
This is a thinking aid, not a complete list. Add rows from your own errors.
Worked example
A drop of coloured dye is placed in still water. After ten minutes the colour has spread through the whole beaker without stirring. Explain this observation.
Step 1, what changes? The dye particles move from where there are many to where there are few.
Step 2, name the model. The particle model, specifically diffusion.
Step 3, build the answer. Particles in a liquid move randomly all the time. Dye particles collide with water particles and spread out. Over time there is a net movement from the region of higher concentration to the region of lower concentration, until the dye is evenly mixed.
Short form: observation (spreading), model (random motion of particles), link (net movement down a concentration difference).
The mistake to watch for
A common slip is to use a model from the topic you revised last, not the one the question needs.
Mistaken answer: “The dye spreads because of convection currents.”
The student has just revised convection in physics. Convection needs a heated fluid rising and falling, and nothing in the stem mentions heating.
The correction is to underline what the stem actually says. If it does not mention heat, convection is a weak guess. The words “without stirring” and “spread” point to diffusion.
Check yourself
All scenarios are invented.
1. Magnesium ribbon reacts more slowly with acid than magnesium powder of the same mass. Which model explains this?
Show answer
Collision theory. The powder has a larger surface area, so more particles are exposed and more collisions happen each second.
2. A metal spoon left in hot tea becomes warm at the handle. Which model, and which idea in that model?
Show answer
The particle model of conduction. Particles at the hot end vibrate more and pass energy to neighbouring particles along the spoon.
3. Potato cylinders gain mass in pure water. Which model, and what direction does the water move?
Show answer
Osmosis. Water moves into the cells because the water concentration outside is higher than inside.
Where this leads next
After you can name the model, practise reviewing errors by underlying skill. Use the scientific investigation critic when a task describes an experiment, and test yourself on the mixed practice set.
If you often reach for the wrong model, a teacher in online one-to-one Combined Science tuition can watch you choose and stop you at that moment.