I cannot change models in mixed original practice

If mixed questions leave you stuck, the gap is usually choosing the model, not knowing the content. A question that joins, say, biology and a calculation gives no label telling you which idea to reach for.

This page gives a routine for switching models and a full worked example. It supports mixed original practice.

What is actually going wrong?

There are three places where students commonly stall:

1. One model for everything. The student uses the same idea for every part, even when the question has changed subject.
2. No decision made. The question is read, nothing is written, and the student waits for the right topic to appear.
3. Mixed-up answer. Two models appear in one sentence, so neither is clear.

The routine below gives each part its own model and a short link between them.

A routine that switches models

1. Underline the data and the question words. Numbers and verbs such as “explain”, “calculate” and “compare” show what is wanted.
2. Split the task into parts. Each part usually asks for one kind of thinking.
3. Name the model for each part. Write a few words: “water movement”, “percentage change”, “particles”.
4. Answer one part at a time. Keep each answer to its own model.
5. Link with one sentence. Say how the result of one part supports the next.
6. Check the units and the direction. Did the quantity rise or fall, and does your sentence say so?

Worked example

A student cuts a potato chip with a mass of 5.0 g and places it in a strong salt solution for one hour. Its mass afterwards is 4.2 g. Explain the change in mass and calculate the percentage change.

Steps 1 and 2, underline and split. The data are 5.0 g and 4.2 g. There are two parts: an explanation and a calculation.

Step 3, name the models. Part one needs a biological water-movement model: water passes through the partially permeable cell membranes. Part two needs a percentage-change calculation.

Step 4a, explain. The salt solution has a lower concentration of water than the potato cells. Water moves out of the cells into the solution by osmosis, through the partially permeable membranes. The chip loses water, so its mass decreases.

Step 4b, calculate. The change is 4.2 − 5.0 = −0.8 g. Percentage change = −0.8 ÷ 5.0 × 100 = −16%.

Step 5, link. The 16% decrease in mass agrees with water leaving the cells.

Independent check. 16% of 5.0 g is 0.16 × 5.0 = 0.8 g, and 5.0 − 0.8 = 4.2 g, which matches the measurement.

The mistake to watch for

A typical slip is to divide by the wrong mass.

Mistaken answer: “−0.8 ÷ 4.2 × 100 = −19%.”

The student divided by the final mass. A percentage change is measured against the starting mass, so 5.0 g is the divisor.

The correction is to write ”÷ original” beside the formula before you substitute. The explanation was correct, but the number would lose marks because the wrong model of “change” was applied.

Check yourself

1. A potato chip of mass 6.0 g is placed in pure water and gains 0.9 g. Give the percentage change in mass and say which way the water moved.

2. A chip starts at 8.0 g and ends at 6.8 g. Calculate the percentage change.

3. Write the model name for each part of this task: “Explain why the chip became firm, then calculate the percentage change in mass of a chip that goes from 4.0 g to 4.6 g.”

What can you do next?

Try the scientific investigation critic to practise linking evidence to an explanation. Then work through the biological mechanisms module for the ideas behind each model, and the original practice hub for more mixed tasks.

When the first line will not come, it helps to have someone ask “which idea fits this part?” at the right moment. That is what online one-to-one Combined Science tuition can provide, and a paid trial is a practical way to find out whether the pace suits you.