Introduction
Graph questions in PSLE Science often compare how two organisms respond to a factor such as temperature. The key idea is to find the overlap where both organisms can grow, then choose the option that is most suitable—usually a temperature that lies within the overlap and is close to each organism’s peak.
The Question
A graph shows number of bacteria (vertical axis) against temperature (°C) (horizontal axis) for Bacteria P and Bacteria Q.
Which temperature is the most suitable for where P and Q grow?
(Options include 20 °C, 25 °C, 30 °C, 35 °C.)
Step-by-Step Explanation
Step 1: Read each curve
- P grows from lower temperatures and peaks around the high-20s to ~30 °C.
- Q grows at slightly warmer temperatures and peaks around the low-30s.
Step 2: Find the overlap region
Both curves are above zero roughly between the high-20s and low-30s.
Step 3: Choose the best discrete option
Among the choices, 30 °C sits inside the overlap and is near both peaks, giving a high population for both P and Q.
✅ Answer: 30 °C.
If the prompt says “where both grow best together,” pick the common high point (or the closest option to it). Avoid temperatures where one curve is near zero.
Quick Concept Guide
| Idea | What it means | How to apply |
| Growth range | Temperatures where a curve is above zero | Pick a temperature inside both ranges |
| Optimum temperature | Peak of a curve (maximum growth) | Prefer a value near both peaks |
| Overlap | Where both curves exist together | That’s where both bacteria can grow |
| Discrete options | You must choose from given values | Pick the closest suitable number in the overlap |
Tips for Students
- Always read axis labels first.
- Circle or shade the overlap where both curves exist.
- When options are spaced out, choose the one inside the overlap and closest to both peaks.
For Parents
This skill blends graph reading and scientific reasoning. We train students to translate visual trends into clear decisions, avoiding guesses at isolated points.