STUDENT SPACE · TOPIC 4

Investigating and evaluating rates

Choose a method, control variables and use evidence from graphs.

The essentials

CO₂ escapes when marble reacts with acid, so the mass of the apparatus decreases.
Sulfur forms when sodium thiosulfate reacts with acid. Time how long it takes to obscure a cross.
Change one independent variable and measure gas volume or mass at regular intervals.

Key vocabulary

English	Chinese
control variable	控制变量
mass loss	质量损失
end point	终点
tangent	切线

Quick practice

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

Question 1

Give one limitation of the gas-syringe method and a specific improvement.

Need a hint for Question 1?

Identify the variable, measurement and control first. Distinguish mass remaining from mass lost.

Check Question 1

Gas may escape before sealing; use a sealed arrangement that allows mixing after sealing, or minimise and standardise the delay. Check connections for leaks.

Question 2

The mass falls from 120.50 to 120.02 g in 60 s. Find the average mass-loss rate.

Need a hint for Question 2?

Subtract final mass from initial mass, then divide by time.

Check Question 2

Mass lost = 0.48 g. Rate = 0.48 ÷ 60 = 0.0080 g s⁻¹.

Question 3

Two cross experiments take 80 s and 40 s at the same endpoint. Compare relative rates.

Need a hint for Question 3?

For the same endpoint, compare 1/time.

Check Question 3

1/40 is twice 1/80, so the 40 s experiment has twice the relative rate.

Use Lesson 4 in your worksheet for written practice. Your teacher will discuss those answers in class.

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