AeroLab Reading room

CHEM / Foundations

Chemical evidence and safe investigation

A fair investigation changes a planned variable and measures a response while controlling relevant conditions. Repeats help estimate random variation; a control experiment tests an alternative explanation.

Learning objectives

The big idea

A fair investigation changes a planned variable and measures a response while controlling relevant conditions. Repeats help estimate random variation; a control experiment tests an alternative explanation.

Chemical observations should distinguish what was seen from what was inferred. Real laboratory work requires teacher supervision, risk assessment, eye protection and approved disposal; these activities use diagrams or supplied data.

Worked example

Three reaction times are 18, 20 and 22 s. Find the mean.

Follow the solution

Add 18+20+22=60 s and divide by three readings.

Investigate and explain

Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.

Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.

Open the interactive investigation

Check your understanding

1. Which is an observation rather than an inference?

  1. A white precipitate appeared
  2. A chloride ion is certainly present
  3. The reaction mechanism is proven
Check the answer and reasoning

A white precipitate appeared

The precipitate is directly observed; its identity requires further evidence.

2. Three reaction times are 18, 20 and 22 s. Find the mean.

  1. 60 s
  2. 20 s
  3. 22 s
Check the answer and reasoning

20 s

Add 18+20+22=60 s and divide by three readings.

3. How should an unknown chemical be tested by taste?

  1. Taste a small amount
  2. Dilute it first
  3. Never taste it
Check the answer and reasoning

Never taste it

Unknown chemicals must not be tasted; use approved supervised analytical methods.