Teach lesson
The Perfect Pink Challenge
A 25–35 minute mission: use a chemical alarm and stop a titration using visual evidence.
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Learning Outcomes
Explain why a liquid's appearance is not enough to identify its chemical properties.
Use an indicator colour change as the signal to stop the drops.
Record minimal evidence before adding drops and at the moment of stopping.
Communicate the result with a two-frame visual card and one sentence.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
1 · The mystery of a clear sample
5 min
Two liquids can fool your eyes
Imagine two beakers holding liquids that look clear and identical. One could be water,
while the other could have very different chemical properties. Looking at them is not enough
to know how they will behave.
That is why scientists never taste, touch or smell unknown liquids. They use tests and
signals. Some substances called indicators act like chemical alarms: they change
colour when the conditions in a liquid change.
You already know two things in this challenge: sample #1 is vinegar, and the indicator can
leave a pink signal. The mystery is spotting the exact moment: is it a flash of colour,
or has the colour spread out and remained? Your mission is to stop a titration — adding
one liquid to another drop by drop — at the first stable indicator signal. This is not a
race. The winning team is the one that observes most carefully. First predict when pink will
start to appear; then use evidence from the beaker to decide when to stop.
The real sample
A rule for safe investigation
This test uses a recording of a titration carried out with real equipment. You remotely control when the drops advance and stop, but the liquids are far away from your desk. Even so, always follow the scientific rule: never taste, touch or smell a laboratory liquid, even when you know its name.
One mission: with a partner or on your own
This mission is normally completed in pairs: one device and one submission. Agree on
your prediction and record together, then write both names on the final card.
If you are working on your own, you will still complete the whole mission: switch
between the two roles and write only your name. Your teacher will tell you if someone will
help by calling out the stopping signal.
Your prediction: when will pink start to appear?
Before opening the laboratory, when do you think you will start to see pink in the beaker?
2 · Remote mission: find the first stable signal
12 min
Before opening the lab: find where to look
In the next image, find the burette — the vertical tube — and the beaker below it.
This is the real equipment used to record the titration. You will remotely control when that sequence advances and stops. The beaker already contains the
sample and a few drops of indicator, so it can change colour. The other liquid will fall
from the burette, and your clue will appear in the beaker.
The real equipment behind your screen
The control you will use
The control on the left is a red faucet. Find it by its colour and position, not by a
label. Under it, the screen may say “Click to turn the faucet on” or
“Click to turn the faucet off”: both captions refer to the same red faucet. The icon changes position:
horizontal means closed, and vertical means open. One click opens it; another click
closes it. Do not use the A button in this mission.
Click to open; click to close
Two people, two pairs of eyes
- Controller: clicks the red faucet to start and stop the drops.
- Observer: watches the whole beaker. Ignore a patch of colour that appears for only a
moment and then disappears as the liquid mixes. Say “Stop!” when the new colour has
spread through the liquid and remains visible after mixing.
If you are working on your own, switch between the roles: prepare the control first, then
focus your eyes on the beaker.
If you find the colour change difficult to distinguish, take the controller role and
follow the spoken signal from your partner — or from your teacher if you are working alone.
The decision to close the faucet is still yours: you are assessed on using evidence to make
a decision, not on naming an exact shade.
Two on-screen words, in plain language
You do not need to memorise technical vocabulary for this mission. You only need to
recognise these two labels:
- Titrant: the liquid that will fall from above. Under Choose the titrant, select
NaOH 0.151 ± 0.002 mol/L.
- Analyte: the sample already in the beaker. Under Choose the analyte, select
Acetic acid #1 (the vinegar in the image).
If something does not go as expected
- The image freezes, disappears or stops updating: if the faucet was still open, the
controller clicks it once to close it. Do not click again: each click changes the state,
so clicking without seeing could reopen it. If the image returns, check that the faucet is
horizontal and that no drops are falling. Do not continue without a picture or restart the
test: tell your teacher, then record “we could not see it clearly”.
- The pink becomes strong before you close the faucet: do not repeat the test or invent
a perfect result. Close the faucet straight away with one click and check that no drops
are falling. Then record “the colour became strong before we closed the faucet” and
continue.
Open the laboratory and select “2 - Configuration”. The activity has already chosen the correct mode: colour will be your main clue.
Under “Choose the titrant”, select NaOH 0.151 ± 0.002 mol/L. Under “Choose the analyte”, select Acetic acid #1. You only need to recognise these two labels; you do not need to calculate anything.
Select “Start measurement”. Find the beaker camera labelled “Close view”. This is the close-up view to watch. Observe how the beaker looks before adding any drops, and check that the red faucet is horizontal and closed.
The controller clicks once: the faucet turns vertical and the drops begin. Keep your hand ready on the same control. Do not look away from the beaker: the change can happen quickly. Depending on the run, the signal may take from 1 to 5½ minutes to appear.
The observer watches the whole beaker. If a pink patch disappears as it mixes, wait. When the colour spreads through the liquid and remains visible after mixing, say “Stop!” The controller clicks again, then checks that no drops are falling and that the faucet is horizontal again.
Return to this tab, select “Mark practice as done” beside the laboratory and continue to “3 · Quick record”. In the “perfect pink”, *perfect* means stopping at the first signal that remains. Careful control and evidence matter, not speed.
3 · Quick record: two moments and one decision
6 min
Quick record
With your partner or on your own, complete the two fields with what you observed during
the run. Each field offers only the answers that make sense for that moment.
Before the drops and when you said ‘Stop!’
Complete one row only. Choose what you actually observed; if there was a problem with the image, do not invent a colour.
| Before adding drops | When you said ‘Stop!’ |
|---|---|
The moment to close the faucet
During the drops, a pink patch appeared but disappeared as the liquid mixed: you do not stop
yet. In which of these two states should you close the faucet? Choose the image that shows
the moment to stop. In the real laboratory, the colour may look lighter or darker than in
these drawings.
Return to your prediction
Complete one line using this structure: “My prediction was [your idea]. In the test [what you
saw]. Now I know [what you learnt].”
4 · Create your ‘Chemical Alarm’ card
7 min
Two-frame card
Create an individual or shared card, depending on how you worked. Write your name — or
both partners' names — and add two frames. You can draw them on paper and add a clear photo,
or represent them on screen using the words and brackets in the template:
1. BEFORE: show the colourless beaker.
2. ALARM: show the same beaker with the first pink that remains while it mixes, and add a
STOP signal.
Underneath, write one sentence only that begins:
> Looking at it at the start was not enough, but the indicator was our chemical alarm because…
You can copy and complete this template:
> Name(s): [write here]
>
> BEFORE: [colourless beaker]
>
> ALARM: [pink beaker] STOP
>
> Sentence: Looking at it at the start was not enough, but the indicator was our chemical alarm because…
If you make the card on screen, use the plain-text template. The card must show what you
observed, not a made-up number of drops.
5 · Ages 12–13 extension: what happened between the drops?
5 min
A qualitative model
The beaker contains an acidic solution and an indicator called phenolphthalein. A
base, NaOH, is added from the burette. When they mix, the acid and base react in a
process called neutralisation.
Phenolphthalein is colourless in acidic conditions and becomes pink when the mixture enters
its basic colour-change range. That is why the first pink that remains while mixing can be
used as the observed endpoint. It is a practical signal; on its own, it does not prove
that the liquid is exactly pH 7. The screen does not display pH in this mode: colour is
the clue, just as it was in the core mission.
What happened between the drops?
Which description best matches what happened?
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