Science 7–10 · Year 10

Chlorine, bromine and iodine: reactivity down group 17 by displacement

Reactions

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

A more reactive halogen displaces a less reactive one from its salt, so a grid of halogen against halide solutions orders the group.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

chlorine and bromine vapours irritate the airways; both waters are harmful
iodine solution stains and is harmful

Controls

the whole tile is set out and read in a fume cupboard, with learners adding their drops there in turn (CSIS Appendix F: carry out all reactions which may result in evolution of chlorine in a fume cupboard); microscale drops only
eye protection; learners with asthma kept away from the fume cupboard while the halogen waters are in use
tile rinsed into a sodium thiosulfate quench beaker in the fume cupboard

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson. CSIS Appendix F lists bromine (pure liquid) and chlorine (gas) under chemicals to be used with caution, advises buying bromine water ready-made where that is all that is needed, and calls it the relatively harmless form for demonstrating bromine reactions; liquid bromine is never used here. CSIS Appendix F also warns that adding acid to bleach gives vigorous evolution of chlorine, so chlorine water is prepared only in a fume cupboard, never on the open bench.

What you need

chlorine water 0.1 percent w/v, prepared or dispensed only in a fume cupboard, and bromine water 0.1 percent w/v (bought ready-made, as CSIS advises), 2 drops per well
iodine solution 0.1 mol/L (in potassium iodide), 2 drops per well
potassium chloride, potassium bromide and potassium iodide solutions 0.1 mol/L and deionised water, 2 drops per well
spotting tile with at least 12 wells (3 rows of 4), dropping pipettes, white paper under the tile

How to do it

Set the tile out in a fume cupboard. Label the four columns chloride, bromide, iodide and water, and put 2 drops of each halide solution, or of deionised water, into the wells of its column.
Add 2 drops of chlorine water to each well of the first row, bromine water to the second row and iodine solution to the third row. The water column is the blank: the colour of each halogen solution on its own.
After 1 minute, compare each well with the blank at the end of its row against the white paper; a well that is clearly darker orange or brown than its blank shows that a halogen has been displaced.
Order the halogens by the number of displacements each causes and explain the trend using the outer electrons and atomic size.

What you should see

Against their blanks, chlorine water turns the bromide well orange and the iodide well brown (two displacements); bromine water turns only the iodide well brown, darker than its orange-yellow blank (one); every well in the iodine row matches its brown blank (zero); no halogen displaces its own halide. The learner knows it worked when the grid is a clean staircase: chlorine, bromine, iodine.

What changes

What you change: halogen and halide pairing
What you measure: colour change in the well
What you keep the same: drop counts
the same solutions in every row, with a water blank for each halogen
time before reading
white background

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Reactivity increases down every group.
The colour change is the halide salt dissolving.
Iodine is unreactive because it is a solid.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-RXN-01 SC5-WS-06
Australian Curriculum v9AC9S10U06AC9S10U07

Sources

The pages the author read to write this activity.

All Concept Studio activities