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Group 17: Question 1

Syllabus 11.1, 11.2, 11.3, 11.4

Multiple choice AS 1 mark

A resealable glass ampoule found in a school stock cupboard has lost its label. At room temperature it contains a lustrous grey-black solid. When the ampoule is warmed gently in a water bath, the solid does not melt but instead gives off a distinctive violet vapour, which condenses back into dark solid crystals on the cooler glass above.

Which halogen is inside the ampoule?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the physical properties of the three Group 17 elements

At room temperature and pressure:

  • Cl2\text{Cl}_2 is a pale yellow-green gas.
  • Br2\text{Br}_2 is a red-brown, fuming liquid.
  • I2\text{I}_2 is a lustrous grey-black solid that readily sublimes on gentle heating, giving off a distinctive violet vapour which re-solidifies as dark crystals on a cooler surface.

Step 2: Explain why volatility falls down the group

Going down Group 17 from chlorine to iodine, the number of electrons in each diatomic molecule increases. This increases the strength of the instantaneous dipole–induced dipole (van der Waals’) forces between neighbouring molecules, since larger electron clouds are more easily distorted into temporary dipoles. Stronger intermolecular forces mean more energy is needed to separate the molecules, so melting and boiling points rise steadily down the group, and volatility (the tendency to vaporise) decreases. This is why chlorine is a gas, bromine a liquid, and iodine a solid at room temperature.

Step 3: Match the clues to iodine specifically

The ampoule contains a grey-black solid, ruling out chlorine (a gas) and bromine (a liquid) immediately. On warming, this solid gives off a violet vapour rather than melting into a liquid first: this sublimation behaviour, together with the characteristic violet colour of the vapour, is unique to iodine among the three halogens.

Why the other options are wrong

  • A (chlorine): chlorine cannot exist as a solid under normal laboratory conditions; it is a gas.
  • B (bromine): bromine is a liquid, not a solid, at room temperature, and its vapour is red-brown, not violet.
  • D (any of the three): while all three halogens follow the same trend in volatility, the specific solid state and specific violet vapour colour described are not shared by chlorine or bromine, they identify iodine uniquely.

Final answer

  • The ampoule contains iodine, option C.