6.7 - Physical property trends in halogens

Physical properties down Group 7

A physical property can be observed or measured without making a new substance. Colour, state, melting point and boiling point are physical properties; reactivity is a chemical property.

From chlorine to bromine to iodine, the elements become darker. At room temperature chlorine is a pale green gas, bromine a red-brown liquid and iodine a grey-black solid. Both melting point and boiling point increase down Group 7. These trends describe the pure elements, rather than solutions of their compounds.

Using melting and boiling points

At ordinary atmospheric pressure, compare the temperature with the melting and boiling points:

• Below the melting point: solid.
• Between the melting and boiling points: liquid.
• Above the boiling point: gas.

These are rounded teaching values.

[DIAGRAM: asset_name: halogen_melting_boiling_trend: Halogen melting and boiling point trends - diagram 1; asset_slug: 153_6_7_physical_property_trends_in_halogens_diagram1; file: diagram_assets/153_6_7_physical_property_trends_in_halogens_diagram1.png; recommended_method: deterministic_drawn; description: Exact monochrome line graph showing chlorine, bromine and iodine on the x-axis and temperature in degrees C on the y-axis. Plot rounded melting points: chlorine -101, bromine -7, iodine 114. Plot rounded boiling points: chlorine -34, bromine 59, iodine 184. Include a horizontal room-temperature reference line at 20 degrees C. Use different monochrome line styles and direct text labels, not colour coding. Students should see that both melting point and boiling point increase down Group 7 and use the 20 degrees C line to explain why chlorine is a gas, bromine is a liquid and iodine is a solid at room temperature. Deterministic drawing is required because the axes, values and trend positions are assessed data content.]