Question Constellation Sign up for free AQA · GCSE · Chemistry · Higher
Hydrogen halide boiling trend
| Hydrogen halide | Boiling point in °C |
|---|---|
| HF | 20 |
| HCl | −85 |
| HBr | −67 |
| HI | −35 |
Describe how the boiling points of the hydrogen halides change as the relative formula mass changes.
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Marking points
- 1 Describes the general increase in boiling point with increasing relative formula mass.
- 2 Identifies HF as the anomaly with the highest boiling point.
Full-mark answer
From HCl to HI, boiling point increases as relative formula mass increases. HF is anomalous because it has the highest boiling point instead of following this trend.
Why this answer loses marks
The boiling point increases as relative formula mass increases.
This gains the trend mark but misses that HF is anomalous and has the highest boiling point.