To test for pure water physically, which property is checked?

• A. Boiling point exactly 100°C.
• B. Melting point exactly 0°C.
• C. Density exactly 1.0 g/mL at all temperatures.
• D. Refractive index exactly 1.33.

Answer: (A)

The main idea here is that pure water has a characteristic boiling point when the pressure is standard (about 1 atmosphere). Measuring boiling point is a direct physical test for purity because impurities raise the boiling point, so pure water should boil at around 100°C under 1 atm. If a sample boils at 100°C, it strongly suggests it’s very pure; if it boils at a higher temperature, there are dissolved substances that raise the boiling point.

Other properties aren’t as reliable for purity checks. The density of water changes with temperature, not a fixed value at all temperatures. The melting point is 0°C for pure water, but impurities lower that point, so it’s not a stable indicator of purity under typical conditions. The refractive index of water is around 1.333, but it also varies with temperature and impurities, so it’s not as straightforward a purity test as the boiling point at 1 atm.