Water is unsuitable for use as a thermometric liquid because it

Analyze the question: A thermometric liquid must change volume regularly (uniformly) with temperature so the scale can be evenly marked. We need the reason water fails this requirement.

Explain the concept: Most liquids, such as mercury and alcohol, expand steadily as temperature rises. Water does not. Between 0°C and 4°C, water contracts when heated, so its density rises and reaches a maximum at 4°C. Above 4°C it begins to expand, and the rate of expansion is not uniform. This is the anomalous expansion of water. Because the expansion is irregular, a thermometer with equal divisions on its scale would give wrong readings.

Evaluate the options:

• A. Expands unevenly between 0°C and 40°C: This is correct. Water contracts from 0°C to 4°C and then expands non-uniformly, so equal temperature changes do not give equal volume changes.
• B. Has narrow temperature range: Water's liquid range of 0°C to 100°C is fairly wide, and this is not the main reason it is unsuitable.
• C. Has a concave meniscus: Water does wet glass and form a concave meniscus, but this is a reading inconvenience and not the fundamental reason. Mercury has a convex meniscus yet is still usable.
• D. Maintains a fixed density: This is false. Water's density changes with temperature, and it is greatest at 4°C.

Answer: The correct option is A.

Exam tip: Remember the phrase "water is anomalous". Whenever a question asks why water is a poor thermometric liquid, think of its irregular expansion around 4°C. A good thermometric liquid needs uniform expansion, a wide liquid range, visibility, and quick response to temperature changes.