I  It wets glass II It needs to be coloured III It has a low density Water is not suitable for use as a thermometric liquid because

Analysis of the Question

The question asks why water is unsuitable for use as a thermometric liquid (a liquid used inside a thermometer to measure temperature). We need to evaluate the three statements provided to see which ones represent valid reasons for water's unsuitability.

Concept Explanation

A good thermometric liquid must possess specific physical properties to be effective:

1. It should not wet the glass: If a liquid wets the glass, it leaves a film behind as it moves, making it difficult to get an accurate reading of the meniscus.
2. It should be easily visible: Thermometric liquids are usually colored (like alcohol) or opaque (like mercury) so the level can be read clearly against the scale.
3. It should have a uniform expansion: The liquid must expand and contract linearly with temperature changes.
4. It should have a high boiling point and a low freezing point: This ensures it remains liquid over a wide range of temperatures.

Step-by-Step Evaluation

• Statement I: It wets glass. This is true. Water has strong adhesive forces with glass, causing it to stick to the walls of the capillary tube. This makes it a poor choice for a thermometer.
• Statement II: It needs to be coloured. This is true. Pure water is transparent and colorless. To be used in a thermometer, it would need to be dyed to be visible against the scale. This is a practical disadvantage compared to mercury or alcohol.
• Statement III: It has a low density. This is false. Density is not a primary factor in determining whether a liquid is suitable for a thermometer. In fact, water’s density is not the reason it is avoided; its expansion properties and wetting behavior are the main issues.

Conclusion

Since statements I and II are the valid reasons why water is not ideal for thermometric purposes, the correct option is B.

Exam Tip: Remember the "Mercury vs. Alcohol" comparison often asked in JAMB. Mercury is preferred because it doesn't wet glass and has a high boiling point, while alcohol is preferred for very low temperatures because it has a very low freezing point. Water is never used because it wets glass, has a small range of expansion, and is transparent.