Heat & Thermodynamics — Chemistry

Q.1

WBCS Prelims 2022

Q35. Pure water freezes at a temperature—

A. 47 °F

B. 32 °F

C. 0 °F

D. 19 °F

Explanation

Why Correct: Pure water at standard atmospheric pressure freezes at exactly 32 degrees Fahrenheit, which equals 0 degrees Celsius.
Distractor Analysis: 47 °F equals approximately 8.3 °C, a typical cool room temperature. 0 °F equals -17.8 °C, a very cold temperature where saltwater might freeze. 19 °F equals -7.2 °C, a common freezing point for some aqueous solutions.
Takeaway: The Fahrenheit scale sets water's freezing point at 32°F and boiling point at 212°F, creating a 180-degree interval between phase changes.

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Q.2

WBCS Prelims 2018

If we add salt to pure water, its boiling point will

A. increase

B. decrease

C. remains same

D. None of the above

Explanation

Core Formula/Logic: Boiling point elevation: Adding a non-volatile solute increases boiling point because solute particles lower vapor pressure, requiring higher temperature for vapor pressure to equal atmospheric pressure.
Step-by-Step Solution: 1. Pure water boils at 100°C at 1 atm pressure. 2. Adding salt (NaCl) introduces Na+ and Cl- ions. 3. These ions interfere with water molecules escaping into vapor phase. 4. This lowers vapor pressure at any given temperature. 5. To reach atmospheric pressure (760 mmHg), temperature must increase above 100°C.
Common Pitfall: Confusing boiling point elevation with freezing point depression leads to choosing decrease. Thinking salt makes water boil faster at same temperature produces remains same.
Shortcut/Takeaway: Non-volatile solutes always elevate boiling point and depress freezing point. For every mole of particles per kg water, boiling point rises ~0.51°C.

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Q.3

WBCS Prelims 2007

What is the main property of mercury for using it in a clinical thermometer?

A. Good conductor of heat

B. High density

C. Good Conductor of electricity

D. High coefficient of expansion

Explanation

Why Correct: Mercury expands uniformly and significantly with temperature changes due to its high coefficient of expansion, making small temperature differences clearly visible in the narrow capillary of a thermometer.
Distractor Analysis: Good conductor of heat helps mercury respond quickly but isn't the primary reason for thermometer design. High density makes mercury settle but doesn't aid temperature measurement. Good conductor of electricity is irrelevant for clinical thermometers which measure temperature, not electrical properties.
Takeaway: Alcohol thermometers use colored alcohol with high expansion coefficient for low-temperature measurements where mercury would freeze.

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