Heat Questions and Answers – Page 6

The formula for heat absorbed or released by a substance is a fundamental equation in thermodynamics. Option A (Q = mcθ) is CORRECT — this is the correct formula where Q is the heat energy absorbed or released (in joules or calories), m is the mass of the substance (in kg or grams), c is the specific heat capacity of the substance (in J/kg·K or cal/g·°C), and θ (theta) is the change in temperature. This formula tells us that the heat absorbed depends on three factors: mass, specific heat, and temperature change. Option B (Q = mc²) is incorrect — this is Einstein's mass-energy equivalence formula (E = mc²) where c represents the speed of light (3 × 10⁸ m/s). It has no relation to heat absorbed by a substance in calorimetry. Option C (Q = mθ/c) is incorrect — dividing temperature change by specific heat gives the wrong relationship. Option D (Q = c/mθ) is incorrect — this inverted formula has no physical meaning in thermodynamics.

Normal human body temperature is a medically and scientifically important value. Option A (36°C) is incorrect — 36°C (96.8°F) is slightly below the normal range. While some individuals may have temperatures in the 36°C range (hypothermia threshold begins near 35°C), this is not the standard normal body temperature used in medical and exam contexts. Option B (37°C) is CORRECT — the normal human body temperature is 37°C = 98.6°F = 310 K. This is the internationally accepted standard normal core body temperature (oral measurement). The normal range is typically 36.1°C to 37.2°C, but 37°C is the standard value used in all competitive exams. A temperature of 38°C or above indicates a fever condition. It is above the normal body temperature and indicates a significant infection or illness.

Option A (0°C, 32°F, 273 K) is incorrect — these three values represent the freezing point of water, not the boiling point. 0°C = 32°F = 273 K is the freezing/melting point equivalence. Option B (37°C, 98.6°F, 310 K) is incorrect — these values represent the normal human body temperature, not the boiling point of water. 37°C = 98.6°F = 310 K is an extremely important set for health and medical topics in competitive exams. Option C (100°C, 180°F, 373 K) is incorrect — 100°C and 373 K are correct for the boiling point, but 180°F is WRONG. 180 is the number of Fahrenheit degrees between freezing (32°F) and boiling (212°F), not the boiling point itself. The boiling point in Fahrenheit is 212°F, not 180°F. Option D (100°C, 212°F, 373 K) is CORRECT — the boiling point of pure water at standard atmospheric pressure is 100°C = 212°F = 373.15 K (≈ 373 K). This is a fundamental fact tested in SSC, Railway, UPSC, and all government exams. Conversion check: F = (9/5)×100 + 32 = 180 + 32 = 212°F ✓; K = 100 + 273 = 373 K ✓.

The Kelvin scale is the SI unit of temperature and is used in all scientific calculations. Option A (273 K) is incorrect — 273 K (more precisely 273.15 K) is the freezing point of water, not the boiling point. 273 K = 0°C = 32°F. Students frequently confuse 273 K (freezing) with 373 K (boiling). Option B (300 K) is incorrect — 300 K equals approximately 27°C, which is normal room temperature in warm conditions. It has no significance as the boiling point of water. Room temperature is typically taken as 300 K in many physics calculations for convenience. Option C (373 K) is CORRECT — the boiling point of pure water at standard atmospheric pressure on the Kelvin scale is 373.15 K (rounded to 373 K in competitive exams). The formula: K = °C + 273; so 100 + 273 = 373 K. Option D (473 K) is incorrect — 473 K equals 200°C, well above the boiling point of water. 473 K has no special significance in standard thermometry. Exam trick: K = C + 273. Freezing = 0 + 273 = 273 K. Boiling = 100 + 273 = 373 K. Body temp = 37 + 273 = 310 K.

The Kelvin scale is the absolute temperature scale used in scientific calculations. Absolute zero is NOT the freezing point of water on the Kelvin scale; it is a fundamental minimum temperature. Option B (100 K) is incorrect — 100 K equals -173.15°C, which is an extremely cold temperature (below the boiling point of nitrogen at -196°C but not as cold as liquid helium). It has no special significance as the freezing point of water. Option C (273.15 K) is CORRECT — the freezing point of pure water on the Kelvin scale is 273.15 K (often approximated as 273 K in competitive exams). The Kelvin scale starts at absolute zero (0 K = -273.15°C) and its degree increments are the same size as Celsius degrees. The scale was developed by William Thomson (Lord Kelvin) in 1848. Option D (373.15 K) is incorrect — 373.15 K is the boiling point of water (100°C), not the freezing point. Freezing = 273.15 K; Boiling = 373.15 K.

The Reamur scale uses a specific range for its temperature divisions. Option A (40°R) is incorrect — 40°R on the Reamur scale equals 50°C, which is well below the boiling point of water. It has no special significance as a fixed point. Option B (60°R) is incorrect — 60°R equals 75°C, still below the boiling point of water. This is not the upper fixed point on any standard temperature scale. Option C (80°R) is CORRECT — the upper fixed point (boiling point of pure water) on the Reamur scale is 80°R. The Reamur scale has exactly 80 equal divisions between 0°R (freezing of water) and 80°R (boiling of water). 80°R = 100°C = 212°F = 373.15 K. The scale was proposed by René Antoine de Réaumur in 1730 and was widely used in France, Germany, and Russia before the Celsius scale became universal. Option D (100°R) is incorrect — 100 is the total number of divisions in the Celsius scale, not the boiling point in the Reamur scale.

The Reamur scale is a lesser-known temperature scale, but it appears in competitive exam content. Option A (0°R) is CORRECT — on the Reamur scale, the lower fixed point (freezing point of pure water) is 0°R. This is similar to the Celsius scale where 0°C is also the freezing point. The Reamur scale was proposed by René Antoine Ferchault de Réaumur in 1730. The scale has 80 equal divisions between the freezing point (0°R) and the boiling point (80°R) of water. Option B (32°R) is incorrect — 32 is the freezing point of water on the Fahrenheit scale, not the Reamur scale. Option C (80°R) is incorrect — 80°R is the upper fixed point (boiling point) of water on the Reamur scale, not the lower fixed point. The boiling point = 80°R corresponds to 100°C. Option D (273°R) is incorrect — 273 is associated with the Kelvin scale (0°C = 273 K), not the Reamur scale. There is no 273°R in standard thermometry. Exam tip: Reamur scale: 0°R (freezing) to 80°R (boiling) — 80 equal divisions.

The boiling point of water is a key fixed point on temperature scales. Option A (100°F) is incorrect — 100°F corresponds to approximately 37.8°C, which is close to normal human body temperature but is NOT the boiling point of water. 100°C (not 100°F) is the boiling point on the Celsius scale. Students frequently confuse 100°C with 100°F. Option B (180°F) is incorrect — 180 is significant on the Fahrenheit scale because it is the total number of divisions between the freezing point (32°F) and boiling point (212°F) of water: 212 - 32 = 180. Option C (200°F) is incorrect — 200°F ≈ 93.3°C, which is below the boiling point of water. Water would not yet be boiling at 200°F at standard atmospheric pressure. Option D (212°F) is CORRECT — the boiling point of pure water at standard atmospheric pressure (1 atm) on the Fahrenheit scale is 212°F. This corresponds to 100°C and 373.15 K. The Fahrenheit scale has 180 divisions between 32°F (freezing) and 212°F (boiling).

The Fahrenheit scale was developed by Daniel Gabriel Fahrenheit in 1724 and is commonly used in the United States. Option A (0°F) is incorrect — 0°F on the Fahrenheit scale corresponds to the freezing point of a brine solution (a mixture of ice, water, and ammonium chloride), which Fahrenheit used as the lower reference point in his original scale. It does NOT represent the freezing point of pure water. Option B (32°F) is CORRECT — the freezing point of pure water on the Fahrenheit scale is 32°F. This is the lower fixed point of the Fahrenheit thermometer scale used for calibration. Pure water freezes at 32°F = 0°C = 273.15 K. Option C (212°F) is incorrect — 212°F is the upper fixed point (boiling point of pure water) on the Fahrenheit scale. The Fahrenheit scale has 180 equal divisions between 32°F (freezing) and 212°F (boiling). Option D (273°F) is incorrect — 273 is associated with the Kelvin scale (0°C = 273 K), not Fahrenheit. 273°F would be extremely hot and has no fixed-point significance on the Fahrenheit scale. Exam tip: Always remember — Fahrenheit freezing = 32°F; Celsius freezing = 0°C; Kelvin freezing = 273 K.

The Celsius scale is the most commonly used temperature scale in everyday life and science. Option A (0°C) is incorrect — 0°C is the lower fixed point of the Celsius scale, which is the freezing/melting point of pure water at standard atmospheric pressure (1 atm). This is not the boiling point. Option B (32°C) is incorrect — 32°F is the freezing point of water on the Fahrenheit scale. Students often confuse Fahrenheit values with Celsius values. 32°C in Celsius is a warm temperature (~89.6°F) but is neither the freezing nor boiling point of water. Option C (100°C) is CORRECT — the upper fixed point of the Celsius scale is 100°C, which is the boiling point of pure water at standard atmospheric pressure (1 atm = 101.325 kPa). The Celsius scale was devised by Anders Celsius in 1742. It has 100 equal divisions between 0°C (freezing) and 100°C (boiling). Option D (212°C) is incorrect — 212°F is the boiling point of water on the Fahrenheit scale. Understanding the difference between Fahrenheit and Celsius boiling points is crucial for competitive exams.