Heat Questions and Answers – Page 3

Different refrigeration systems use different refrigerants based on their scale and efficiency requirements. Option A (CFC) is incorrect — CFC (chlorofluorocarbon), also known as Freon, is used in DOMESTIC (household) refrigerators and small air conditioners. CFC is being phased out globally because it depletes the ozone layer. Option B (Ammonia NH₃) is CORRECT — the image states: बड़े शीतगृह संयंत्रों में प्रशीतक के रूप में अमोनिया का उपयोग करते हैं. Ammonia has excellent thermodynamic properties for large-scale refrigeration: very high latent heat of vaporization, low boiling point (-33°C at atmospheric pressure), and high coefficient of performance (COP). It is used in industrial cold storage, ice plants, food processing facilities, and large refrigeration systems. Option C (Nitrogen gas) is incorrect — liquid nitrogen is used for ultra-cold freezing (cryogenic applications like quick-freezing food, medical applications) but not as a standard refrigerant in cold storage refrigeration cycles. Option D (Carbon dioxide) is incorrect — CO₂ has been used historically as a refrigerant (R-744) and is making a comeback in some modern systems, but the standard answer for large cold storage in Indian competitive exams is Ammonia.

The temperature maintained in a refrigerator for food preservation is scientifically significant. Storing food at exactly 0°C risks freezing the food. While 0°C slows bacterial growth significantly, the image specifically gives 4°C as the safe refrigerator temperature. The standard safe refrigerator temperature is 4°C (40°F). At this temperature, bacterial growth is significantly slowed (most bacteria grow optimally between 4°C and 60°C — the danger zone). Additionally, 4°C is significant because it is the temperature of maximum density of water (water is densest at 4°C). Option C (10°C) is incorrect — 10°C is too warm for safe food storage. At this temperature, bacterial growth occurs rapidly. This is the temperature of a wine cooler or root cellar, not a standard refrigerator. Option D (-18°C) is incorrect — -18°C is the standard temperature for a FREEZER compartment, not the main refrigerator section. At -18°C, food freezes solid.

This question is about the physics of evaporation and latent heat. Option A (Cap prevents air from entering) is partially correct in the sense that the cap does seal the pen, but the scientific explanation given in the image is more specifically about latent heat of evaporation. While preventing air contact does reduce evaporation, the key concept tested is latent heat. Option B (Cap maintains high pressure) is incorrect — the cap does not significantly increase internal pressure. The pen is not an airtight pressure vessel. Pressure effects play a negligible role in keeping pen ink from drying compared to the latent heat mechanism. Option C (Ink absorbs latent heat from surroundings to evaporate, prevented by cap) is CORRECT — the image states: पेन का टुकड़ा ठंडा नहीं होने देता, क्योंकि — क्योंकि पेन से गुप्त ऊष्मा प्राप्त कर बर्फ पिघलने लगती है तथा ऊष्मा का हास होने से पेन ठंडा हो जाता है. When the cap is on, the air space inside is quickly saturated with vapor, stopping further evaporation. The cap essentially prevents the heat transfer mechanism needed for evaporation. Option D (Cap keeps pen warm) is incorrect — the cap is not a heater. Keeping the pen warm would actually increase evaporation, not prevent it. The explanation involves latent heat absorption, not warmth.

The behavior of biscuits in a refrigerator is a fascinating application of thermodynamics. Option A (Cold temperature hardens sugar) is incorrect — while cold can affect sugar crystallization, the primary reason biscuits become crispy in a refrigerator is related to moisture removal, not sugar hardening. Sugar chemistry alone does not explain the crispiness phenomenon. Option B (Moisture is absorbed and taken out in dry form) is CORRECT — the image states: मीठी बिस्कुट को फ्रिज के अन्दर रखने पर वह कुरकुरे हो जाते हैं, क्योंकि — आर्द्रता अवशोषित होकर द्रव रूप में बाहर निकाल दी जाती है. Soft or soggy biscuits contain moisture. When placed in a refrigerator, the cold, dry air inside absorbs this moisture from the biscuits. The moisture is then collected as condensation and drained. This is the same principle by which a refrigerator acts as a dehumidifier. Option C (Refrigerator adds preservatives) is incorrect — refrigerators do not add any chemical preservatives to food. They simply lower temperature to slow bacterial and enzymatic activity. Option D (Cold air makes biscuit porous) is incorrect — cold air does not create porosity in biscuits. The porosity in biscuits comes from the baking process, not from refrigeration.

The refrigerator is an essential household appliance working on thermodynamic principles. Option A (To freeze all food items) is incorrect — a regular refrigerator does NOT freeze food items. The main refrigerator compartment maintains temperatures above 0°C (usually 2-8°C) to preserve food without freezing. Option B (To maintain a uniform low temperature) is CORRECT — the image states: रेफ्रिजरेटर में थर्मोस्टेट का कार्य होता है — एक समान तापमान को बनाए रखना. The refrigerator's primary function is to maintain a uniformly low temperature inside to slow down bacterial growth and food spoilage. It uses a refrigerant (CFC in domestic models, ammonia in large cold storage) that absorbs heat from inside and releases it outside through a thermodynamic cycle involving evaporation and condensation. Option C (To kill bacteria using high heat) is incorrect — refrigerators use LOW temperature to slow bacterial activity, not high heat to kill bacteria. Pasteurization and autoclaving use high heat to kill bacteria, not refrigeration. Option D (To purify air inside) is incorrect — refrigerators do not purify air. They simply reduce temperature to preserve food. Some modern refrigerators have air purifiers but this is not the primary function.

A thermostat is an automatic temperature control device widely used in everyday appliances. Option A (To measure temperature) is incorrect — measuring temperature is the function of a thermometer, not a thermostat. A thermostat does sense temperature but its primary function is control, not measurement. Option B (To stabilize and maintain constant temperature) is CORRECT — the image states: थर्मोस्टेट यंत्र के क्या कार्य हैं — तापक्रम का स्थिरीकरण. A thermostat automatically controls heating or cooling equipment to maintain a set temperature. When temperature rises above the set point, the thermostat switches off the heater (or switches on the cooler). It uses a bimetallic strip that bends at a specific temperature to make or break an electrical circuit. Option C (To convert heat into electricity) is incorrect — converting heat to electricity is the function of a thermocouple (Seebeck effect) or thermoelectric generator, not a thermostat. Option D (To transfer heat from cold to hot) is incorrect — transferring heat from a cold body to a hot body (against natural flow) is the function of a refrigerator or heat pump, which requires external work.

When a substance is heated, thermal expansion occurs. Option A is incorrect — heating causes molecules to vibrate faster and move farther apart, so volume INCREASES, not decreases. Density = mass/volume; since mass stays constant but volume increases, density must DECREASE, not increase. Option B is incorrect — while volume does increase on heating (correct), density also increasing is wrong. Since density = mass/volume and mass is constant, when volume increases, density must decrease. Volume and density change in opposite directions upon heating. When heated, molecules gain kinetic energy and move farther apart, increasing volume while mass remains unchanged. Since density = mass/volume, with mass constant and volume increasing, density decreases. This explains why hot air rises (less dense), why ice floats on water (ice is less dense than liquid water), and why heated metals expand. Option D is incorrect — volume decreasing on heating would mean the substance is contracting, which is opposite to normal thermal expansion. Water between 0°C and 4°C is a notable exception where it contracts on heating, but the general rule is volume increases on heating.

Understanding the ratio of temperature scale divisions is essential for solving conversion problems in competitive exams. Option A (C : F : R = 5 : 9 : 4) is CORRECT — the number of divisions between the two fixed points (freezing and boiling of water) for each scale is: Celsius: 100 - 0 = 100 divisions, Fahrenheit: 212 - 32 = 180 divisions, Reamur: 80 -

The Fahrenheit equivalent of normal human body temperature is a frequently tested value in competitive exams. Option A (96.6°F) is incorrect — 96.6°F corresponds to approximately 35.9°C, which is slightly below the lower end of the normal body temperature range. A body temperature of 96.6°F would begin to indicate mild hypothermia or the person may be running a slightly subnormal temperature. Option B (97.6°F) is incorrect — 97.6°F corresponds to approximately 36.4°C, which is at the lower end of the normal range but is not the standard accepted normal body temperature value used in competitive exams and medical textbooks. Option C (98.6°F) is CORRECT — the normal human body temperature is 98.6°F = 37°C = 310 K. Option D (100°F) is incorrect — 100°F corresponds to approximately 37.8°C, which is above the normal range. 100°F is close to fever territory but is not the normal body temperature.

The freezing point of water is a fundamental fixed point used to define and calibrate all temperature scales. Option A (100°C, 212°F, 80°R, 373 K) is incorrect — these are the BOILING point values of water across all four scales, not the freezing point. Option B (37°C, 98.6°F, 0°R, 310 K) is incorrect — 37°C = 98.6°F = 310 K represents the normal human body temperature, not the freezing point of water. Option C (0°C, 32°F, 0°R, 273 K) is CORRECT — the freezing point of pure water at standard atmospheric pressure is: Celsius = 0°C (the lower fixed point of Celsius scale), Fahrenheit = 32°F (the lower fixed point of Fahrenheit scale), Reamur = 0°R (the lower fixed point of Reamur scale), Kelvin = 273.15 K ≈ 273 K (using K = C + 273 = 0 + 273 = 273). The image table shows Jal ka Ubalna (water boiling) and Jal ka Ubalna columns confirm these values. Option D (0°C, 32°F, 32°R, 273 K) is incorrect — 32°R is wrong. The Reamur scale's lower fixed point (freezing of water) is 0°R, not 32°R. The value 32 belongs to the Fahrenheit scale's freezing point.