NEET Physics - Thermometry, Thermal Expansion and Calorimetry
Thermometry – Complete Notes, Revision, Important Questions & Downloads
Thermometry is the measurement framework behind the subtopic Principle of Thermometry, where NEET tests whether you can choose a valid thermometric property, recognise why it must change uniformly with temperature, and calibrate the thermometer from fixed points. The core textbook relation is t°C = (x - x₀)/(x₁₀₀ - x₀) × 100, so a question may give capillary length, gas pressure at constant volume, or platinum-wire resistance and ask for the unknown temperature. NEET usually turns this topic into a short interpolation numerical or a concept check on standard thermometers rather than a long derivation. Read the symbol x as the measurable property, not as temperature itself; that is the step that prevents most option-level mistakes.
10 SubtopicsCalibration FormulaThermometric Property
Expected QuestionsQ
0-1
Thermometry is rarely isolated as a long standalone NEET problem, but the calibration equation and the idea of a standard thermometer do appear in direct concept MCQs.
Time Required⏱
35-45 min
Enough for one reading pass, one formula drill, and four to five calibration numericals using different thermometric properties.
Difficulty⚡
Easy-Medium
The algebra is short; the real difficulty is reading the given physical property correctly and mapping it to the calibration relation without mixing it up with temperature.
NRI USA Curriculum GapUS
Medium
Students from AP Physics 1 or sensor-based US school labs often know temperature measurement qualitatively, but NEET expects explicit fixed-point calibration using x₀, x₁₀₀, x, and sometimes the triple-point based modern form.
10Subtopics
5Practice Questions
4Free Downloads
45 minPrep Time
NEET Weightage - Thermometry
Thermometry, Thermal Expansion and Calorimetry (Chapter 12)| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 0 | 0 | |
| 2023 | 0 | 0 | |
| 2022 | 0 | 0 | |
| 2021 | 0 | 0 | |
| 2020 | 0 | 0 | |
| 2019 | 0 | 0 | |
| 6-Year Trend | 0-1 | 0-4 |
The direct NEET use of Thermometry is usually the interpolation formula t°C = (x - x₀)/(x₁₀₀ - x₀) × 100 when the thermometric property x is a length, pressure, volume, emf, or resistance reading.
The textbook emphasises that the thermometric property must vary linearly or at least uniformly with temperature; if the property is erratic, the calibration loses meaning.
Old thermometry uses the ice point and steam point, while modern thermometry uses the triple point calibration relation T = 273.16 (x/x_Tr); NEET can ask you to identify which fixed-point logic is being used.
The textbook emphasises that the thermometric property must vary linearly or at least uniformly with temperature; if the property is erratic, the calibration loses meaning.
Old thermometry uses the ice point and steam point, while modern thermometry uses the triple point calibration relation T = 273.16 (x/x_Tr); NEET can ask you to identify which fixed-point logic is being used.
0-0.2
Avg Questions / Year
0-4
Total Marks (6 yrs)
Irregular
Pattern
Easy
Difficulty
How to Revise Thermometry for NEET
1
Fix the two calibration equations in memory Memorise both t°C = (x - x₀)/(x₁₀₀ - x₀) × 100 and T = 273.16(x/x_Tr). Before substituting, check which reference points the question gives. The common trap is to use the Celsius equation when the data are referenced to the triple point instead of 0°C and 100°C.
2
Map x to the actual measurable property When the question gives capillary length, gas pressure at constant volume, gas volume at constant pressure, emf, or resistance, mark that quantity as x and write x₀ and x₁₀₀ beside it. The trap is reading x as temperature and then mixing the numerator and denominator.
3
Check uniform variation before trusting the scale Remember that thermometry works only when the chosen property changes regularly with temperature. If the option asks which property can define a good thermometer, choose the one with reproducible, nearly linear behaviour. The trap is selecting a property that changes with temperature but not in a stable, calibratable way.
4
Keep standard thermometers separate from ordinary liquid thermometers Gas thermometers and platinum resistance thermometers are treated as standards because their behaviour is reproducible over a wide range. If a NEET MCQ asks for the most reliable reference thermometer, recognise that this is a standards question, not a mercury-range question.
Download Study Notes - Thermometry
PDF · Cheat Sheet · MCQ Set · PYQFull Notes
Topic notes on Thermometry covering the principle of thermometry, thermometric properties, old fixed-point calibration, modern triple-point calibration, and standard thermometer choices.
Download PDFFormula Sheet
One-page sheet with the Celsius calibration relation, the modern thermometry relation, the fixed points used in each method, key formulas, conditions, and one worked example per subtopic.
Download PDFMCQ Practice
A compact MCQ set on choosing thermometric properties, calibrating an unknown thermometer, comparing old and modern thermometry, and selecting standard thermometers.
Download PDFPYQ
A PYQ-style revision pack focused on thermometer calibration, temperature-scale interpolation, and thermometer-selection traps from thermal-properties questions.
Download PDFSubtopics in Thermometry
2-Column Table| Column A | Column B |
|---|---|
Rapid Revision - Thermometry
Concept → Trap → Example1) Principle of Thermometry
Calibration and Standard ReferenceThe basic principle of thermometry is linear or regular variation of a measurable property x with temperature; in old thermometry t°C = (x - x₀)/(x₁₀₀ - x₀) × 100, and in modern thermometry T = 273.16(x/x_Tr).
- Choose x as the thermometric property itself: capillary length, gas pressure at constant volume, gas volume at constant pressure, resistance, or emf.
- Use the Celsius form only when the readings are given relative to the ice point and steam point; use the Kelvin form when the reading is given relative to the triple-point calibration.
- Common NEET trap: substituting temperature values directly in place of x and then inverting the ratio, which gives a numerically neat but physically wrong answer.
Example (NEET-style)If a liquid thermometer shows x₀ = 2.0 cm at 0°C, x₁₀₀ = 12.0 cm at 100°C, and x = 6.0 cm for an unknown state, then t = [(6.0 - 2.0)/(12.0 - 2.0)] × 100 = 40°C. If a modern thermometer has x = 20 and x_Tr = 80, then T = 273.16 × (20/80) = 68.29 K.
US Curriculum Gaps - Thermometry
NRI students often know temperature scales, but NEET Thermometry expects a more explicit calibration workflow than most US high-school courses emphasise.AP Physics 1: thermometer use is usually qualitative, not fixed-point calibration
AP Physics 1 treats temperature measurement as an experimental tool, but it usually does not ask students to derive a thermometer equation from x₀, x₁₀₀, and x. NEET expects that interpolation step to be immediate and calculator-light.
- You must translate the measured property into x before doing any temperature calculation.
- NEET questions may hide the property in a capillary-length or resistance reading rather than saying 'thermometric property' explicitly.
- Practise solving one old-thermometry and one modern-thermometry calibration question back to back.
Integrated Math III / Algebra 2: linear interpolation is taught mathematically, but not as thermometer calibration
US math courses teach proportional reasoning, yet they rarely connect it to lower fixed point, upper fixed point, and triple-point calibration. In NEET Physics, that algebra is embedded inside a physical measurement model and must be recognised instantly.
- Treat the thermometer scale as a line segment between two reference states, not as an isolated ratio problem.
- Write the numerator and denominator in the same physical units before simplifying.
- Memorise why standard thermometers are preferred: reproducibility matters as much as proportionality.
NEET-style Practice Questions - Thermometry
4 scenario-based MCQs1A liquid-in-glass thermometer has capillary length 3.0 cm at 0°C and 18.0 cm at 100°C. If the column length at an unknown temperature is 9.0 cm, what temperature should be assigned to the body?Calibration numerical
30°C
40°C
50°C
60°C
For old thermometry, use t = [(x - x₀)/(x₁₀₀ - x₀)] × 100. Here x = 9.0 cm, x₀ = 3.0 cm, and x₁₀₀ = 18.0 cm. Therefore t = [(9.0 - 3.0)/(18.0 - 3.0)] × 100 = (6/15) × 100 = 40°C, so option 2 is correct. The trap in this kind of question is to use x/x₁₀₀ = 9/18 and forget to subtract x₀. Because the lower fixed point is not zero length, you must work with the interval above x₀, not the absolute length from the bulb.
2A platinum resistance thermometer has resistance 100 Ω at 0°C and 138 Ω at 100°C. When placed in a bath, its resistance becomes 119 Ω. Assuming linear calibration in the working range, the bath temperature is:Resistance thermometer
45°C
50°C
55°C
60°C
Treat resistance as the thermometric property x. Then t = [(R - R₀)/(R₁₀₀ - R₀)] × 100 = [(119 - 100)/(138 - 100)] × 100 = (19/38) × 100 = 50°C. This is exactly why platinum resistance thermometers are used as standard thermometers: their change is reproducible and nearly linear over a useful temperature range. Options 45°C and 55°C come from arithmetic slips in the ratio, while 60°C is what you get if you mistake the interval as 19/32 or round carelessly before finishing the calculation.
3In modern thermometry a thermometer reads x = 24 units for an unknown state and x_Tr = 96 units at the triple point of water. The absolute temperature assigned to the unknown state is:Modern thermometry
54.63 K
68.29 K
81.95 K
96.00 K
Modern thermometry uses T = 273.16(x/x_Tr). Substituting the data gives T = 273.16 × (24/96) = 273.16 × 0.25 = 68.29 K. Option 54.63 K would follow from using 273 instead of 273.16 and another ratio mistake, option 81.95 K comes from taking 24/80 or 0.3 by guesswork, and option 96 K incorrectly treats x itself as the temperature. The point of the question is that the measured property is only proportional to temperature after the reference calibration is applied.
4A laboratory must choose one thermometer as a standard over a wide range because the measured property should be highly reproducible and closely tied to temperature. Which option best fits that requirement?Standard thermometer logic
Ordinary colored alcohol clinical thermometer
Mercury thermometer used only near room temperature
Platinum resistance or gas thermometer
Any thermometer whose bulb is large
The textbook and standard laboratory practice treat platinum resistance thermometers and gas thermometers as standard references because their thermometric properties are reproducible and can be calibrated very accurately. The question is not asking which thermometer is cheapest or most common; it is asking which one gives the most reliable thermometric property for defining temperature. Ordinary alcohol and mercury thermometers are useful working instruments, but they are not the reference standard for precision measurement over wide ranges. The large-bulb option is irrelevant because sensitivity alone does not guarantee standardisation or accuracy.
Practice Problems - Thermometry
Click "Reveal Answer" after attempting1A thermometer uses gas pressure at constant volume as its thermometric property. The pressure is 80 kPa at 0°C, 120 kPa at 100°C, and 92 kPa for an unknown body. Find the body temperature.
20°C
30°C
40°C
50°C
👁 Reveal Answer
Option (b): 30°C. Take x as pressure. Then t = [(x - x₀)/(x₁₀₀ - x₀)] × 100 = [(92 - 80)/(120 - 80)] × 100 = (12/40) × 100 = 30°C. The pressure itself is not the temperature; it is the calibrated property. This is why the subtraction from the lower fixed-point reading is essential.
2The resistance of a platinum wire is 50 Ω at 0°C and 90 Ω at 100°C. At what temperature will its resistance be 74 Ω if the thermometer is assumed linear in this interval?
55°C
60°C
65°C
70°C
👁 Reveal Answer
Option (b): 60°C. Using resistance as the thermometric property, t = [(74 - 50)/(90 - 50)] × 100 = (24/40) × 100 = 60°C. The calculation is a direct application of the calibration law. A common mistake is to divide 74 by 90 and ignore the fact that the scale begins at 50 Ω, not at zero resistance.
3A liquid thermometer reads x₀ = 1.5 cm at 0°C and x₁₀₀ = 11.5 cm at 100°C. If a feverish patient gives x = 5.5 cm, what is the thermometer reading?
35°C
40°C
45°C
50°C
👁 Reveal Answer
Option (b): 40°C. Substitute into t = [(x - x₀)/(x₁₀₀ - x₀)] × 100 = [(5.5 - 1.5)/(11.5 - 1.5)] × 100 = (4/10) × 100 = 40°C. The problem is a standard interpolation on a calibrated scale. It tests whether you can use the interval above the lower fixed point rather than the total observed length from zero on a ruler.
4For a modern thermometer, x_Tr = 64 units at the triple point and the reading at an unknown state is x = 48 units. What is the absolute temperature of the state?
136.58 K
204.87 K
273.16 K
307.31 K
👁 Reveal Answer
Option (b): 204.87 K. Use T = 273.16(x/x_Tr) = 273.16 × (48/64) = 273.16 × 0.75 = 204.87 K. The triple-point reading acts as the reference value, so the thermometer scale is proportional to x/x_Tr. Picking 273.16 K would mean x = x_Tr, which is not the case here.
5A student says that any property that changes with temperature can be used to build a good thermometer. Which correction is physically correct?
Correct, because change alone is enough
Correct, because only sensitivity matters
Wrong, because the property must change regularly and reproducibly with temperature
Wrong, because only liquids can be used as thermometric substances
👁 Reveal Answer
Option (c): Wrong, because the property must change regularly and reproducibly with temperature. The principle of thermometry is not mere variation but usable calibration. If the property changes irregularly or depends strongly on history, pressure, or contamination, the same reading will not correspond to a unique temperature. That is why gas pressure, gas volume, platinum resistance, and similar properties are preferred over arbitrary effects.
Physics - Thermometry Revision Checklist
Check off chapters as you revise
Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.
Frequently Asked Questions - Thermometry
Notes · Downloads · Revision · Important QuestionsWhat exactly is thermometry in this chapter?
Thermometry is the science of measuring temperature by linking temperature to a measurable physical property of a substance. In this topic, that property may be liquid length, gas pressure, gas volume, resistance, or emf. NEET expects you to treat temperature as the calibrated output and the measured property as the input variable x in the thermometer equation.
Why must a thermometric property vary regularly with temperature?
A thermometer is useful only if one reading corresponds to one temperature in a stable way. If the chosen property changes irregularly, the same reading may represent different temperatures under slightly different conditions. Regular and reproducible variation makes calibration possible, which is why thermometry is built on properties that follow a dependable trend with temperature.
Can any temperature-dependent property be used as a thermometer?
No. The property must not only depend on temperature but must do so in a measurable, reproducible, and nearly unique way over the working range. A property that changes with contamination, pressure changes, or hysteresis may respond to temperature, but it will not define a reliable temperature scale. In exam questions, the correct choice is the property that supports calibration, not merely sensitivity.
Why are gas thermometers and platinum resistance thermometers treated as standard thermometers?
They are treated as standards because their thermometric properties are highly reproducible and can be calibrated accurately over a broad range. A gas thermometer uses pressure or volume in a controlled condition, and a platinum resistance thermometer uses a resistance-temperature relation that is stable and precise. NEET may ask this as a concept question to separate standard reference thermometers from ordinary laboratory thermometers.
What is the difference between old thermometry and modern thermometry?
Old thermometry uses two fixed points, the ice point and steam point, and then interpolates linearly between them. Modern thermometry uses a reference connected with the triple point and writes temperature on an absolute scale. For NEET, the operational difference is simple: look at the data given in the question and choose the calibration formula that matches those reference readings.
Why do students lose marks in thermometry even though the formula is short?
Most errors come from reading the thermometer property incorrectly rather than from difficult algebra. Students often use x/x₁₀₀ instead of (x - x₀)/(x₁₀₀ - x₀), or they forget that x is the measured property and not temperature. Because the numbers are usually clean, a wrong setup can still produce an attractive option, which is why careful variable mapping matters more than speed here.
Is a temperature difference the same in Celsius and Kelvin?
Yes. Although the zero points of the Celsius and Kelvin scales are different, one degree Celsius and one kelvin have the same size. So a rise of 10°C is also a rise of 10 K. This matters when a NEET problem talks about change in temperature rather than absolute temperature, because the numerical difference stays the same on both scales.
How should I identify the thermometric property x in a question quickly?
Read the statement and underline the quantity that the thermometer actually measures: length, pressure, volume, resistance, or emf. Then rewrite the problem in the form x₀, x₁₀₀, and x before doing any calculation. This small translation step turns an unfamiliar-looking question into the same calibration formula every time.
Does modern SI still define kelvin through the triple point alone?
Current SI defines the kelvin through the fixed value of the Boltzmann constant, but school-level thermometry problems still use the triple-point calibration idea because it gives a clear operational reference for building the temperature scale. So for NEET preparation, solve the textbook thermometry equations exactly as given while keeping in mind that the modern SI definition is more fundamental.
For NRI / OCI / U.S.-Based Families
NEET NRI Counseling & Admission eBook Download
A practical guide covering sponsor rules, document checklist, verification traps, NRI quota reality, and step-by-step counselling flow. Designed to prevent last-minute rejections and wrong choice filling.
Sponsor + Proof ClarityDocuments ChecklistState-wise Traps
Tip: Keep this eBook open during verification + choice filling week for quick cross-checking.
NEET Prep (India + NRI-USA)
Schedule Trial Session For NEET Prep
Get a short diagnostic + study roadmap: syllabus gaps (NCERT vs U.S. curriculum), weak chapters, and the exact weekly plan needed to improve accuracy under time.
Gap MappingWeekly PlanAccuracy Fix
Best for: Students in Grade 10–12 (U.S. / India) who want a clear NEET timeline and daily practice structure.