Big picture
Temperature helps us describe how hot or cold something is. Our hands give useful everyday impressions, but they cannot measure temperature reliably or safely. A thermometer provides a reading on a defined scale so observations can be compared.
Temperature measurement is used in weather records, cooking, experiments and health care. This chapter explains suitable instruments, careful reading and the difference between a measured temperature and a guess based on touch. Practical safety and the instrument's instructions are part of correct measurement.
Learning goals
Distinguish temperature from heat and touch sensation.
Choose suitable thermometers and temperature scales.
Calculate least count and read scales correctly.
Describe safe laboratory and clinical measurement practices.
Interpret records and perform simple conversions.
Core ideas
1. Temperature and the limits of touch
Temperature measures how hot or cold a body is in a scientific sense. Heat is energy transferred because of a temperature difference; the two terms are not interchangeable. Heat usually transfers from a hotter body to a colder one when they can exchange energy.
Touch can mislead because the feeling depends on the condition of your hand and the rate of energy transfer. A metal object and a wooden object in the same room may feel different even when their temperatures are similar. Never test a possibly hot liquid by putting a finger into it. A teacher may demonstrate the limits of sensation using safely lukewarm and cool water, but no activity requires ice-cold or hot extremes. The conclusion is that a standard instrument gives a more dependable comparison than sensation alone.
2. Thermometers and common scales
A thermometer is an instrument for measuring temperature. Its design determines its usable range, resolution and method of use. Celsius, written °C, is common in school and weather readings. Fahrenheit, written °F, appears on some thermometers. Kelvin, written K without a degree sign, is the SI temperature unit.
For pure water near ordinary atmospheric pressure, melting ice is near 0°C and boiling water is near 100°C. These are reference conditions, not a claim that every sample boils at the same temperature everywhere. A temperature is incomplete without its scale. The number 40 alone could mean quite different conditions on Celsius and Fahrenheit scales. Write the value and unit together, and do not attach cm or kilograms to a thermometer reading.
3. Clinical and laboratory instruments
A clinical thermometer is designed for the body-temperature range and the method of use specified by its manufacturer. Digital clinical thermometers display an electronic reading. Some instruments use a non-contact infrared method with model-specific positioning and environmental requirements.
A laboratory thermometer is intended for experimental materials across a suitable range. Many school liquid-in-glass laboratory thermometers have a range approximately from −10°C to 110°C, but read the actual instrument rather than assuming every model is identical. A traditional mercury clinical thermometer may have a constriction that retains a reading; an ordinary laboratory thermometer need not. Do not shake or treat a digital model like a traditional liquid-in-glass one. Choose the instrument for the task instead of assuming that any thermometer can safely measure any object.
4. Reading a marked scale
To determine a scale's least count, divide the difference between two labelled marks by the number of equal intervals between them. Count spaces, not lines. If 20°C and 30°C have ten equal intervals, each interval is (30 − 20) ÷ 10 = 1°C.
If a liquid column ends at the sixth small interval above 20°C, the reading is 20 + 6 × 1 = 26°C. If the same labelled gap has only five intervals, each represents 10 ÷ 5 = 2°C. A third interval above 20°C would then also be 26°C. This shows why counting marks without calculating their value can give a wrong answer. View the scale at eye level to reduce parallax error, in which the apparent position changes because the eye is angled away from the level being read.
5. Using a laboratory thermometer safely
For an appropriate liquid-temperature activity, choose an instrument whose range covers the expected values. Lower its bulb into the liquid while keeping it clear of the vessel's sides and bottom. Keep the instrument upright when that is the intended design and wait until the reading stabilises.
For a thermometer that does not retain its reading, observe while the bulb remains in the liquid. Removing it lets the reading change towards the surrounding temperature. Do not hold the bulb in your fingers during measurement because your hand affects it. Support the instrument to avoid breakage and never use it as a stirring rod. If glass breaks, tell the teacher and do not handle fragments. Traditional mercury instruments require special care and appropriate adult handling; students should not attempt to collect spilled mercury.
6. Body temperature and responsible interpretation
Human body temperature varies with person, time, activity, measurement site and method. About 37°C is a commonly taught reference, not an exact value that every healthy reading must equal. A measurement should be interpreted in context and according to appropriate health guidance.
For a digital clinical thermometer, follow its instructions, use an approved measurement site, keep it clean and wait for the completion signal where applicable. Do not use a laboratory thermometer in the mouth. A non-contact thermometer also needs the specified distance and technique. Repeating a measurement incorrectly does not make it accurate. This chapter teaches instrument use, not diagnosis: do not infer a specific illness from one number or change treatment based on a classroom example. Seek responsible adult or qualified support for real health concerns.
7. Air temperature, weather and records
Air temperature should be measured under appropriate conditions rather than by leaving a thermometer on a sun-heated surface. Direct sunlight, nearby hot walls and handling can affect a reading. Weather observations require consistent methods so that comparisons reflect the air rather than different instrument exposure.
A maximum–minimum record shows the highest and lowest measured temperatures over a period. Suppose a day's maximum is 32°C and minimum is 21°C. The temperature range is 32 − 21 = 11°C. This range is not the same as a new temperature of 11°C at which the whole day occurred. In a table, record date, time, place, instrument and units where relevant. Compare like with like: an indoor afternoon reading is not automatically comparable with an outdoor early-morning reading.
8. Conversions and a worked measurement report
When conversion is requested, use °F = (9 ÷ 5 × °C) + 32. For 25°C, first calculate 9 ÷ 5 × 25 = 45; then add 32 to obtain 77°F. Conversely, °C = (°F − 32) × 5 ÷ 9. For 68°F, subtract 32 to get 36 and calculate 36 × 5 ÷ 9 = 20°C. Kelvin is related by K = °C + 273.15, so 0°C corresponds to 273.15 K.
A clear report might state: “Using a scale with one-degree intervals, the liquid column stabilised at 26°C while the bulb was immersed without touching the vessel.” This includes instrument resolution, observation and method. Do not claim more precision than the scale supports. If the task expects only Celsius at this level, the conversion examples are extension practice rather than a reason to overlook accurate basic readings.
A reliable approach to questions
Read the command word before choosing an answer. A definition needs the distinguishing feature; a comparison needs matching criteria on both sides; an application needs a reason linked to the situation.
Extract the supplied evidence and assumptions. Keep a small working table rather than relying on memory. If information is absent, state the limit instead of inventing a value or claiming certainty.
Work through the relevant rule one step at a time. For a program, record the changing state; for a scenario, connect the requirement to the feature that meets it. Explain why the closest alternative fails.
Check the result against the original question, including boundary cases, units and wording. Present a concise final answer followed by enough reasoning for another student to reproduce your conclusion.
Connections that matter
States of water links temperature with melting and boiling.
Mathematics supplies scale intervals, subtraction and conversion formulas.
Scientific investigation requires reliable measurements and recorded methods.
Common mistakes
Counting marks instead of intervals: use the spaces between labelled values.
Reading from an angle: keep the eye level with the scale.
Using every thermometer in the same way: follow the instrument’s design.
Treating 37°C as a universal exact value: body readings vary with context.
Revision checklist
I can distinguish temperature from heat and touch sensation.
I can choose suitable thermometers and temperature scales.
I can calculate least count and read scales correctly.
I can describe safe laboratory and clinical measurement practices.
I can interpret records and perform simple conversions.