Temperature and its Measurement

After a hand has been in warm water, tap water may feel cool. After the other hand has been in cold water, the same tap water may feel warm. Activity 7.1 compares predictions with these observations: the two sensations do not mean that the same water has two temperatures.

Our recent experience affects the sensation of hotness or coldness. Touch alone is therefore not a reliable numerical measurement. If trying the comparison, use comfortably warm and cool water with supervision; never test dangerously hot or cold objects by touch.

Temperature expresses how hot or cold a body is. A thermometer measures it; the hotter of two bodies has the higher temperature when compared on the same scale. State the unit with the reading. Temperature is not a measure of the total amount of heat in an object.

A clinical thermometer is designed for human body temperature. A digital clinical thermometer uses a sensor, battery and display. A liquid-in-glass laboratory thermometer serves other measurements and is not a substitute for a clinical device in the mouth.

Older clinical thermometers contained mercury. A digital device avoids a mercury spill if the sensor housing breaks; it still needs correct use and care. Do not handle spilled mercury or broken glass—tell the responsible adult or laboratory staff. Non-contact infrared thermometers sense without touching the body; suitable equipment and its specific instructions matter.

NCERT uses 37.0 °C, equivalent to 98.6 °F, as an average reference for a healthy person. It is not an exact target for every person at every time. Age, activity, time of day, measurement site and method can affect readings. A single departure from 37.0 °C does not by itself establish illness or recovery.

The chapter describes a usual clinical range around 35 °C to 42 °C. Always check the actual device: this is not a claim that human temperatures can never fall outside that range. Ice water and boiling water are outside the intended range and purpose of an ordinary clinical thermometer.

Activity 7.2 records body temperatures using a digital clinical thermometer. The following is the chapter’s oral-measurement example for an appropriate user and device; other sites and devices have different instructions. Consent and hygiene matter when measuring another person.

1. 1

   Prepare the device

   Read its instructions, wash your hands and clean the tip as directed. Keep the display and non-waterproof parts out of water. Switch on or reset as instructed; do not hold the sensing tip.

2. 2

   Position and wait

   For a device intended for oral use, place the tip correctly under the tongue and close the lips. Wait for the specified signal rather than removing it early. Follow professional guidance for children or anyone unable to use the device safely.

3. 3

   Read and record

   Read the display with its °C or °F indication. Record the time and measurement site as well as the value so comparisons are meaningful. Do not apply a fixed correction to every underarm reading.

4. 4

   Clean and store

   Clean and dry the tip according to the manufacturer’s instructions and store the device safely. Use repeated observations to identify variation, not to make a diagnosis from a classroom record.

A numerical temperature is incomplete without its scale. The same temperature can have different numerical values: 37.0 °C and 98.6 °F describe the same reference, not two different degrees of hotness.

To convert a Celsius temperature to kelvin, add 273.15 to the Celsius number: 25 °C corresponds to 298.15 K. Going back, subtract 273.15. A change of 1 °C is the same size as a change of 1 K; do not add 273.15 when converting a temperature difference.

Write kelvin with a lower-case initial when naming the unit, and K as its symbol, without a degree sign. Leave a space between number and unit: 25 °C, 98.6 °F, 298.15 K. Symbols do not take a plural s.

Absolute zero is 0 K, equivalent to −273.15 °C. This is very different from 0 °C. The final box contrasts this with the Sun’s core, which NCERT describes as roughly 15 million degrees Celsius; the core is not the Sun’s visible surface.

A liquid-in-glass thermometer has a bulb connected to a narrow tube and a marked scale. The coloured liquid expands or contracts with temperature, moving the column up or down. Read the top of that column. Red liquid need not be mercury: the chapter also describes coloured alcohol.

Range means the lowest to highest temperatures the instrument is designed to measure. The laboratory example spans −10 °C to 110 °C. Other thermometers differ, so inspect the actual scale rather than memorising one range as universal.

The value of one division is the difference between two labelled values divided by the number of equal intervals between them. Between 0 °C and 10 °C with ten intervals, each interval is 1 °C. Count the gaps, not the number of boundary marks.

A scale with 50 equal intervals from 0 °C to 100 °C has 2 °C per interval. To draw a 0.5 °C scale from 10 °C to 20 °C, draw twenty intervals and twenty-one boundary marks. Finer divisions help resolve a reading but do not by themselves guarantee accuracy.

Activity 7.5 measures warm water with a laboratory thermometer. Handle glass carefully, avoid holding the bulb, and use a suitable safe temperature. The sensing bulb must measure the water, not the beaker wall or your fingers.

1. 1

   Place the bulb correctly

   Hold the thermometer vertically. Immerse the bulb as directed for the instrument, clear of the beaker’s bottom and sides.

2. 2

   Let the column settle

   Wait until the liquid column stops changing. Do not hurry the reading or assume the initial value is the water temperature.

3. 3

   Read at the correct level

   Keep your eye in line with the top of the liquid column to avoid a slanting view. Read the temperature while the bulb remains in the water.

4. 4

   Record before removing

   Record the value and unit. After removal from warm water into cooler air, the column can fall; that later value no longer represents the original water temperature.

NCERT’s teacher-led demonstration compares melting ice and boiling water. Under steady conditions, a pure substance can change state while its temperature remains approximately constant. Ice need not always be at 0 °C: it can be colder before melting. Boiling-water demonstrations require teacher supervision; do not handle the heated setup.

Activity 7.6 provides the readings below from Shillong. Their spread is 98.1 − 97.8 = 0.3 °C. Reading technique is one possible reason for a difference; timing, instrument resolution and conditions also need checking. The table alone does not identify which student was correct.

Pure water boils at about 100 °C at one atmosphere of pressure. Lower surrounding pressure lowers its boiling temperature. Do not mark every reading below 100 °C as a mistake, or infer an exact altitude from this classroom table. Pressure and dissolved substances matter.

A room thermometer gives an approximate air temperature at its location. A weather report’s daily maximum and minimum are the highest and lowest recorded over its stated period, not necessarily the temperature when you read the report.

For Activity 7.7, record the maximum and minimum for the same place for ten successive days, including dates, units and the source. Compare the day-to-day values; a daily range is maximum minus minimum. No real weather values should be filled in without observations or a dated report.

Temperature usually changes with weather and season. A ten-day record is not enough to establish a long-term climate trend. Weather stations measure several variables, not temperature alone, to support forecasting.

The further activity compares a room thermometer with a laboratory thermometer in nearby water at three times a day for two weeks. Record the actual readings and allow for different responses and local conditions; do not assume they must agree immediately.

Anna Mani and measurement

NCERT’s profile describes Anna Mani (1918–2001), known as the Weather Woman of India, and her work on weather instruments and wind and solar energy. Her example connects dependable measurements with scientific work and reduced reliance on imported instruments.

Question 9 provides a three-day temperature record for Vaishnavi. The same data are reorganised below for a narrow screen. Every recorded value is retained; this is a textbook exercise, not a live health record.

The highest recorded value is 40.0 °C on Day 1 at 7 pm. On Day 3 at 4 pm the record reaches the chapter’s 37.0 °C reference, and later readings are lower. That answers the table-reading question; it does not establish a clinical diagnosis or prove recovery.

Use the end questions to explain a choice, not merely recall a number. Refer to the instrument’s purpose, scale, range, intervals and measuring conditions.

1. Units and purpose · questions 1–4, 8 and 14

   Explain why 37 °C is a reference rather than an exact value for everyone. Name the scale when someone says “101 degrees”: in the body-temperature example Fahrenheit is intended, not Celsius. Explain why a clinical thermometer is unsuitable for ice water and why laboratory glass must not be used for body measurement.

2. Instrument readings · questions 5–7 and 10–13

   Check vertical placement, immersion and clearance from glass before reading. Practise drawing columns for 14 °C, 17 °C and 7.5 °C. Choose a suitable range and sufficiently fine divisions for 22.5 °C. Read each original figure from its own intervals rather than assuming every small mark means 1 °C.

3. Patterns and limits · question 9 and further learning

   Find the maximum and its time in the supplied record. For local temperature extremes, retain the station, date, source and measurement conditions. A warm or cold day is not automatically a record.

4. Further investigations

   Learn about animal temperature measurements from a veterinary professional rather than trying invasive procedures. For planets, compare authoritative temperature data with distance from the Sun, checking whether each value refers to a surface, atmosphere or average. Investigate differences rather than inventing a simple distance-only rule.

Choose an answer, check the explanation, and revisit the relevant concept if your reasoning differs.