Temperature and its Measurement — NCERT Solutions, AI Tutor & Practice

Understand: Temperature is a measure of the degree of hotness or coldness of an object or body.

Analyze: Our sense of touch can only tell us whether something feels hot or cold — it cannot give an exact value. It can also mislead us. For example, if you dip one hand in warm water and the other in cold water, and then place both hands in the same lukewarm water, the two hands sense different temperatures for the same water.

Conclude: Because our senses are unreliable and give only a rough, sometimes false idea, we use a thermometer to measure temperature accurately.

Instrument: A thermometer is used to measure temperature.

Units:

• The SI unit of temperature is kelvin (K).
• The commonly used unit in everyday life is degree Celsius ($^\circ$C).
• Another unit, degree Fahrenheit ($^\circ$F), is used in some countries.

Weather reports and doctors usually report temperature in $^\circ$C.

Structure:

• A clinical thermometer has a long, narrow glass tube called the stem.
• At one end is a bulb containing a liquid (earlier mercury; modern ones use a safer coloured liquid or are digital).
• Inside the stem is a fine capillary tube through which the liquid rises.
• There is a small kink (bend) near the bulb that prevents the liquid from flowing back quickly, so the reading can be noted after removing it.
• A scale is marked on the stem.

Range: A clinical thermometer usually measures from about $35\,^\circ$C to $42\,^\circ$C, because the human body temperature stays within this range.

Normal body temperature:

• In degree Celsius: about $37\,^\circ$C
• In degree Fahrenheit: about $98.6\,^\circ$F

This value may vary slightly from person to person and at different times of the day, but it stays close to $37\,^\circ$C. A temperature clearly above this may indicate fever.

Comparison:

| Feature | Clinical thermometer | Laboratory thermometer | |---|---|---| | Use | To measure body temperature | To measure temperature of other objects/liquids in experiments | | Range | About $35\,^\circ$C to $42\,^\circ$C | About $-10\,^\circ$C to $110\,^\circ$C | | Kink | Present (holds the reading) | Absent | | Reading method | Removed and then read | Read while the bulb is still in contact |

Conclude: The clinical thermometer is designed for the narrow range of body temperatures, while the laboratory thermometer covers a much wider range.

Precautions:

1. Wash the thermometer before and after use, preferably with an antiseptic solution.
2. Before using, check that the liquid level is below $35\,^\circ$C; if not, give it a few gentle jerks to bring it down.
3. Do not hold the thermometer by the bulb.
4. Read the thermometer keeping your eye directly in line with the level of the liquid (to avoid parallax error).
5. Do not use it to measure the temperature of very hot or very cold objects — it can break.
6. Handle it carefully so that it does not fall or get damaged.

Given: Patient's temperature $= 39\,^\circ$C

Normal body temperature $= 37\,^\circ$C

Difference: $39\,^\circ\text{C} - 37\,^\circ\text{C} = 2\,^\circ\text{C}$

The patient's temperature is $2\,^\circ$C higher than normal.

Conclusion: Since the temperature is above the normal value, it indicates the patient likely has a fever.

Understand: Mercury was used in thermometers because it is a shiny liquid metal that expands evenly on heating.

Analyze: However, mercury is poisonous (toxic). If a mercury thermometer breaks, the mercury can escape as harmful vapour or droplets, causing danger to health and to the environment.

Conclude: For safety, mercury thermometers are being replaced by:

• Digital thermometers, which show the reading directly on a screen.
• Thermometers that use a safer coloured liquid (such as alcohol-based liquid) instead of mercury.

Reading: Pure water boils at about $100\,^\circ$C, so a laboratory thermometer would read close to $100\,^\circ$C.

Why not a clinical thermometer:

• A clinical thermometer only measures up to about $42\,^\circ$C.
• The temperature of boiling water ($100\,^\circ$C) is far beyond this range.
• Using it for such a high temperature would cause the liquid to expand too much and the thermometer would break.

Conclude: A laboratory thermometer, with its wide range, is the correct choice for measuring boiling water.

Understand: To read a thermometer correctly, the eye must be kept exactly in line (at the same level) with the top of the liquid column.

Analyze:

• Riya kept her eyes above the level, so her line of sight is at an angle. This causes a parallax error, giving a wrong reading.
• Rahul kept his eyes in line with the liquid level, which is the correct way.

Conclude: Rahul's reading is correct, because reading in line with the liquid avoids parallax error and gives an accurate value.

Given:

• Afternoon temperature $= 40\,^\circ$C
• Night temperature $= 28\,^\circ$C

Fall in temperature: $40\,^\circ\text{C} - 28\,^\circ\text{C} = 12\,^\circ\text{C}$

Conclusion: The temperature fell by $12\,^\circ$C from afternoon to night.

Understand: A thermometer contains a liquid (like a coloured spirit or, earlier, mercury) sealed inside a thin tube.

Analyze: When the bulb is placed in contact with a hot object, the liquid gets heated. On heating, the liquid expands (its volume increases). Since it is inside a very narrow tube, this expansion makes the liquid rise up the tube.

Conclude: The rise of the liquid level shows a higher temperature; a fall shows a lower temperature. This expansion and contraction of the liquid is the working principle of a thermometer.