Thermometers & Temperature

• How thermometers measure the hotness of a body using the expansion of liquids (mercury or alcohol).
• Difference between clinical and laboratory thermometers — range, kink, and safe use.
• Reading the Celsius scale (°C): water freezes at 0 °C, boils at 100 °C; normal body temperature ≈ 37 °C.
• Distinguishing heat (energy in joules) from temperature (degree of hotness in °C).
• Converting between °C and kelvin: K = °C + 273.
• Safe handling of mercury thermometers and why digital thermometers are preferred in hospitals.

Level 1 — Core idea

1. Temperature — measure of hotness; SI unit kelvin (K); everyday use degrees Celsius (°C).

2. Clinical thermometer — range about 35 °C to 42 °C; has a kink near the bulb to hold the reading after removal from the mouth.

3. Laboratory thermometer — wider range (often −10 °C to 110 °C); no kink; must be read while bulb remains in liquid.

Level 2 — Process and representation

1. Mercury / alcohol — expand uniformly when heated; rise in a narrow capillary tube.

2. Normal body temperature — about 37 °C (98.6 °F); fever when reading stays above ~37.5 °C.

3. Heat flow — always from hotter body to colder body until thermal equilibrium.

Level 3 — Applications and NCERT links

1. Diagram (text) — bulb dipped in water → liquid column rises; mark at meniscus read at eye level.

2. Parallax error — avoided by reading with eye level with top of liquid column.

3. Real world (India) — digital thermometers in PHCs; mercury thermometers being phased out for safety.

NCERT Activity: Measuring temperature of warm and cold water

Materials: two beakers, warm water (~45 °C), cold water (~15 °C), laboratory thermometer.
Step 1 — Touch beakers (qualitative): warm feels hotter; touch is unreliable for exact values.
Step 2 — Dip thermometer bulb fully in warm water; wait 10–15 s; read level → e.g. 45 °C.
Step 3 — Transfer same thermometer to cold water; column falls → e.g. 15 °C.
Step 4 — Calculate difference: 45 − 15 = 30 °C.
Step 5 — Convert warm reading to kelvin: 45 + 273 = 318 K.
Step 6 — Never use lab thermometer for body; never hold clinical thermometer by bulb while reading.
Step 7 — Jerk clinical thermometer before use so mercury falls below 35 °C.
Conclusion: thermometer gives objective, comparable temperature readings.

• State normal human body temperature in °C.
• Why does a clinical thermometer have a kink?
• Water boils at ___ °C at sea level.
• Convert 25 °C to kelvin.
• Why should you not hold a clinical thermometer by the bulb?
• Convert 37 °C to °F (approx 98.6 °F).

Open the Practice tab for graded questions on Thermometers & Temperature.