Hot or cold, and temperature
After holding one hand in hot water and the other in cold water, the same room-temperature water feels cool to one hand and warm to the other.
Temperature tells how hot or how cold something is. Our sense of touch is not reliable for this because it compares with what the skin felt just before. A thermometer measures temperature correctly.
In the figure: The same water feels warm or cool depending on the hand’s past.
Rule 1: Why touch misleads — Skin compares, it does not measure.
- Easy: A steel plate feels colder than a wooden plate in the same room.
- Medium: Both plates actually have the same temperature.
- Exam-type: So we need an instrument, not fingers.
Rule 2: Temperature and heat — Temperature is a measure; it is not a material.
- Easy: Temperature says how hot; it is measured in degrees.
- Medium: A bigger vessel of water at the same temperature holds more heat.
- Exam-type: Use a thermometer whenever the exact value matters.
Never judge temperature by touch when the exact value is needed.
Saying that a steel plate is colder than a wooden one in the same room.
"Touch compares, a thermometer measures."
Practice questions
- 1Phiban measured Lambok's temperature using a —
- 2In Activity 7.1, the right hand was kept in warm water and the left in ice-cold water, then both in tap water. The tap water feels —
- 3A hotter body has —
- 4In Activity 7.1, container C holds —
- 5Activity 7.1 is done in two parts —
- 6What does Activity 7.1 show?
- 7A reliable measure of the hotness or coldness of a body is its —
- 8A device that measures temperature is called a —
Show answers (try them yourself first)
- 1thermometer
- 2cool to the right hand and warm to the left hand
- 3a higher temperature than a colder body
- 4ice-cold water
- 5prediction and observation
- 6We cannot always rely on our sense of touch to judge hotness or coldness
- 7temperature
- 8thermometer
Clinical thermometer
When you have a fever, the doctor or parent places a small thermometer under your tongue or arm to see how high your body temperature is.
A clinical thermometer measures the temperature of the human body. It reads from 35 °C to 42 °C. Normal body temperature is about 37 °C. It has a kink near the bulb that stops the mercury from falling back, so we can read it after taking it out. We must shake it down before use.
In the figure: The mercury stops at 37 °C; the kink holds it there until it is shaken.
Rule 1: Using it correctly — Follow the steps in order.
- Easy: Wash it with antiseptic and shake the mercury below 35 °C.
- Medium: Keep the bulb under the tongue or in the armpit for about a minute.
- Exam-type: Take it out and read the level of the mercury.
Rule 2: Care and safety — Mercury and glass need careful handling.
- Easy: Never wash it in hot water; it can break.
- Medium: Never use it to measure the temperature of hot water.
- Exam-type: If it breaks, keep away and tell an adult; mercury is harmful.
Read the thermometer with your eye level with the top of the mercury.
Not shaking the mercury down before taking the next reading.
"Shake, hold a minute, read, clean."
Practice questions
- 1A digital clinical thermometer runs on —
- 2A clinical thermometer measures temperature when it is placed —
- 3Mercury thermometers are being replaced by digital ones because mercury —
- 4The normal temperature of a healthy human body is taken to be —
- 5For small children or old people, a digital thermometer can also be placed in the —
- 6Clinical thermometers generally use the —
- 7The unit of temperature on the Celsius scale is denoted by —
- 8A digital thermometer determines temperature with the help of —
Show answers (try them yourself first)
- 1batteries
- 2in contact with the person's body
- 3is extremely toxic and difficult to dispose of if the thermometer breaks
- 437.0 °C
- 5armpit
- 6Celsius scale
- 7°C
- 8heat sensors
Temperature scales and units
The weather report in your city says 32 °C, while an American website may say 90 °F for the same day; both describe the same heat.
Temperature is measured in degree Celsius (°C) in most countries, in degree Fahrenheit (°F) in some, and in kelvin (K) in science. On the Celsius scale ice melts at 0 °C and water boils at 100 °C. A kelvin value is the Celsius value plus 273.
Formula: K = °C + 273
In the figure: The same three points read in °C, °F and K.
Rule 1: Fixed points — Two easy points define the Celsius scale.
- Easy: Ice melting is 0 °C (32 °F, 273 K).
- Medium: Body temperature is 37 °C (98.6 °F, 310 K).
- Exam-type: Water boiling is 100 °C (212 °F, 373 K).
Rule 2: Convert Celsius to kelvin — Add 273 to the Celsius value.
- Easy: 25 °C becomes 25 + 273 = 298 K.
- Medium: 0 °C becomes 273 K.
- Exam-type: The kelvin scale has no degree sign: write K, not °K.
Write the unit with every temperature: 37 °C, 98.6 °F, 310 K.
Mixing up °C and °F in the same calculation.
"Zero ice, hundred steam, add 273 for K."
Practice questions
- 1The unit of temperature on the Fahrenheit scale is denoted by —
- 237.0 °C is equivalent to —
- 3The symbol of kelvin is —
- 4The three temperature scales are named after —
- 5For temperatures more than one degree, the full form of the unit uses the plural —
- 6The scale generally used in scientific work is —
- 7The SI unit of temperature is —
- 8To convert Celsius to kelvin, we —
Show answers (try them yourself first)
- 1°F
- 298.6 °F
- 3K
- 4the scientists who developed them
- 5degrees
- 6Kelvin scale
- 7kelvin
- 8add 273.15
Temperature scales and units
In the app you can move this figure with your finger.
Open in the appLaboratory thermometer
To check how hot the water for a science experiment is, we dip a long thermometer in it, much longer than the one used for fever.
A laboratory thermometer measures the temperature of liquids and air. It reads from about −10 °C to 110 °C and has no kink, so the mercury falls when it is taken out; therefore it is read while the bulb is still in the liquid. Hold it upright, keep the bulb in the liquid but off the walls and base, and read at eye level.
In the figure: Bulb in the liquid, thermometer upright, eye level with the mercury.
Rule 1: Correct way to read — Each habit avoids a different error.
- Easy: Bulb touching the glass reads the glass temperature, not the liquid’s.
- Medium: Read with the eye in line with the top of the mercury to avoid a wrong reading.
- Exam-type: Do not take it out before reading; the mercury falls at once.
Rule 2: Clinical or laboratory? — Compare the two thermometers.
- Easy: Clinical: 35–42 °C, has a kink, used for the body.
- Medium: Laboratory: −10 to 110 °C, no kink, used for liquids and air.
- Exam-type: A clinical thermometer would break in boiling water.
Before reading, wait until the mercury stops rising.
Touching the bottom or wall of the beaker with the bulb.
"Upright, bulb in, eye level, read in place."
Practice questions
- 1A laboratory thermometer consists of a sealed narrow glass tube with —
- 2The liquid used in a laboratory thermometer is generally —
- 3The range of the laboratory thermometer in the chapter is —
- 4While immersed in water, the thermometer bulb should not touch —
- 5When the thermometer is taken out of the warm water, the liquid level —
- 6The temperature reading is the mark of the scale with which —
- 7The liquid column rises or falls with —
- 8Between 0 °C and 10 °C there are 10 small divisions. The smallest value this thermometer can read is —
Show answers (try them yourself first)
- 1a bulb at one end containing a liquid
- 2alcohol (coloured red) or mercury
- 3–10 °C to 110 °C
- 4the bottom or sides of the beaker
- 5begins to fall
- 6the top level of the liquid column coincides
- 7change in temperature
- 81 °C
Air temperature and weather
The morning is cool, the afternoon is hot and the night cools again; the day’s weather report gives the highest and lowest temperatures.
Air temperature changes through the day. It is usually highest in the afternoon and lowest just before sunrise. A maximum–minimum thermometer records both. To measure air temperature, keep the thermometer in the shade, away from walls, and out of direct sunlight.
In the figure: Temperature rises towards the afternoon and falls after it.
Rule 1: Reading a day’s record — A line graph shows the rise and fall clearly.
- Easy: The highest value is the maximum; here 35 °C at 3 pm.
- Medium: The lowest value is the minimum; here 22 °C early in the morning.
- Exam-type: Range of the day = 35 − 22 = 13 °C.
Rule 2: Why shade, why a weather record — Weather records help plan farming and travel.
- Easy: In the Sun the thermometer would show the Sun’s heat, not the air’s.
- Medium: Daily records, kept for many days, show the season’s pattern.
- Exam-type: Weather forecasts also use these temperature records.
Take readings at the same times each day to compare days fairly.
Keeping the thermometer in the Sun when measuring air temperature.
"Shade, same time, note max and min."
Practice questions
- 1Weather reports mention the —
- 2A room thermometer on a wall gives —
- 3Air temperature is monitored at —
- 4Her work reduced India's reliance on other nations for —
- 5Vaishnavi's temperature returned to normal on —
- 6As we approach the summer season, temperature generally —
- 7During the winter season, temperature generally —
- 8Air temperature data, with other parameters, is used for —
Show answers (try them yourself first)
- 1maximum and minimum air temperature of the day
- 2an approximate idea of the room temperature
- 3weather stations all over the world
- 4weather measurement instruments
- 5Day Three
- 6rises
- 7falls
- 8making weather forecasts