Temperature and its Measurement
Hotness, Coldness and Temperature
We often describe things as hot or cold — hot tea, cold ice, a warm day. Temperature is the measure of how hot or how cold an object is. The hotter an object, the higher its temperature.
Our sense of touch can tell us whether something is hot or cold, but it cannot give a reliable measurement, and it can even fool us. Try this: keep one hand in warm water and the other in cold water for a minute, then put both into the same bowl of normal water. The same water feels cold to one hand and warm to the other! This shows our skin is not a trustworthy way to measure temperature.
We also should never judge a very hot object by touching it — it could burn us. For these reasons we use an instrument called a thermometer to measure temperature accurately and safely.
Heat normally flows from a hotter object to a colder one until both reach the same temperature. That is why a hot cup of tea slowly cools down to room temperature, and a cold drink slowly warms up.
Touch gives only a feeling, not a number, and it can be misled.
- The same water can feel warm to one hand and cold to the other, depending on what the hand felt before.
- So touch cannot give a reliable measurement.
Heat flows from the hotter object to the cooler surroundings.
- Heat leaves the hot tea and goes to the cooler air and table.
- So the tea cools down towards room temperature.
Define the quantity.
- Temperature tells us how hot or cold an object is.
Key Points
- Temperature measures how hot or cold an object is.
- Touch can sense hot/cold but is not reliable and can be fooled.
- We use a thermometer to measure temperature accurately and safely.
- Heat flows from a hotter object to a colder one until both are equally warm.
Thermometers: Clinical and Laboratory
A thermometer is the instrument used to measure temperature. A common type has a thin glass tube with a small bulb at one end containing a liquid. When the bulb is warmed, the liquid expands and rises up the tube; when cooled, it falls. The level of the liquid against a marked scale gives the temperature.
There are two thermometers you should know:
- Clinical thermometer — used to measure the temperature of the human body. Its scale usually runs from about 35°C to 42°C, because body temperature stays within a small range. The normal temperature of a healthy human body is about 37°C (98.6°F). It has a small bend (a kink) near the bulb that stops the liquid from slipping back, so the reading can be read after the thermometer is taken out. It must be washed before and after use, and the reading shaken down before the next use.
- Laboratory thermometer — used in laboratories to measure the temperature of other things, such as liquids being heated. Its scale covers a much wider range, often −10°C to 110°C, and it has no kink.
Older thermometers used mercury, but because mercury is poisonous, modern thermometers often use a coloured alcohol liquid or are digital (showing the temperature as numbers on a screen), which are safer and easier to read.
Recall the standard body temperature.
- The normal human body temperature is about 37°C.
The kink controls the liquid after the thermometer is removed.
- It stops the liquid from flowing back down on its own.
- So the reading stays in place and can be read after taking the thermometer out of the mouth.
Match the instrument to its range.
- A clinical thermometer reads only up to about 42°C.
- Boiling water is at 100°C, far beyond this range, and could break the thermometer.
Key Points
- A thermometer measures temperature; the liquid rises as the bulb is warmed.
- A clinical thermometer (35–42°C, with a kink) measures body temperature; normal body temperature ≈ 37°C.
- A laboratory thermometer (about −10 to 110°C, no kink) measures other things.
- Modern thermometers use safer alcohol liquid or are digital, replacing poisonous mercury.
Temperature Scales: Celsius and Fahrenheit
To put a number on temperature, we use a scale marked on the thermometer. Two scales are commonly used.
- The Celsius scale (°C) is used most around the world and in science. On it, the temperature at which ice melts (the melting/freezing point of water) is fixed at 0°C, and the temperature at which water boils (the boiling point of water) is fixed at 100°C. The space between is divided into 100 equal divisions, each one degree Celsius.
- The Fahrenheit scale (°F) is used in a few countries (such as the USA). On it, water freezes at 32°F and boils at 212°F. The normal human body temperature of 37°C is the same as 98.6°F.
The two scales describe the same hotness in different numbers, just as a length can be written in metres or in feet. Scientists prefer the Celsius scale because it is simpler — based on the easy numbers 0 and 100 for the freezing and boiling of water.
The unit of temperature in everyday science is the degree (°), and we always write the scale with it, for example 25°C or 77°F, so the reading is clear.
Recall the two fixed points of the Celsius scale.
- Water freezes (ice melts) at 0°C.
- Water boils at 100°C.
Recall the matching values for body temperature.
- 37°C is the normal body temperature.
- On the Fahrenheit scale this equals 98.6°F.
Think about which scale uses simpler fixed points.
- On Celsius, water freezes at 0 and boils at 100 — easy, round numbers.
- This makes it simpler to use than Fahrenheit (32 and 212).
Key Points
- Temperature is read on a scale: Celsius (°C) or Fahrenheit (°F).
- On Celsius, water freezes at 0°C and boils at 100°C.
- On Fahrenheit, water freezes at 32°F and boils at 212°F; body temperature 37°C = 98.6°F.
- Scientists prefer Celsius because of its simple fixed points.