Senior 1 practice book · Unit 6 of 9

Temperature Measurement and Physical Properties of Matter

50 questions that complete the Senior 1 quiz for this unit: 27 core and 23 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

When you touch a cold metal bench, 'cold' flows from the metal into your hand.Only heat (energy) flows, always from the hotter body to the colder one. The metal feels cold because heat flows quickly out of your hand into the metal.
When a solid is heated, its particles get bigger, so the solid expands.The particles stay the same size. They vibrate more strongly and the average spacing between them grows, so the whole solid gets bigger.
If you keep heating boiling water harder, its temperature keeps rising above 100 °C.At normal pressure pure water stays at its boiling point while it boils; the extra energy turns more water into steam instead of raising the temperature.
Evaporation and boiling are the same thing.Evaporation happens at any temperature and only from the surface; boiling happens at one fixed temperature, with bubbles of vapour forming all through the liquid.
Every liquid always expands when it is heated.Water is an exception: between 0 °C and 4 °C it contracts when heated, so it has its greatest density at 4 °C.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Heat and temperature: the difference, heat flow and thermal equilibrium
Liquid-in-glass thermometers: clinical and laboratory types, mercury and alcohol, sensitivity
Other thermometers: digital, thermocouple, maximum-minimum (Six's) and bimetallic dial thermometers
Thermometric properties and choosing a suitable thermometer
Fixed points and calibrating an unmarked or faulty thermometer
Temperature scales and conversions: Celsius, kelvin, Fahrenheit, Réaumur
Kinetic theory of matter and the three states of matter
Brownian motion and diffusion
Changes of state: melting, freezing, boiling, condensation, sublimation and heating curves
Evaporation and boiling compared; cooling by evaporation
Effect of pressure and impurities on melting and boiling points
Thermal expansion of solids, liquids and gases: uses and problems
Anomalous expansion of water

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 23 harder ones are marked ★ Challenge.

1True or false
When a substance is heated, its particles themselves get bigger.
Show answer
Answer: False
Particles keep the same size; heating makes them move faster and spread out, so the spaces grow.
Common mistakeThis idea comes from seeing the whole object expand, but the expansion is due to bigger gaps, not bigger particles.
2Fill in the blank · ★ Challenge
Boiling takes place at a fixed temperature, and bubbles of vapour form ______ the liquid, not only at its surface.
Show answer
Answer: throughout (inside)
In boiling the vapour forms inside the whole liquid and rises as bubbles; evaporation only happens at the surface.
Common mistakeWriting 'on top of' describes evaporation; the bubbles inside the liquid are what make boiling different.
3True or false
When you hold a cold metal spoon, cold flows from the spoon into your hand.
Show answer
Answer: False
Heat flows from your warmer hand into the colder spoon; there is no such thing as 'cold' flowing.
Common mistakeLearners often imagine 'cold' as something that moves; only energy (heat) moves, from hot to cold.
4True or false · ★ Challenge
Water would make a good thermometric liquid for measuring temperatures between 0 °C and 10 °C.
Show answer
Answer: False
Between 0 °C and 4 °C water contracts on heating, so the same length could mean two different temperatures; it is also colourless and wets the glass.
Common mistakeLearners assume any liquid that expands will do; water's unusual expansion below 4 °C makes its readings ambiguous.
5True or false
Dissolving salt in water raises its boiling point and lowers its freezing point.
Show answer
Answer: True
Impurities such as salt raise the boiling point and lower the melting (freezing) point of water.
Common mistakeLearners often think impurities change both points in the same direction; they push them apart.
6Fill in the blank · ★ Challenge
Pollen grains in water show Brownian motion. If much larger pollen grains are used, the random movement becomes ______ noticeable.
Show answer
Answer: less
A large grain is hit by many water molecules on all sides at once, so the pushes nearly balance and it barely jiggles.
Common mistakeThinking bigger grains move more because they 'catch more hits' forgets that the hits from all sides cancel more fully on a large grain.
7True or false
While pure ice is melting, its temperature keeps rising as more heat is supplied.
Show answer
Answer: False
Pure ice stays at 0 °C until all of it has melted; the energy is used to change the state.
Common mistakeLearners link 'heating' with 'temperature rise' every time; during a change of state the temperature is constant.
8True or false
Most digital thermometers use a sensor whose electrical resistance changes with temperature.
Show answer
Answer: True
Digital thermometers contain a thermistor or a similar sensor; a circuit converts its resistance into a number on the display.
Common mistakeSome learners think digital thermometers contain a tiny liquid column; they use an electrical property, not expansion.
9Multiple choice · ★ Challenge
Washing is hung out to dry in Nyagatare. On which kind of day will it dry fastest?
1. AA cool, humid and still day
2. BA hot, humid and still day
3. CA cool, dry, still day
4. DA hot, dry and windy day
Show answer
Answer: D. A hot, dry and windy day
Evaporation is fastest when the temperature is high, the air is dry (low humidity) and wind carries the vapour away.
Common mistakeChoosing 'hot, humid, still' looks only at temperature; humid, still air near the clothes slows evaporation a lot.
10Fill in the blank
The change of state from gas to liquid is called ______.
Show answer
Answer: condensation
Condensation is gas → liquid; it releases energy to the surroundings.
Common mistakeWriting 'evaporation' gives the reverse process (liquid → gas).
11Multiple choice · ★ Challenge
Ammonia gas (light molecules) and hydrogen chloride gas (heavier molecules) are released at the same time at opposite ends of a long glass tube. Where does the white ring form where they meet?
1. ANearer the ammonia end, because heavier particles move faster
2. BExactly in the middle, because all gas particles move at the same speed
3. CNearer the hydrogen chloride end, because ammonia particles move faster
4. DAt either end at random, because diffusion has no fixed direction
Show answer
Answer: C. Nearer the hydrogen chloride end, because ammonia particles move faster
At the same temperature lighter molecules move faster, so ammonia travels further before meeting the hydrogen chloride, and the ring forms nearer the HCl end.
Common mistakeChoosing 'in the middle' assumes all gases diffuse equally fast; lighter particles diffuse faster.
12True or false
Steam above boiling water, rather than the water itself, is used for the upper fixed point because impurities in the water can raise its boiling point.
Show answer
Answer: True
Dissolved impurities raise the boiling point of water, but the steam above it is still at 100 °C at standard pressure.
Common mistakeLearners assume 'boiling water' and 'steam' are interchangeable; impure water may boil above 100 °C, so the steam gives a more reliable point.
13Multiple choice · ★ Challenge
Near the top of Mount Karisimbi (about 4500 m) water boils at about 85 °C. The best explanation is that:
1. AThe air is colder there, so the water cannot be heated above 85 °C
2. BThe water on a mountain contains more impurities than in a valley
3. CGravity is a little weaker on the mountain, so the steam escapes more easily
4. DThe air pressure is lower there, so water boils at a lower temperature
Show answer
Answer: D. The air pressure is lower there, so water boils at a lower temperature
Boiling point falls as the pressure above the liquid falls; at high altitude the atmospheric pressure is lower.
Common mistakeChoosing 'the air is colder' confuses the surrounding temperature with the boiling point; it is the lower pressure that lowers the boiling point.
14Fill in the blank
Ice floats on water because ice is ______ dense than liquid water.
Show answer
Answer: less
Ice has a larger volume than the same mass of water, so its density is lower and it floats.
Common mistakeWriting 'more' would make ice sink; ice floats because its density is about 0.92 g/cm³, less than water's 1.0 g/cm³.
15Multiple choice · ★ Challenge
Which of these temperatures is the highest?
1. A22 °Re
2. B300 K
3. C80 °F
4. D25.5 °C
Show answer
Answer: A. 22 °Re
In °C: 22 °Re = 22 × 5/4 = 27.5 °C; 300 K = 27 °C; 80 °F = 5/9 × 48 ≈ 26.7 °C; so 22 °Re is highest.
Common mistakeChoosing 300 K because 300 is the biggest number ignores that different scales have different zeros and degree sizes; convert all to one scale first.
16Fill in the blank
A flask with a tube dipping into water is warmed with the hands, and bubbles come out of the tube because the air inside ______.
Show answer
Answer: expands
Gases expand a lot even for a small temperature rise, so the warmed air pushes out through the tube.
Common mistakeWriting 'evaporates' confuses air with a liquid; air is already a gas and simply expands.
17Multiple choice · ★ Challenge
An oven has a dial thermometer containing a coiled bimetallic strip. Why does the pointer move as the oven gets hotter?
1. AThe two metals soften slightly and the warm, softened coil sags downwards
2. BMercury inside the coil expands and pushes the pointer round
3. CThe metals become magnetic when hot and attract the pointer
4. DThe coil curls up or unwinds, because one metal expands more than the other
Show answer
Answer: D. The coil curls up or unwinds, because one metal expands more than the other
The two metals expand by different amounts, so the coil winds up or unwinds as temperature changes and turns the pointer fixed to its end.
Common mistakeChoosing the mercury option assumes all thermometers use a liquid; a bimetallic thermometer has no liquid and works by unequal expansion of two solids.
18Multiple choice
Which of these is NOT a thermometric property?
1. AThe volume of a liquid in a thin tube
2. BThe electrical resistance of a platinum wire
3. CThe mass of a fixed amount of mercury
4. DThe pressure of a gas kept at constant volume
Show answer
Answer: C. The mass of a fixed amount of mercury
The mass of a fixed amount of mercury does not change with temperature, so it cannot be used to measure temperature; the other three change steadily.
Common mistakeChoosing 'electrical resistance' forgets that resistance thermometers exist; resistance changes with temperature, but mass does not.
19Short answer · ★ Challenge
State two properties that a good thermometric property should have, and give one example of a thermometric property other than the expansion of a liquid.
Show answer
Model answer: It should change continuously and uniformly (by equal amounts for equal temperature changes) over a wide range, and it should be easy to measure accurately. Example: the electrical resistance of a metal wire (or the voltage of a thermocouple, or the pressure of a gas at constant volume).
Common mistakeLearners often name 'colour' or 'mass' as thermometric properties; a thermometric property must change steadily with temperature.
20Fill in the blank
A thermometer made of two wires of different metals joined together, which produces a small voltage that depends on temperature, is called a ______.
Show answer
Answer: thermocouple
A thermocouple gives a voltage that changes with the temperature difference between its junctions.
Common mistakeWriting 'bimetallic strip' mixes up two devices: a bimetallic strip bends, while a thermocouple produces a voltage.
21Multiple choice · ★ Challenge
A clinical thermometer usually has a scale from only about 35 °C to 42 °C. Why is such a short range chosen?
1. AMercury freezes below 35 °C and boils above 42 °C, so it cannot be used at all outside this range
2. BA short scale makes the thermometer cheaper, but it gives readings that are less accurate
3. CDoctors are only interested in fevers, so temperatures below 37 °C are never measured
4. DHuman body temperature stays within this range, so a short range allows widely spaced, fine divisions
Show answer
Answer: D. Human body temperature stays within this range, so a short range allows widely spaced, fine divisions
A living person's temperature does not go far outside 35–42 °C, so the whole scale is used for this range and each 0.1 °C can be read.
Common mistakeChoosing the mercury option ignores that mercury freezes at −39 °C and boils at about 357 °C; the range is short to make the scale more precise, not because of mercury.
22Multiple choice
Which statement is true of evaporation but NOT of boiling?
1. AIt happens at any temperature, from the surface only
2. BIt needs a source of heat kept at a fixed, high temperature
3. CBubbles of vapour form all through the liquid
4. DIt happens only at the boiling point of the liquid
Show answer
Answer: A. It happens at any temperature, from the surface only
Evaporation happens from the surface at any temperature; boiling happens at one temperature with bubbles throughout the liquid.
Common mistakeChoosing 'bubbles all through the liquid' describes boiling, the opposite of what was asked.
23Multiple choice · ★ Challenge
Data: alcohol freezes at −115 °C and boils at 78 °C; mercury freezes at −39 °C and boils at 357 °C. Which thermometer could measure cooking oil heated to about 250 °C?
1. AAn alcohol thermometer with a scale up to 300 °C
2. BA mercury thermometer with a scale reaching 300 °C
3. CA clinical thermometer, because it holds its reading
4. DAny of these, because all thermometers expand when hot
Show answer
Answer: B. A mercury thermometer with a scale reaching 300 °C
250 °C is below the boiling point of mercury (357 °C) but far above that of alcohol (78 °C), so only mercury stays liquid.
Common mistakeChoosing alcohol because the scale goes up to 300 °C ignores the liquid itself: alcohol would boil at 78 °C and the thermometer could burst.
24Multiple choice
A pressure cooker cooks beans faster than an open pan because:
1. AThe steam inside is colder, so the beans absorb more of the energy
2. BThe lid stops all heat from leaving, so the water never boils
3. CThe higher pressure inside raises the boiling point of the water above 100 °C
4. DThe higher pressure lowers the boiling point of the water, so it boils much sooner
Show answer
Answer: C. The higher pressure inside raises the boiling point of the water above 100 °C
Raising the pressure raises the boiling point, so the water and the beans get hotter than 100 °C and cook faster.
Common mistakeChoosing 'lowers the boiling point' reverses the rule: more pressure means a higher boiling point.
25Multiple choice
Changing directly from a solid to a gas, without becoming a liquid, is called:
1. ASublimation
2. BEvaporation
3. CCondensation
4. DMelting
Show answer
Answer: A. Sublimation
Sublimation is solid → gas directly, as with solid carbon dioxide (dry ice) or iodine crystals.
Common mistakeChoosing evaporation is tempting, but evaporation is liquid → gas, not solid → gas.
26Short answer · ★ Challenge
Use the kinetic theory to explain why sweating cools the body.
Show answer
Model answer: In the sweat, the fastest-moving (most energetic) molecules escape from the surface. The molecules left behind have a lower average kinetic energy, so the sweat is cooler, and it takes heat from the skin.
Common mistakeSaying 'sweat is cold water' misses the point: cooling happens because the most energetic molecules leave during evaporation.
27Multiple choice
A tight metal lid on a glass jar can be loosened by holding it under hot water because:
1. AThe metal lid expands more than the glass, so the lid becomes looser
2. BThe glass expands more than the metal, so the jar pushes the lid off
3. CThe hot water makes the lid softer, so it bends open more easily
4. DThe air inside the jar contracts and pulls the lid down tighter
Show answer
Answer: A. The metal lid expands more than the glass, so the lid becomes looser
For the same temperature rise, metal expands more than glass, so the lid grows away from the rim of the jar.
Common mistakeChoosing 'the glass expands more' reverses the facts; metals generally expand more than glass.
28Short answer · ★ Challenge
Describe how you would mark the lower fixed point on an unmarked mercury thermometer.
Show answer
Model answer: Put the bulb in a funnel of pure melting ice (with the melt water draining away) so the bulb is fully covered. Wait until the mercury thread stops moving, then mark the level of the thread on the stem as 0 °C.
Common mistakeA frequent mistake is to use ice straight from a freezer, which may be below 0 °C; the ice must be melting so its temperature is exactly 0 °C.
29Multiple choice
A thermocouple thermometer is the most suitable choice for measuring the temperature of:
1. AA sick child's mouth at home, where it is at about 38 °C or a little more
2. BThe inside of a furnace in a steel workshop, at about 1000 °C
3. CThe air in a classroom on a warm afternoon
4. DBath water for a baby, at about 37 °C
Show answer
Answer: B. The inside of a furnace in a steel workshop, at about 1000 °C
A thermocouple works over a very wide range, including temperatures far above the boiling point of mercury, so it suits a furnace; simpler thermometers are fine for the other jobs.
Common mistakeChoosing the sick child confuses a thermocouple with a clinical thermometer; a liquid-in-glass thermometer would melt or boil in a furnace, which is why a thermocouple is used there.
30Short answer · ★ Challenge
In cold countries, salt is spread on icy roads in winter. Explain how this makes the roads safer.
Show answer
Model answer: Salt dissolves in the thin layer of water on the ice and lowers its melting (freezing) point below 0 °C. The ice therefore melts even when the air is a little below 0 °C, so the road is less slippery.
Common mistakeSaying 'salt heats the ice' is wrong; the salt lowers the melting point, it does not supply heat.
31Multiple choice
A maximum–minimum (Six's) thermometer is placed in a greenhouse on a tea farm. It is used to:
1. AMeasure the temperature of each tea leaf separately, one at a time
2. BKeep the greenhouse at a fixed temperature by switching a heater on and off
3. CShow only the temperature at the moment when someone reads it
4. DRecord the highest and lowest temperatures reached since it was last reset
Show answer
Answer: D. Record the highest and lowest temperatures reached since it was last reset
Small indexes are pushed along by the liquid and stay at the highest and lowest points reached, so the farmer can read both extremes later.
Common mistakeChoosing 'keeps the greenhouse at a fixed temperature' confuses a thermometer with a thermostat; a Six's thermometer only records, it does not control.
32Multiple choice · ★ Challenge
On a new scale X, pure ice melts at 20 °X and pure water boils at 220 °X. What reading on scale X corresponds to 40 °C?
1. A60 °X
2. B100 °X
3. C80 °X
4. D120 °X
Show answer
Answer: B. 100 °X
200 °X divisions cover 100 °C, so 1 °C = 2 °X. Reading = 20 + 40 × 2 = 100 °X.
Common mistakeChoosing 60 °X just adds 20 and forgets that each Celsius degree is 2 divisions on scale X.
33Multiple choice
Drops of water form on the outside of a cold bottle of juice taken from a fridge. This is because:
1. AJuice seeps out through tiny holes in the glass
2. BThe juice inside evaporates and passes through the bottle
3. CIce from the fridge melts slowly on the bottle
4. DWater vapour in the air condenses on the cold surface
Show answer
Answer: D. Water vapour in the air condenses on the cold surface
The cold bottle cools the air touching it, and water vapour from the air condenses into liquid droplets.
Common mistakeChoosing 'juice seeps out' ignores that the drops are pure water and also form on sealed bottles; they come from the air.
34Short answer · ★ Challenge
Use the kinetic theory to explain why a gas in a closed container exerts a pressure on the walls of the container.
Show answer
Model answer: Gas particles move rapidly in all directions and keep colliding with the walls. Each collision exerts a small force on the wall; the many collisions every second give a steady force on each unit area, which is the pressure.
Common mistakeSome learners say the gas 'pushes because it wants more space'; the pressure comes from particle collisions with the walls.
35Multiple choice
In a Six's thermometer the small steel indexes stay where the liquid has pushed them. They are reset by:
1. APulling them back to the liquid with a small magnet
2. BShaking the thermometer firmly, like a clinical one
3. CHeating the bulb in boiling water for a few minutes
4. DCooling the whole thermometer in melting ice
Show answer
Answer: A. Pulling them back to the liquid with a small magnet
The indexes are made of steel, so a magnet slid along the outside of the glass drags them back to the ends of the liquid thread.
Common mistakeChoosing 'shaking' copies the clinical thermometer method; shaking can break the liquid thread and does not reliably move the steel indexes.
36Multiple choice · ★ Challenge
A faulty thermometer reads 2 °C in pure melting ice and 98 °C in steam at standard pressure. In warm water it reads 26 °C. What is the true temperature of the water?
1. A26 °C
2. B25 °C
3. C24 °C
4. D24.5 °C
Show answer
Answer: B. 25 °C
The faulty scale has 98 − 2 = 96 divisions for 100 °C. True θ = (26 − 2) ÷ 96 × 100 = 25 °C.
Common mistakeChoosing 24 °C only subtracts the zero error and forgets that 96 divisions, not 100, cover the whole range.
37Fill in the blank
A temperature change of 9 °F is equal to a temperature change of ______ °C.
Show answer
Answer: 5
180 Fahrenheit degrees cover the same interval as 100 Celsius degrees, so a change of 9 °F = 9 × 5/9 = 5 °C. The 32 is not used for a change.
Common mistakeSubtracting 32 from a temperature CHANGE is wrong; 32 shifts the zero point and only matters when converting a reading.
38Short answer · ★ Challenge
A wooden bench and a metal bench have both stood in the same shade all night. In the morning the metal bench feels much colder to touch. Are they at different temperatures? Explain.
Show answer
Model answer: No. Both are at the same temperature as the air around them. Metal is a good conductor, so it takes heat away from your hand much faster than wood does, and your hand cools quickly, so the metal feels colder.
Common mistakeMany learners trust their sense of touch as a thermometer; touch senses how fast heat leaves the skin, not the temperature of the object.
39Multiple choice
Why is the bulb of a laboratory thermometer made of very thin glass?
1. ASo that the bulb can hold a much larger amount of the liquid inside it
2. BSo that the thermometer does not break when it is dropped
3. CSo that heat reaches the liquid quickly and the thermometer responds fast
4. DSo that the liquid cannot expand further than the top of the tube
Show answer
Answer: C. So that heat reaches the liquid quickly and the thermometer responds fast
Thin glass conducts heat to the liquid quickly, so the thermometer reaches the temperature of its surroundings sooner.
Common mistakeChoosing 'holds more liquid' confuses the bulb's wall thickness with its size; a thin wall is for quick heat transfer, not for volume.
40Multiple choice
Glass bottles completely full of water often crack when left in a freezer. Why?
1. AWater contracts when it freezes, so the glass is pulled inwards
2. BWater expands when it freezes, so the ice needs more space than the water
3. CThe glass expands in the cold and becomes far too thin to hold the water in
4. DIce is heavier than water, so the bottle cannot support its weight
Show answer
Answer: B. Water expands when it freezes, so the ice needs more space than the water
Ice has a larger volume than the same mass of water, so freezing pushes outwards on the bottle and cracks it.
Common mistakeChoosing 'water contracts when it freezes' applies the usual rule for most liquids; water is the exception and expands on freezing.
41Short answer · ★ Challenge
Describe how the volume of 1 kg of water changes as it is cooled steadily from 10 °C to 0 °C (still liquid).
Show answer
Model answer: From 10 °C down to 4 °C the volume decreases (the water contracts). At 4 °C the volume is smallest (density greatest). From 4 °C down to 0 °C the volume increases again (the water expands as it cools).
Common mistakeMany learners say the volume decreases all the way to 0 °C; water reaches its minimum volume at 4 °C.
42Multiple choice
What happens to the particles of a solid as it is heated, before it starts to melt?
1. AThey stop vibrating and move freely past one another
2. BThey grow larger, so the solid takes up more space
3. CThey vibrate more strongly and their spacing grows a little
4. DThey lose energy and move closer together
Show answer
Answer: C. They vibrate more strongly and their spacing grows a little
Heating gives the particles more kinetic energy; they vibrate more strongly about fixed positions, so the solid expands slightly.
Common mistakeChoosing 'they grow larger' is the classic misconception: the particles themselves do not change size, only the spaces between them.
43Multiple choice · ★ Challenge
Ice at −10 °C is heated steadily until it becomes steam at 100 °C. On the graph of temperature against time, the two flat (horizontal) parts show:
1. AChanges of state: the energy breaks bonds, so the temperature stays constant
2. BTimes when the heater was switched off, so no energy was being supplied
3. CTimes when the ice or water lost as much heat as it gained from the heater
4. DPlaces where the thermometer could not read the temperature correctly
Show answer
Answer: A. Changes of state: the energy breaks bonds, so the temperature stays constant
During melting (0 °C) and boiling (100 °C) the energy is used to separate the particles, not to raise their kinetic energy, so the temperature does not change.
Common mistakeChoosing 'the heater was switched off' assumes no rise means no energy input; energy is still supplied but goes into changing the state.
44Multiple choice
A thermometer is put into a cup of hot tea. Its reading rises and then stops at 65 °C. Why does the reading stop rising?
1. AAll the heat in the tea has passed into the thermometer, so the tea has no heat left
2. BThe liquid in the thermometer cannot expand any further once it reaches 65 °C
3. CThe thermometer and the tea are at the same temperature, so no more heat flows
4. DThe tea has started to boil, so its temperature can no longer change at all
Show answer
Answer: C. The thermometer and the tea are at the same temperature, so no more heat flows
Heat flows only while there is a temperature difference. When both reach 65 °C they are in thermal equilibrium and the reading is steady.
Common mistakeChoosing 'all the heat has passed into the thermometer' treats heat as a stored substance; the flow stops because the temperatures are equal, not because the tea is empty of energy.
45Short answer · ★ Challenge
A maximum–minimum thermometer in a greenhouse in Musanze was reset yesterday morning. Now the bottom of the maximum index is at 31 °C, the bottom of the minimum index is at 9 °C and the liquid shows 24 °C. State the highest and lowest temperatures since it was reset, and the temperature range.
Show answer
Model answer: Highest = 31 °C, lowest = 9 °C (read at the ends of the indexes nearest the liquid). Range = 31 − 9 = 22 °C. The present temperature is 24 °C.
Common mistakeA common error is to give 24 °C as the maximum; the present reading is only the temperature now, while the indexes hold the extremes.
46Short answer
Before using a clinical thermometer again, a nurse at a health centre shakes it firmly. Explain why this is necessary.
Show answer
Model answer: The constriction stops the mercury from flowing back into the bulb by itself, so the thread still shows the last patient's reading. Shaking forces the mercury back into the bulb so that the new reading starts from below body temperature.
Common mistakeSome learners think shaking 'resets the scale'; it only returns mercury past the constriction, otherwise an old high reading would be read again.
47Short answer · ★ Challenge
A student heats solid wax and records its temperature every minute: 40, 48, 55, 60, 60, 60, 60, 66, 72 °C (from 0 to 8 minutes). State the melting point of the wax and explain what happens to the energy supplied between 3 and 6 minutes.
Show answer
Model answer: Melting point = 60 °C (the temperature stays constant from 3 to 6 minutes). During this time the energy supplied is used to break the forces between the particles so the solid changes to liquid (latent heat); the kinetic energy and hence the temperature do not rise.
Common mistakeSome learners give 72 °C (the last reading) as the melting point; the melting point is the flat part of the data, not the highest reading.
48Short answer
Water kept in a traditional unglazed clay pot (ikibindi) stays cooler than water kept in a plastic bucket. Explain why.
Show answer
Model answer: Water seeps slowly through the tiny pores of the clay and evaporates from the outside surface. Evaporation takes energy from the pot and the water inside, so the water cools. Plastic has no pores, so there is no evaporation from its outside.
Common mistakeLearners often say clay is 'naturally cold'; the cooling comes from evaporation through the pores, not from the material being cold.
49Multiple choice · ★ Challenge
A steel bridge is 50 m long. Steel expands by 0.012 mm per metre for each 1 °C rise. By how much does the bridge lengthen when the temperature rises from 15 °C to 35 °C?
1. A0.6 mm
2. B12 mm
3. C21 mm
4. D9 mm
Show answer
Answer: B. 12 mm
Rise = 35 − 15 = 20 °C. Expansion = 0.012 × 50 × 20 = 12 mm.
Common mistakeChoosing 21 mm uses 35 °C instead of the temperature rise of 20 °C; expansion depends on the change in temperature.
50Short answer
A student writes: 'Pure water freezes at 0 K.' Explain the error and give the correct value.
Show answer
Model answer: 0 K is absolute zero (about −273 °C), the lowest possible temperature. Pure water freezes at 0 °C, which is 0 + 273 = 273 K.
Common mistakeMixing up the zero of the Celsius scale with the zero of the Kelvin scale; the two scales have equal degree sizes but zeros 273 apart.