States and Changes of State of Matter

Lessons

Senior 1 · Chemistry

States and Changes of State of Matter

5 lessons

Lesson 3 of 5

Changes of State and Heating Curves

25 min

Why do beans take longer to cook in Musanze?

A cook who moves from Rusizi, beside Lake Kivu, to Musanze, high in the north, notices that beans take longer to soften, even though the water is bubbling hard. A thermometer in the boiling pot explains it: in Musanze the water boils at about $94\,^\circ\mathrm{C}$ , not $100\,^\circ\mathrm{C}$ . To understand why, you first need to see what happens to the temperature of water as it changes state.

Experiment: heating ice until it boils

Half fill a beaker with crushed ice and set it up as in Figure 3.1.
Stir, and record the temperature every minute.
Heat gently with a small flame. Keep stirring and recording until the water has been boiling for three or four minutes.
Plot a graph of temperature against time.

Figure 3.1: Apparatus for heating ice.

Warning

Safety. Steam and boiling water scald. Do not lean over the beaker; switch off the burner before reaching across. Stir with the glass rod, never with the thermometer, which breaks easily. Let the apparatus cool before you move it.

Table 3.1: Results of a class in Kigali

Time in min	Temperature in $^\circ\mathrm{C}$	Time in min	Temperature in $^\circ\mathrm{C}$	Time in min	Temperature in $^\circ\mathrm{C}$
0	−6	6	8	12	90
1	−2	7	22	13	95
2	0	8	36	14	95
3	0	9	50	15	95
4	0	10	64	16	95
5	0	11	78

Figure 3.2: Heating curve from the results in Table 3.1.

Reading the heating curve

A (0 to 2 min): the ice warms from $-6\,^\circ\mathrm{C}$ to $0\,^\circ\mathrm{C}$ .
B (2 to 5 min): the ice melts. The temperature stays at $0\,^\circ\mathrm{C}$ even though heat is still going in: the energy is used to break the particles out of their fixed positions, not to make them move faster. $\mathrm{H_2O(s) \rightarrow H_2O(l)}$
C (5 to 13 min): all the ice has melted and the liquid water warms steadily.
D (from 13 min): the water boils. Again the temperature stays constant while the energy separates the particles completely to form a gas. $\mathrm{H_2O(l) \rightarrow H_2O(g)}$

Definition

Melting and melting point: melting is the change from solid to liquid; the melting point is the temperature at which a solid melts. Pure ice melts at $0\,^\circ\mathrm{C}$ . Freezing is the reverse change, and a pure substance freezes at the same temperature as it melts.

Definition

Boiling and boiling point: boiling is the change from liquid to gas that happens all through the liquid, with bubbles of vapour forming inside it; the boiling point is the temperature at which this happens. It depends on the air pressure: pure water boils at $100\,^\circ\mathrm{C}$ at sea level.

Definition

Evaporation and condensation: evaporation is the change from liquid to gas that happens only at the surface, at any temperature below the boiling point; condensation is the change from gas to liquid, as when steam forms drops on a cold lid.

Definition

Sublimation and deposition: sublimation is the change from solid straight to gas without melting, $\mathrm{I_2(s) \rightarrow I_2(g)}$ ; deposition is the reverse, from gas straight to solid.

Figure 3.3: The six changes of state. Each pair of arrows shows a change and its reverse.

Worked example

Problem. Why is the boiling point of water only about $95\,^\circ\mathrm{C}$ in Kigali and $94\,^\circ\mathrm{C}$ in Musanze?

Solution.

Kigali is about 1,500 m above sea level and Musanze nearly 1,900 m, so there is less air above them and the air pressure is lower.
A liquid boils when its particles can escape against the pressure of the air pushing on its surface.
With less pressure pushing down, water particles escape at a lower temperature.

Answer: lower air pressure at altitude lowers the boiling point. Food in boiling water is therefore cooked at a lower temperature and takes longer; a pressure cooker raises the pressure, and so the boiling point, to cook faster.

Table 3.2: Melting and boiling points

Substance	Melting point in $^\circ\mathrm{C}$	Boiling point in $^\circ\mathrm{C}$ (sea level)
Oxygen	$- 218$	$- 183$
Ethanol	$- 114$	78
Mercury	$- 39$	357
Water	0	100
Iodine	114	184 (sublimes when warmed gently)
Sodium chloride	801	1465
Iron	1538	2862

Worked example

Problem. Use Table 3.2. What is the state of (a) ethanol at $25\,^\circ\mathrm{C}$ , (b) ethanol at $-120\,^\circ\mathrm{C}$ , (c) mercury at $400\,^\circ\mathrm{C}$ , (d) sodium chloride at $1000\,^\circ\mathrm{C}$ ?

Solution. Below the melting point a substance is solid; between the melting and boiling points it is liquid; above the boiling point it is a gas.

$- 114 < 25 < 78$ , so ethanol is a liquid.
$- 120 < - 114$ , so it is a solid.
$400 > 357$ , so mercury is a gas.
$801 < 1000 < 1465$ , so sodium chloride is a liquid.

Changes of state in daily life

Washing hung in the sun dries by evaporation, faster when it is warm and windy. Coffee beans on raised racks and salt in open pans lose their water the same way. Early-morning mist over the valleys and dew on the grass form by condensation as moist air cools overnight. A hot cup of tea cools partly because its fastest particles evaporate from the surface, taking energy with them; the same cooling happens when sweat evaporates from your skin.

Warning

Common mistake: thinking that the temperature keeps rising while water boils. Turning the flame up makes the water boil faster, but it does not get any hotter than its boiling point. That is why boiling water hard wastes fuel.

Tip

On a heating curve: slopes mean one state is warming up; flat parts mean the state is changing. Read the temperature of a flat part to find a melting or boiling point.

FuseSchool: Changes of State (5 min). Watch what the particles do on the flat parts of the heating curve.

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Practice

Name the change of state when (a) butter melts in a hot pan, (b) steam forms drops on a cold window, (c) solid iodine turns to vapour.
What is the difference between evaporation and boiling?
Why does the temperature stay the same while ice is melting, even though it is being heated?
Use Table 3.2 to give the state of (a) oxygen at $-190\,^\circ\mathrm{C}$ , (b) iodine at $25\,^\circ\mathrm{C}$ , (c) iron at $2000\,^\circ\mathrm{C}$ .
A pure solid melts sharply at $80\,^\circ\mathrm{C}$ . Sketch the part of its heating curve around $80\,^\circ\mathrm{C}$ and label it.
Suggest why clothes dry faster on a sunny, windy day than on a cool, still one.
A cook in Musanze boils water hard for longer to try to make it hotter. Evaluate this idea and suggest a better one.

Show answersHide answers

(a) melting (b) condensation (c) sublimation.
Evaporation happens only at the surface and at any temperature below the boiling point; boiling happens throughout the liquid, with bubbles, at the boiling point.
The energy is used to break the particles out of their fixed positions, not to make them move faster, so the temperature does not rise until all the ice has melted.
(a) liquid (b) solid (c) liquid.
A sloping line (solid warming) up to $80\,^\circ\mathrm{C}$ , a flat line at $80\,^\circ\mathrm{C}$ labelled 'melting: solid + liquid', then a sloping line (liquid warming).
Heat gives more water particles enough energy to escape, and wind carries the vapour away so more water can evaporate.
It will not work: water cannot get hotter than its boiling point, about $94\,^\circ\mathrm{C}$ there, so extra boiling only wastes fuel. A pressure cooker raises the boiling point; soaking beans first also shortens cooking.

Sources and credits

Physical Sciences Grade 10, Chapter 3: States of matter and the kinetic molecular theory (the changes of state and their definitions, and the heating-curve experiment) by Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Image: Iodine sublimation- final product.jpg by Kate7234, via Wikimedia Commons, licensed under CC0.
Video: Changes of State, FuseSchool - Global Education, embedded from YouTube.
The table of results is illustrative. The diagrams, graph, worked examples, Practice items and exercises were written and drawn by Ganzaa.

1Why do beans take longer to cook in Musanze?

2Experiment: heating ice until it boils

3Reading the heating curve

4Changes of state in daily life

Why do beans take longer to cook in Musanze?

Experiment: heating ice until it boils

Reading the heating curve

Changes of state in daily life