Phase Change
Properties of Matter

Changes of phase

for 14-16

The experiments in this collection include some that stimulate qualitative thinking about the motion of particles that accompanies a material changing phase. And it includes some that lead to a quantitative estimate of the relative spacings of particles in gases as compared to liquids.

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Examples of change of phase

Phase Change
Properties of Matter

Examples of change of phase

Practical Activity for 14-16

Class practical

Apparatus and Materials

For each pair of students

  • Pyrex test-tube (e.g. 75 mm x 10 mm)
  • Tripod
  • Bunsen burner
  • Microscope slide
  • Therometer: 10°C to 110°C
  • Heat-resistant mat
  • Solder, not cored
  • Lead strip
  • Iron wire or steel wool
  • Copper wire, 15 cm lengths
  • Glass, small pieces
  • Snow
  • Naphthalene
  • Sulfur, 2 g powdered roll sulfur per group

Health & Safety and Technical Notes

Open windows to ensure good ventilation.

Sulfur should not be melted in an open shallow dish or tin lid because it will catch fire. In a deep crucible, it may be melted with care, but have a lid ready to cover it if ignition occurs. Melting sulfur is best done in a borosilicate test-tube with a mineral wool plug to contain the vapour.

The naphthalene must be heated in a test tube in a water bath. In the open, it may catch fire.

Read our standard health & safety guidance


Resin-cored solder, when melted in a flame, may make small flashes of flame. Use pure solder. When heating metal samples, the Bunsen should be at 45° to avoid molten metal falling back into it.

When trying to melt snow, good results are unlikely unless the snow is very fine.

Procedure

The following experiments could be tried, in any order.

  1. Put ice in a beaker and stand it on a tripod over a Bunsen. Watch it melt.
  2. Repeat with sulfur using a slightly hotter Bunsen flame. If time allows, watch the sulfur cool and see monoclinic crystals of sulfur forming.
  3. Hold the Bunsen at 45° so that the centre of the flame is vertically over the centre of the heat resistant mat. Adjust the Bunsen flame for the highest temperature. Stick the end of a short length of solder into the flame. Molten solder will splash onto the mat.
  4. Repeat step 3 using a short length of lead strip either held in a Bunsen flame or placed in a tin lid with the flame directed on it.
  5. Repeat step 3 using iron wire.
  6. Repeat step 3 using copper wire.
  7. Boil a little water in the beaker or test tube. It takes a few minutes to boil dry. This experiment could obviously be a continuation of 1, provided only a little ice was used.
  8. Put a small quantity of naphthalene into the test-tube to a depth of about 2.5 cm. Heat some water in the beaker over a Bunsen burner and hold the test tube in it with the test tube holder. Observe the melting and solidifying of the naphthalene.
  9. If snow is available, a melting experiment could be carried out. Heat a 400 ml beaker, crammed with snow, over a small Bunsen flame that is kept burning at a steady rate. Hold the Bunsen under the container of snow for only a quarter of a minute at a time. After each quarter of a minute ‘dose’ of heating, move the Bunsen away (but keep it burning) and stir carefully until the temperature stops changing. Then give another dose of heating. Continue thus, dose after dose, until the temperature is 30°C or higher.
  10. Put a drop of alcohol on the back of a hand and feel what happens as it evaporates.

Teaching Notes

  • Although this activity may seem trivial, many children will not be aware that some metals melt at a relatively low temperature.
  • These experiments demonstrate that the phase of matter depends on temperature. The process of melting and freezing, and evaporation and condensing, should be discussed in terms of the change in structure of the materials as well as change in volume. Link it to the students’ growing understanding of atomic and kinetic theory.
  • Perfume, or other smelly substances, diffusing throughout the laboratory will also illustrate that gases and vapours move.
  • There may be suggestions that gases are made of particles farther apart than those in solids and liquids. At this point it is useful to say, ‘Suppose each of you were a particle (atom or molecule). Suppose every student is a molecule of a piece of some stuff. If the stuff is solid, how would you all look? Stand in the middle of the room and show how you would be arranged in a solid … (crowded close in regular array, e.g. with arms touching shoulders).
  • Now show me how you would be arranged in a liquid …(crowded close but arranged irregularly and able to move around.)
  • Now show how you would look if you were particles of a gas … (spread out.)
  • It is, of course, possible to measure the temperature of cooling naphthalene and to plot a cooling curve showing solidification. Other materials, such as cetyl alcohol, produce good cooling curves. This could lead to a useful discussion of what happens to the energy of the particles during change of phase.
  • Heating a solid brings it to a point where the springy forces can no longer hold the atoms together. Atoms break loose and move about close to neighbours with random motion. This is the process of melting, in which atoms gain energy. When a solid is formed from a liquid, atoms lose energy and the surroundings are warmed.
  • A simple but effective experiment is to wipe some alcohol on the back of the hand, using cotton wool. Students will feel the temperature fall as it evaporates. Do NOT issue alcohol to a class while they are using Bunsen burners.

This experiment was safety-tested in January 2005

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Solid carbon dioxide turning into a gas

Phase Change
Properties of Matter

Solid carbon dioxide turning into a gas

Practical Activity for 14-16

Demonstration

Apparatus and Materials

Health & Safety and Technical Notes

Whether you use a gas cylinder or solid CO2, you should do a risk assessment.

Solid carbon dioxide must not be handled with bare fingers: use tongs or wear thick gloves. Wear eye protection before breaking a large block (with a hammer and cold chisel).

Read our standard health & safety guidance


You may be able to locate a local supplier for solid CO2. If you have a dry ice attachment and a CO2 cylinder, follow the instructions to get about half a teaspoon of carbon dioxide snow.

For further information about dry ice, see the apparatus entry for:

CO2 cylinder (syphon type)


Procedure

If using a balloon, stretch the neck of the balloon and hold it open with several fingers whilst about half a teaspoon of carbon dioxide snow is scraped in. Quickly flatten the balloon and knot the neck firmly. A polythene bag is easier to fill but more awkward to seal well.

Teaching Notes

  • Solid carbon dioxide is unusual in that, at room temperature and pressure, it does not turn into a liquid before changing into a gas as it warms up – it sublimes. The change in volume from solid to gas can be seen to be as much as 600 times if the container in which it is expanding is well-sealed and has no holes.
  • Alternately you could place a small piece of solid carbon dioxide (say, 0.5 cm3 ) under the mouth of a gas jar or measuring cylinder which is full of water and inverted over a tank of water so that the bubbles of gas can be collected.

This experiment was safety-tested in January 2005

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Change of volume with nitrogen

Phase Change
Properties of Matter

Change of volume with nitrogen

Practical Activity for 14-16

Demonstration

This shows the volume change as liquid nitrogen changes into a gas.

Apparatus and Materials

    Method 1
  • A supply of liquid air (or liquid nitrogren)
  • Measuring flask, 100ml
  • Chemical balance
  • Method 2
  • Trough
  • Water
  • Measuring cylinder (must have a volume 750 times greater than that of the test tube)
  • Rubber tubing
  • Small test tubes, similar, 2
  • A supply of liquid air and liquid nitrogen

Health & Safety and Technical Notes

A special risk assessment is required for this activity. LEA schools and other subscribers can obtain one from the CLEAPSS School Science Service (01895 251 496, email science@cleapss.org.uk).

This will require eye protection and non-absorbent leather gloves for setting up, and rubber gloves when collecting the gas.

Liquid nitrogen presents the following particular risks:

  • asphyxiation in oxygen-deficient atmospheres
  • fire in oxygen-enriched atmospheres
  • cold burns, frost bite and hypothermia from the intense cold
  • over-pressurisation from the large volume expansion of the liquid
  • manual-handling accidents if using 25 litre volumes

Read our standard health & safety guidance


With step 3 once the nitrogen has started boiling, it is not possible to stop it whilst one refills a box or cylinder with water. Therefore it is necessary to have two or more receivers ready.

Procedure

Method 1: Indirect measurement of volume change

  1. Pour liquid air or nitrogen quickly into the 100 ml measuring flask of known mass.
  2. Once the jar has been cooled sufficiently, the violent bubbling will cease. Add more liquid to top the level up to the 100 ml mark so that the volume is known.
  3. Weigh the whole on the balance and find the mass of liquid.
  4. Calculate the density of liquid nitrogen.
  5. Measure the density of air (Air is 80% nitrogen. Oxygen and nitrogen have a similar density.)

Method 2: Direct measurement of the volume change

  1. Fill the trough and measuring cylinder with water. Invert the measuring cylinder in the trough.
  2. Find the volume of the test tube. Do this by filling it with water and measuring the volume of water.
  3. Pour liquid nitrogen into the second, dry test tube. When the liquid has stopped bubbling, top up the test tube and quickly fit the rubber tubing.
  4. Insert the rubber tube into the open end of the measuring cylinder and collect the nitrogen gas.
  5. Compare the ratio of the volume of nitrogen gas with the volume of liquid nitrogen of the same mass.

Teaching Notes

  • Expect a change in volume of about 1 to 750\. 10 ml of liquid nitrogen will release several litres of gas very quickly. Catching the gas with take some practice.
  • The density of liquid air/nitrogen is about 90 g/100 ml which is 900 kg /m3. The density of air/nitrogen is about 1.2 kg /m 3 .
  • Hence the volume change when liquid nitrogen becomes gaseous nitrogen is 900/1.2 = 750.
  • If there is no liquid air/nitrogen available, then solid carbon dioxide can be allowed to sublime. A small solid lump of carbon dioxide can be put into a measuring tube filled with water, and the volume of the gas measured. It is better to get the carbon dioxide from a solid block because that produced by a cylinder is not compacted enough.

This experiment was safety-tested in November 2006.

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Examination of boiling

Phase Change
Properties of Matter

Examination of boiling

Practical Activity for 14-16

Class practical

This is a magnificent experiment, which at the outset may not appear very exciting.

Apparatus and Materials

For each student group

  • Bunsen burner
  • Pyrex beaker
  • Tripod, gauze and heat-resistant mat
  • Thermometer - 10°C to 110°C

Health & Safety and Technical Notes

Students must not sit down to watch this experiment: serious scalding has occurred when the beaker breaks or falls and the pupil has been unable to move away instantly.

Read our standard health & safety guidance


Procedure

Half fill a beaker with water and then bring it gently to the boil. Watch the process carefully, observing the formation of bubbles.

Teaching Notes

  • This experiment could begin with a block of ice in the beaker which is allowed to melt.
  • Students see small bubbles forming from dissolved air; but when boiling starts there is a quite different formation of water vapour (steam) in bubbles.
  • Students are apt to have very careless views of the essential nature of boiling:
    • a fixed (!) boiling point (it depends on atmospheric pressure);
    • vapour pushing the outer air away (when in fact vapour molecules diffuse through air easily);
    • a vague story of more copious evaporation with no clear reason for the constancy of boiling temperature.
  • Ask: "what tells you water is boiling?"; and insist on the clear answer, "bubbles of water".
  • Bubbles cannot form and grow in the liquid until the vapour pressure in them matches the outside atmospheric pressure. The liquid boils away as fast as heating provides the 'exit-taxi' of latent heat. Once the liquid is boiling, further heating simply equips more molecules with speed needed to evaporate into vapour bubbles. Therefore, the temperature stays constant at the boiling point.
  • Thus, evaporation acts as a thermostat for a boiling liquid. (The energy needed to pay the 'exit-taxi' makes distilling an expensive business.)
  • Let students carry out the experiment the first time without a thermometer in the water. A very able group could plot temeperature-time graphs.

This experiment was safety-tested in December 2004

Up next

Change of volume: water to water vapour

Phase Change
Properties of Matter

Change of volume: water to water vapour

Practical Activity for 14-16

Class practical

Apparatus and Materials

  • Syringe kit (large glass syringe, rubber cap, hypodermic syringe with short needle)
  • Tripods, gauzes and heat-resistant mats, 2
  • Bunsen burners, 2
  • Beakers, deep, 2
  • Brine

Health & Safety and Technical Notes

Gloves, even just rubber washing-up gloves, will give some protection from the high temperature when extracting the large syringe from the hot water.

Read our standard health & safety guidance


The glass syringe has a rubber cap fitted over its end when the piston is pushed down to zero volume (as shown in the diagram). As it takes a long time to heat the syringe, it is recommended that it be heated before the arrival of the class by immersing the whole thing in a deep container of water brought nearly to boiling. The second large beaker, containing brine, should also be brought to boiling before the lesson begins.

It is essential that the large syringe be internally dry before use; the caps must be tight fitting, otherwise they will be blown off.

Salt water getting into the very narrow space between piston and barrel may cause the syringe to jam. A smear of silicone grease prevents this. Vaseline contains water droplets and is unsuitable.

Procedure

  1. Partially fill the hypodermic syringe with water and then remove the large syringe from the hot water. If helpful, hold in a clamp attached to a retort stand.
  2. Invert the hypodermic and eject any air in it.
  3. Inject 0.1 ml of water through the rubber cap into the syringe. The cap seals on removal of the needle.
  4. Immerse the large syringe in the boiling brine. The water will turn to steam and the volume change can be observed.
  5. After all the water has turned to vapour, remove the syringe from the beaker. The syringe will cool down and the water vapour condenses back to water.

Teaching Notes

  • Twisting the piston as the volume changes may be helpful, though students will doubtless call this cheating unless the decrease in volume on condensation is also shown.
  • Precise results will not be obtained from this experiment. Its accuracy will show an order of magnitude: 0.1 ml of water becoming at least 100 ml of steam. (The recognised value is a change of 1 to 1650.)
  • Everyday use of the word steam confuses students. Steamed up windows are in fact covered in water droplets; the water vapour condenses into water on the cold window so the steam is water not a gas (vapour).

This experiment was safety-tested in August 2006

Up next

Change of volume: petrol to petrol vapour

Phase Change
Properties of Matter

Change of volume: petrol to petrol vapour

Practical Activity for 14-16

Demonstration

This experiment works more easily than that with water and is less expensive.

Apparatus and Materials

  • Change of volume kit (as shown in the diagram) with long needle (8 to 10 cm)
  • Beaker, 400 ml
  • Wire stirrer to fit inside the tall beaker and go round the measuring cylinder
  • Plastic tube to carry over to beaker
  • Plastic tube to extend to the bottom of the measuring cylinder
  • Petroleum ether, 40 – 60 boiling fraction

Health & Safety and Technical Notes

Ordinary petrol must not be used as it contains benzene (carcinogenic) proportions. Petroleum ether can be regarded as pure petrol.

The security of the syringe with the long needle is an issue to be watched.

Read our standard health & safety guidance


The change of volume kit should include:

  • 100ml measuring jar
  • Rubber stopper with two tubes inserted
  • Rubber cap
  • Hypodermic syringe and needle
  • Syringe and needle

If the measuring cylinder has a pouring lip, the rubber stopper, with its two glass tubes, must extend below the lip to prevent leakage there.

Procedure

  1. Place the measuring cylinder (without its stopper) in the tall beaker.
  2. Put about 200 ml of water into the small beaker.
  3. Fill the tall beaker and the measuring cylinder with boiling water from an electric kettle.
  4. Close the cylinder with its bung and ensure the outlet plastic tube is in the water in the small beaker.
  5. Push the needle through the cap on the short tube in the bung and inject 0.1 ml of petroleum ether. As the petroleum ether turns to vapour, the displaced hot water runs over into the small beaker.

Teaching Notes

  • To reverse the change, take the measuring cylinder out of the large beaker, ensuring the end of the plastic tube stays under the water in the small beaker. You have to wait for the temperature to fall to about 60°C.
  • The expected volume change is 1 to 800.

This experiment was safety-tested in August 2006

Up next

The separation of molecules in a gas

Phase Change
Properties of Matter

The separation of molecules in a gas

Teaching Guidance for 14-16

As a crude picture that will lead to a rough estimate, assume that each molecule in a liquid occupies a cubical box of side, d , the diameter of the molecule. (Of course real molecules are not hard lumps like billiard balls and certainly not spherical.)

At first glance, this may seem to place the molecules too close together for liquid behaviour. However, the volume of space occupied, d 3 , is almost twice the volume of a sphere with diameter d , so the assumed cubical spacing would have liquid behaviour.

In the closely packed array we imagine for a liquid, the spacing for molecules, neighbour to neighbour, is therefore d , one molecule diameter.

'How much greater is the spacing in a gas such as air?'

If the spacing in the gas is D , then a volume of D 3 is needed for each gaseous molecule.

The ratio d 3 / D 3 = volume occupied by a liquid gas/volume occupied by the same mass of the gas.

So, the average separation of the gas molecules is the (volume of gas/volume of liquid) 1/3 x the molecular diameter.

The change in volume from the liquid to the gaseous phase for:

  • air (nitrogen) is about 1: 750
  • carbon dioxide about 1: 850
  • petrol at 90°C about 1: 800
  • water at 100°C about 1: 1650 (water is unusually high)

If we assume that the volume change is about 1: 1000, then we can conclude that in a gas, the molecules are about 10 molecular widths apart, on average. Nine diameters apart would give a volume change of 729 and 12 would give 1728.

This shows the powerful effect of a cube root in estimating molecular separations.

Up next

Avogadro's number and the mass of an air molecule

Number of Moles
Properties of Matter

Avogadro's number and the mass of an air molecule

Teaching Guidance for 14-16

Theory, modelling, guessing and experimenting are all intertwined. Each step progressing from one idea to the next. However, this is a very cleaned up view of the progress of science. Science is much messier than this and many ideas lead to dead ends and wrong predictions.

Knowing:

  • The diameter of an air molecule, 4 x 10-10m,
  • The space occupied by a molecule in liquid, (d3= {4 x 10-10}3= 64 x 10-30m3),
  • The change of volume from a liquid to gas

You can calculate how many molecules there are in a room, (4m x 3m x 3m = 24 m3) giving about 5 x 1026 molecules.

This is in fact an estimate of the Avogadro number for a kilo-mole. A kilogram mole of any gas contains 6 x 1026molecules. It occupies 22.4 m3at 0 °C, or about 24 m3at room temperature, and atmospheric pressure.

Mass of an air molecule

Number of molecules in a room 24 m3, N = 5 x 1026

Mass of air molecules in a room 24 m3, M = Vp = 24 x 1.2 kg = 28.2 kg

Therefore, Mass of an air molecule = 28.2 / 5 x 10-26 = 5.6 x 10-26kg

When students know more about the structure of air (mainly nitrogen and oxygen) then the mass of their atoms can be estimated (they are fairly close in mass).

All this comes from imagining a theoretical picture, guided by the things we know about nature, such as Newton's Laws of Motion, and then making estimates and measurements.

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