IOP Institute of Physics

# Experiments

Solids, liquids and gases: Banana hammers and soft nails

1. Bring in a bunch of flowers and fix them somewhere prominent.

 Activity Discussion They will be used later.

2. Then empty a heap of clothing onto a table.

Aims/Facts

• From our very cold weather to our very hot weather the temperature varies by about 30 degrees Celsius.
 Activity Discussion View Apparatus ListSelect a large, very thick pair of gloves and ask for a volunteer to try them on. Then choose a sun hat and ask for a volunteer. Pick up other objects from the heap and ask the class to choose which should be worn by which child. (If appropriate, point out that school uniform is designed for about 20˚C.) Ask the children when they would wear the clothing from the bag. Move the discussion from winter and summer to actual temperatures. 0˚C for a cold winter day in the UK and 30˚C for a very hot summer day in the UK.

3. Rope activity.

 Activity Discussion Ask the two children to hold the rope at the knots which represent 0˚C and 30˚C and use the pegs to attach the pictures for 0˚C and 30˚C to the rope.View Apparatus List Continue the discussion emphasising that the length of rope represents a very large change in weather conditions. That at 0˚C water freezes - i.e. changes from a liquid to a solid so ponds freeze.Discuss hot weather conditions and lead into the next section.

4. Ask where on Earth it is very hot.

 Activity Discussion After a discussion, point out that the hottest temperature recorded in the Sahara desert is 57˚C. Attach the desert picture at the knot representing 57˚C. (Frequently over 40˚C).Ask where on Earth it is very cold. The South Pole is a usual suggestion. Tell them the average temperature in Antarctica is -57˚C. Attach the picture at the appropriate knot. (Coldest recorded temperature at the South Pole is -80˚C). The children may suggest Spain for a very hot place. Lead the discussion by explaining that we are used to lots of green trees and plants around. Ask about hot places so that they think of deserts.When cold places are discussed make sure they understand negative temperatures are a way of showing temperatures colder than 0˚C.

5. Show the picture of the boiling kettle and ask what temperature on the rope it should be placed.

Aims/Facts

• At 100˚C water boils. i.e. changes from liquid to gas.
 Activity Discussion They may know that water boils at 100˚C. Ask a child to peg on the picture and hold it.Show the animation of water splashing on a hot plate. Alternatively, if the risk assessment has been agreed, splash a few drops of water onto a hot iron which has been unplugged from the mains socket. Emphasise that this is not something to try at home! As soon as this has been done, place the iron on an insulating mat where the children will be not able to touch it.See safety notes Ask what is happening when water boils. Encourage discussion of the change from a liquid to a gas. Explain that this happens at different temperatures for different liquids. Explain that because the hotplate or iron is very much hotter than 100˚C the water boils off very quickly. Droplets have steam beneath them so they skid around.(Some children may know that water vapour exists above water at temperatures below 100˚C but this complication is best avoided if possible.)

6. Tell the children that you have some liquid air which is very cold.

Aims/Facts

• Nitrogen changes from a gas to a liquid at a temperature very, very much lower than water. It will therefore boil at a much lower temperature than water.
• Different liquids change from liquids to gases at different temperatures.
• Different liquids boil at different temperatures.
 Activity Discussion See safety notesAsk what they think will happen as it lands on the table which is at room temperature. Pour a small quantity (a few ml) liquid nitrogen onto the bench or table. The appearance is very similar to that of the water on the hotplate. Ask them to guess the temperature of the liquid air (nitrogen). (Nitrogen liquefies at -196˚C.).View Apparatus ListShow the position on the rope representing -196˚C. Ask a child to hold the knot. Explain that the table at 20˚C is very, very much hotter than the boiling temperature of the liquid nitrogen so the nitrogen gas boils off vigorously.Spend time answering their questions and emphasising how very cold this is. Use the rope to help to explain.

7. Ask where the temperature is extremely cold.

 Activity Discussion Explain that the temperature in outer space is -269˚C. Show the knot on the rope and emphasize the long length of rope indicating how very cold this is. Ask a child to peg on and hold the picture which represents space. Make it clear that you want them to think of places that are colder than the South Pole. The might guess on the outer planets. Some will suggest places in space. Explain that away from objects in space the temperature is very, very cold. Again use the rope to help to explain.

8. Ask if they know of even colder places.

Aims/Facts

• There is a lowest possible temperature. (-273˚C).
 Activity Discussion Ask whether we can go on finding colder and colder temperatures. Have a vote. Depending on the class, the next section could be omitted and the minimum temperature just stated after the vote. Ask if they know what is happening when substances get colder. For some able groups, this is an opportunity to discuss atoms moving more slowly as the temperature drops.

9. Following the discussion linking atom vibration with temperature - explain that at very low temperatures the atoms are hardly vibrating at all.

 Activity Discussion Eventually as they get extremely cold the atoms stop moving and then it is not possible to go colder. This temperature is -273˚C. Show the end of the rope. Spend time on this because it helps to put the liquid nitrogen into perspective as not being magic but being fully explainable.(Science often explains effects that appear to be magic.)

10. Ask whether they know of any very, very hot places.

Aims/Facts

• Very, very, very high temperatures are possible.
• Activities so far provide a contrast between our everyday understanding of hot and cold and the huge range of possible temperatures.
 Activity Discussion The sun is often suggested. Extend the hot end of the rope and ask them to guess where a knot should be put to represent the centre of the Sun. (15.6 million K) Show that this would need an enormously long rope. It would need to be long enough to stretch out of the building 150 km i.e. to a city that is ~100 miles from the school. Spend time reinforcing the ideas using the temperature scale slide. Or by drawing the chart on a black or white board.Emphasise:The UK seasonal temperatures are small compared to other parts of the Earth. The very cold temperatures in Space- nearly at the minimum possible temperature.The very, very high temperatures possible e.g. in stars.(Note: the outer layer of the Sun has a temperature of 6,000˚C.)

11. Ask what happens if we touch something very hot?

Aims/Facts

• Very hot and very cold can hurt us.
 Activity Discussion Also what happens to mountaineers who get very cold? Explain that you will do some experiments using the liquid air (or nitrogen). Liquid nitrogen is very, very cold so it can hurt us if we touch it so we must be very careful.

12. View Apparatus List

Aims/Facts

• A change in temperature can change the properties of substances.
 Activity Discussion See safety notesTap a hand bell or other metal object with a banana and let them hear the dull tap that is made. Then dip one end of the banana into liquid nitrogen for a few minutes. Holding the unfrozen end with a gloved hand, remove it and again tap the bell. The banana rings the bell. Explain that the water in the banana has frozen very hard. The unfrozen banana contains liquids which have changed to solids. The banana is very hard because it is so very cold.

13. Hold an elastic band up and show that it stretches.

Aims/Facts

• Properties of materials change at very low temperatures.
 Activity Discussion Then using insulated tongs dip the band into liquid nitrogen and then quickly hold it up and show that it is no longer stretchy but that its elastic property returns as it warms up.

14. Dip a length of bicycle inner tube into the liquid nitrogen for a moment the hold it up to show that it looks stiff.

Aims/Facts

• This provides an example of a reversible change.
 Activity Discussion (Be careful to put it in with the upper end folded over so that it is not possible for a liquid spurt to form.) Use it to ring the bell. Then lay it on the table and hit the frozen end with a hammer. The rubber shatters into small fragments with jagged edges. The frozen rubber is brittle and shatters like glass when hit.After a short discussion hold up the fragments of rubber and show that they are flexible again. Once the fragments have warmed up for several minutes, pass them round for the class to see the edges and feel that the rubber is flexible again.

15. Hold up a couple of small soft toys and ask them what will happen if they are dipped into the liquid nitrogen.

Aims/Facts

• The properties of the fibrous materials do not change as they are cooled.
 Activity Discussion Get them to vote which is dunked. Use the tongs to lower the toy into the liquid nitrogen and then lift it out. Allow it to drain over the Dewar taking care not to get liquid nitrogen onto your hand and to avoid any surplus liquid nitrogen falling on the floor. Sit the toy on the table and hit it with the hammer or try to ring the bell with it. It is soon clear that the toy is unchanged and not damaged by the hammer. A variation on this is to ask the teacher for a tie or scarf (usually these are undamaged but rarely colours are affected so it is a good idea to bring one for this purpose) After seeing the rubber tube become brittle the children expect the toy or tie to be damaged, when hit, and are surprised at the result.Discuss change of properties of materials at low temperatures.

16. Lower a small inflated balloon, which has been tied, into the liquid nitrogen and as it shrinks feed it down slowly using an insulating glove.

Aims/Facts

• Gases change into liquids when they are cooled. The liquid changes back into a gas as it warms up.
 Activity Discussion Using insulated tongs lift the balloon out with your gloved hand so they can see that the balloon has shrunk to be very small. Place the balloon on the table and at once it begins to expand until it is soon fully inflated. Air in the balloon has cooled and the volume has decreased – some of the air will have liquefied and it may be possible for the children to see the liquid. Discuss changes of gases to liquids as they cool. Encourage the children to explain what is happening as the balloon reflates.

17. Shape a small lump of blu-tack to look like a nail.

 Activity Discussion Then dip it into the liquid nitrogen for about 2 minutes holding it with the insulated tongs in your gloved hand. Hold the blu-tack carefully with the tongs and use a frozen banana as a hammer to hit it into a block of expanded polystyrene. Discuss this and allow it to warm up. Then slice through the polystyrene with a sharp knife to show the blu-tack ‘nail’ inside the block. Once warmed up, they can pass this round to feel the soft ‘nail’.

18. Bounce a ball of bouncing putty.

Aims/Facts

• Properties change when substances are very, very cold.
 Activity Discussion Then dip it into the liquid nitrogen and remove using a plastic spoon. Pick the ball up with a gloved hand and show that it is no longer bouncy it may be so brittle that it shatters. This is fun but could be omitted.

19. Cool a frying pan by pouring some liquid nitrogen into it.

Aims/Facts

• Liquids change to solids when cooled and then back to liquids when warmed.
• Changes from liquid to solid are reversible.
 Activity Discussion Then crack an egg into the pan and add some more liquid nitrogen. The albumen turns white and the egg appears to ‘cook’. The children will be puzzled and ask whether the egg is actually cooking. Get them to think what is happening. However it is important that they do not touch the cold egg. Follow this by the next activity and then return to the egg to let them see that the change is reversible so the egg did not ‘cook’.

20. Pieces of orange frozen in the liquid nitrogen shatter when hit with the hammer.

Aims/Facts

• Freezing vegetable matter can cause changes that cannot be reversed.
 Activity Discussion Rubbery celery works well also. Talk about reversible change and explain that if a finger or other living object is frozen cells are burst and so changes are not reversible.

21. Plenary session.

 Activity Discussion Take time to recap to ensure that each child has grasped at least the basics.

22. Dip a bunch of flowers into the liquid nitrogen: they appear unchanged.

 Activity Discussion Then crush them using a gloved hand. The sound is like crunching paper and tiny fragments will be scattered. It is fun to repeat this. Discuss that the appearance is unchanged. The colour stays. After the petals have warmed for a few minutes they will still feel cool. At this stage they can be passed round and the children will notice that they are cool. Ask them to make sure all the class has a few. Then as they watch the petals turn brownish because the cells have been burst.Aims/Facts The changes to the petals are not reversible. Cells have been burst so irreversible changes have happened.

23. Encourage the children to ask questions.

Aims/Facts

• Physics is everywhere.
 Activity Discussion They may talk about freezing embryos and body parts - explain that they must not be so cold that the cells are ruptured. This is a good area to allow them to ask more questions.