Solids, liquids and gases: Banana hammers and soft nails
Electric Iron Demonstration
- There is danger of hot steam splashing on a child or on the demonstrator. Safety goggles should be worn and the children should be at least two metres from the demonstration.
- An electric iron is heavy and should not be placed so that it can fall on a child.
- The flex should be in a position so that it cannot be reached or pulled by a child.
- After the demonstration, the hot iron should be placed on an insulating mat placed so that the children cannot reach it.
Liquid Nitrogen Activities
When planning to use liquid nitrogen during a school visit contact your local source of nitrogen and ask for a practice session with an experienced user.
Please read the CLEAPSS advice on handling liquid nitrogen in schools (Word, 35 KB). This is an extract from Section 11 of the CLEAPSS Laboratory Handbook which is supplied to all member secondary schools and colleges (but not to primary schools) on the CLEAPSS Science Publications CD-ROM and on paper.
Essential kit list:
- Dewar containing liquid nitrogen with venting lid – rated for nitrogen
- Thick, loose fitting leather gloves
- Safety goggles (not spectacles)
- Small Dewar flask – with a lid. (Special expanded-polystyrene flasks are available which eliminate the hazard of broken glass.)
The risks to be controlled are:
- 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 large (25 litre) containers
- Those associated with the transport of liquid nitrogen particularly in private cars
- Thick gloves and goggles (not safety glasses) should be worn at all times when handling liquid nitrogen because liquid nitrogen can cause severe burns. (Do not wear metal watches or jewellery, which could be in contact with the liquid nitrogen.)
- Guard against build up of pressure* by using a venting lid for all containers.
- Plan ahead how much liquid nitrogen will be needed and do not take excess to the school i.e. transport a small amount (not exceeding 2 litres).
- Transport the liquid nitrogen in a Dewar rated for liquid nitrogen with a venting lid to guard against the build up of pressure. Do not use an ordinary vacuum flask.
- On leaving the supplier, place the Dewar in the boot of a car – and ensure that the lid can vent. Ensure that the Dewar cannot fall over or spill by standing it in a large deep cardboard box filled with packing such as crumpled newspaper.
- On arrival, place the Dewar used for transport in a place that the children cannot access.
- Inform the school about class seating arrangements before your visit.
- Arrange that the children in the class are seated at least 2 metres away from the demonstration bench and do not ask for volunteers during the liquid nitrogen activities. Explain that the liquid is dangerous and so the children should not approach the bench. (Good behaviour is essential – if the children misbehave stop the presentation at once.)
- Before a presentation decant some liquid nitrogen into a small Dewar flask and keep the lid on the flask at all times except when accessing the liquid. This prevents the build up of liquid oxygen in the flask** and the possibility of a serious fire.
- Perform the demonstrations in a well ventilated room or hall to ensure that the percentage of oxygen*** in the air is not reduced dangerously. (If anyone suffers breathing difficulties they should be taken to fresh air at once.)
- Do not touch any objects that have been dipped into the liquid nitrogen with bare hands. Such objects remain cold for considerable lengths of time so do not pass them to the children. (One exception is petals which can be passed round about 5mins after the flowers have been taken out of the liquid nitrogen. They will still be cold to the touch and as they warm, in the hands, they turn brown.)
- Avoid dipping tubes into liquid nitrogen because a spurt of liquid can be caused.
*As the nitrogen vapourises the pressure build up would cause an explosion.
**Nitrogen liquefies at 77K, Oxygen liquefies at 90K
***Nitrogen is not poisonous but can asphyxiate. Nitrogen makes up 78% of air, Oxygen only 21%. A small drop in the concentration of oxygen is very noticeable.