I strongly recommend that everyone does this age old experiment as a ‘whole class demonstration and discussion’ type exercise!
The ‘spoons’ experiment is all about the conduction of heat through wood, metal and plastic.
It is a fantastic introduction to the conductivity of heat but really a very poor experiment from the point of view of accurate results. The fact that it is a rather poor experiment is brilliant however, because it allows for a detailed discussion about fair testing and leads (through step by step improvement) to a really excellent conductivity of heat investigation.
It is excellent for a good old fashioned ‘let’s all gather round the table and discuss it’ session in which every class member can contribute. It covers some great general knowledge and observations from home as well as getting to the heart of the investigative process.
This is a very simple experiment which requires only very common kitchen utensils to perform. As with all experiments it can be taken further and improved and several suggestions are provided. Even the simplest experiment can have pitfalls and it is important to make fair comparisons in order to obtain meaningful results.
Knowledge of Conductors and Insulators of heat is very important because it helps engineers and designers decide which material to use for a particular job eg. the parts of an electric iron – the base of the iron must get very hot in order to do it’s job but the handle must remain cool so that we can hold it without burning ourselves. We use conductors and insulators of heat every day in the kitchen or when we sit down to eat.
Let us see what we can find out about different types of materials as found in the kitchen at home. All sorts of materials are used in the kitchen:
Wood for large stirring and serving spoons.
Glass and pottery for plates and bowls
Metal for cutlery and pans.
Plastic for storage containers and a child’s plate etc.
Fabrics and fibres for oven gloves.
We are going to try and find out about wood, metal and plastic and how they are affected by heat. The reason we have chosen spoons for our investigation is that all three materials are used to make spoons so they are easy to obtain.
The basic experiment
Select a variety of spoons made from different materials (wood, metal and plastic) as shown and place them all into hot water in a jug.
For safety the jug should be placed on a tray in case of spillage and pupils should not be allowed to lean in too close!
The jug should be about half full and the temperature about 50 to 60 degrees Celsius (hotter water works best but you must consider safety and whether or not pupils will be doing this or observing a demonstration).
A safe temperature for pupils is probably about 40deg C maximum.
Quickly touch the handle of each spoon in turn – the metal spoon may well feel colder than the others but none of them will have warmed up.
After about 5 to 10 minutes touch the handles again – little or no change can be detected in the wooden or plastic spoons but the metal teaspoon will have noticeably, although not greatly, warmed up.
The metal spoon warms up before the plastic or wooden spoons (the plastic and wooden spoons may not noticeably warm up).
Metals allow heat to travel through them more easily than wood or plastic.
We say that metals are good conductors of heat and wood and plastics are poor conductors of heat.
Another way to say this is that wood and plastic are good insulators of heat (thermal insulators).
We now have good reasons for using plastic for the handle of an electric iron and metal for the base. Old fashioned irons which were heated on a coal fire or gas stove had wooden handles because plastics weren’t invented then!
Problems with the experiment
As a simple experiment it is fine but pupils may be able to spot ways of improving it. For example: The spoons may not be exactly the same size; Steam from the hot water rises up and may warm the handles of the spoons; You may feel the warmth from the steam on your fingers as you reach in to touch the handles; Some parts of your fingers are more sensitive to heat than others.
One possible source of error which can result in some confusing observations is if the person touching the spoons keeps their fingers in contact with a spoon for long periods or repeatedly touches one spoon then another. The heat from your own hands may interfere with the observations you are trying to make! In other words you will end up warming the spoons yourself simply by touching them for too long.
You can tell more quickly that the metal spoon has warmed up by having an identical metal spoon on the bench beside the jug. If you touch the spoon which is not in the jug and then quickly touch the one that is in the jug you can instantly notice a difference in temperature.
A piece of card, plastic foam or cloth with a hole in it can be used to prevent the steam from the water warming the handles and your fingers as you touch the spoons. This makes it easier to sense the temperature rise of the spoon with your fingers.
The most sensitive way to detect the temperature rise in the handle of the spoon is to lightly grip the handle between, and at the base of, the fingers.
A further improvement could be using stick-on temperature strips which change colour and indicate that the temperature has risen.
Rather than buy special temperature strips why not keep your eyes open for novelty give-aways like these heat sensitive picture cards from a breakfast cereal. Simply cut into strips and tape onto the spoon handle.
When cold the strip is black
As it warms the strip changes colour
Finally the heat reaches the end of the handle and the whole strip changes colour revealing part of a cartoon picture.
Pupils can now be challenged to completely redesign the investigation so that as much unfairness as possible is removed and other materials can be tested also eg. Different metals such as aluminium, copper, brass etc. as well as different types of plastic.
Next: see ‘Conductors’ page