by Stuart Fieldhouse – Science Teacher, Outside Education
Important Safety Caution:
This experiment can produce large flames of up to and in excess of 2m long, 10 – 50cm in diameter. Keep the experiment away from flammable chemicals, materials and fabrics. Do this experiment outside as ceilings may be damaged.
Wear safety goggles and have appropriate first aid and fire protection equipment available.
Please be aware there may also be generated clouds of dust and fine particulate matter that could irritate the respiratory pathways and eyes.
Please be aware of any allergies to the materials used as fine particulate matter could enter the respiratory pathway or make contact with skin and eyes and thus trigger an allergic response.
General Safety for all experiments:
- Experiments should be carried out with the guidance and help of an adult.
- A first aid kit should be readily available.
- The experimental surface needs to kept clear of unnecessary items.
- Flammable liquids and materials need to be kept clear of the experimental surface.
- Long hair should be tied back.
- Sharp instruments and equipment should be used in accompaniment with an adult.
- Hot materials or liquids should be used with aid and guidance from an adult.
- Naked flame should only be used with an adult present.
- Used biological samples should be carefully disposed of in general waste unless specified otherwise in the experimental method.
- After completing an experiment hands should be thoroughly washed with water and soap.
- All waste materials should be disposed of according to the local rules on recycling and waste disposal.
Experiment: Custard Powder Burner (Flame Thrower)
- Bellows foot pump (the type used to pump up an air bed).
- 10cm diameter funnel
- Family sized baked bean tin or any tin that has a diameter greater than 10 cms.
- Duck tape.
- Glue gun.
- Candle/ Fire lighter blocks/tea-light candle
- Metal spoons
- Cooking Tongs
- Custard Powder
- Iron filings/ Iron powder* (easy to order off of the internet)
- Magnesium powder* (not easily obtainable, also quite flammable)
Cut away about two thirds to about three quarters of the bottom of the tin. I used student scissors to do this so any scissors stronger than this will do. Leaving a small lip folded back inwards will help with wax or materials spilling from ledge that has been created into the funnel and thus blocking the airflow.
The ignition sources for the custard powder needs to be placed here. Tea-lights present too feeble a flame to be the primary ignition source however the metal cup that the tea-light is in is ideal for holding a small sugar cub sized fragment of the fire lighter brick.
A larger candelabra candle can be used instead of fire lighter brick. It will need to be cut to about on third the height of the tin. The candle will likely be extinguished by the custard powder reaction and need relighting each time.
Please ensure you have read the safety caution before proceeding!
Assemble the equipment as shown in the diagram. If using a firelighter brick as the ignition source then gluing the cup in place may help though the glue may melt due to the heat. If using a candle drip some molten was in in first to secure the it.
Secure the apparatus against a solid object in an upright position i.e. Leg of a stool or chair. Attach the hose to the funnel and without any flame present give a few good pushes on the pump to check the air is flowing.
Preparation to fire:
Take table spoon of custard powder and carefully tip it down the cone of the funnel spillage else where is quite acceptable. Light the candle or place the lit piece of firelighter in the tube.
Step back from the apparatus. Check the area is safe and no flammable materials are near to the mouth of the apparatus. Push firmly and quickly on the foot pump. DO NOT STAMP. A constant, even and strong flow of air is needed to create the dust cloud and mix the particles with the oxygen.
The following should happen. WOOOSH, ROOOAAR but hopefully not BOOM.
There may be other problems that I have not encountered however these are the main issues:
- The ignition flame went out before the cloud of powder was created. Perhaps try a bigger candle or larger piece of lighter.
- The cloud did not ignite. As above try a larger candle flame, or two candles or larger piece of firelighter.
- The air flow is not strong enough to create a large enough cloud.
- Too little custard powder was used.
- Too much custard powder was used.
- The piece of lighter brick can be blown out of the apparatus by the venturi effect. Use a 1cm piece and replace it when it has been use a few times.
- The can will heat up and will melt the funnel and duck tape holding it together. Give cool down time between each attempt. Remove or extinguish the ignition source also between attempts.
- The funnel or pipe becomes blocked. Wax or molten custard powder will start to block the funnel. It will need cleaning out regularly. A chopstick usually does a good job.
WARNING: Avoid serious injury:
- Do not look over the top or into the loaded apparatus once the ignition source is present.
- Ensure no one is near or tempted to press the foot pump whilst loading.
- Beware of allergies to the components of the powders used.
The Science and More
The Fire Triangle:
The fire triangle gives the three factors that are need to have successful combustion. The powder is the fuel, the heat is the ignition source and the air flow provides the oxygen.
Try putting a lit match into a table spoon of custard powder and see what happens? Why does this happen? What part of the triangle is missing?
Rates of Reaction:
There are several factors that affect the rate of reaction. The one demonstrated here is surface area. A heap of custard powder on the table will not ignite give the same heat sources a in effect it has a very low surface area to volume ratio. Despite being made up of millions of microscopic particle they are all in close contact to each other so the surface are to volume is very low. Once the particles are in the air and separated from each other the surface area to volume ratio has been increased massively. The ignition source sets the reaction going and with each particle that combusts more energy is released until all the powder is burning.
Custard powder is quite a complex mixture of chemicals, however the main components are carbohydrates. Effectively sugar molecules linked together to form starches. Theses are cyclic carbon oxygen molecules with large amounts of energy being stored in the chemical bonds between the carbon atoms.
This then leads to the following questions:
- What other powders would react in the same way?
- Which powders would give the most energy?
- How does the chemical composition and structure of the powders constituents affect the reaction?
- Would powders that are not carbon based react in the same way?
Science in action
There are real-life issues that arise from this experiment and one of them is the preparation of flour and baking of bread. There are some schools of thought that the great fire of London may have been caused by a “flour” explosion. Also a factory making Custard powder did blow up. Here are some interesting links to videos and articles about this.
- using their observations and ideas to suggest answers to questions
- describe the simple physical properties of a variety of everyday materials
- reporting on findings from enquiries, including oral and written explanations, displays or presentations of results and conclusions
- compare and group materials together, according to whether they are solids, liquids or gases
- evaluate risks
- use appropriate techniques, apparatus, and materials during fieldwork and laboratory work, paying attention to health and safety
- exothermic and endothermic chemical reactions (qualitative)
- applying a knowledge of a range of techniques, apparatus, and materials to select those appropriate both for fieldwork and for experiments
- evaluating methods and suggesting possible improvements and further investigations
- factors that influence the rate of reaction: varying temperature or concentration, changing the surface area of a solid reactant or by adding a catalyst