What a wonderful gadget is a chemical hand warmer during winter days! You are cold→you bend the metal piece inside→your hands get warm and you can get some semblance of comfort while removing snow from your car at -20°C.
Prof. Robert Musioł – Director of the Institute of Chemistry and Ambassador of the Chemistry Degree Programme – told us all about how hand warmers work. In his work, he focuses on medical chemistry and drug design. The results of his research appeared in over 100 scientific publications and 30 patents.
Hand warmers
Chemical hand warmers are small and practical tools that help us deal with harsh winter conditions. They work by releasing heat as a result of a chemical reaction or physical change. The former ones are referred to as disposable because when the reaction is initiated it cannot be stopped or reversed.
Hand warmers – how do they work?
The basic chemical ingredient used in this type of heating device is silica gel, which is intended to provide an environment for the reaction to take place and to increase the heat capacity of the hand warmer – that is, to make it give off heat moderately but for a longer time. Chemical substances such as fine iron filings, table salt, additives such as activated carbon, or water-soaked vermiculite are suspended in this gel. Once mixed, the entire product is vacuum-packed in air-tight film. Crushing or tearing the packaging allows the substance to come in contact with oxygen, which reacts with the iron filings. An oxidation takes place resulting in the creation of iron oxide, i.e. rust. Due to the addition of salt and the high degree of fragmentation, this reaction takes place much faster than, for example, on a nail sticking out of the wall. This is why the excess energy created in its course is given off as heat. Scientists call it an exothermic process. The incredible simplicity and effectiveness of these heating devices make them relatively inexpensive and easy to use in situations where we need a quick source of heat outdoors, such as during winter walks or outdoor activities. Chemical hand warmers serve as a great example of how taking advantage of a chemical reaction can bring us many practical benefits in our everyday lives. However, their fundamental flaw is that once activated they cannot be brought back to their original state. Used ones are just thrown away.
Towards sustainability – reusable hand warmers
A far more eco-friendly option is hand warmers using a physical change of a highly concentrated solution. Most often it is an aqueous sodium acetate solution encased in a bag with a piece of metal inside. Sodium acetate is a simple and completely safe substance with the interesting property of producing a supersaturated solution – that is, one in which more of the substance is dissolved than the maximum solubility at a given temperature. Reaching this seemingly impossible-to-achieve state can be done by dissolving salt at a high temperature. This way you can usually dissolve more of the substance than when it’s cold – it is apparent to anyone who has tried to sweeten cold tea. The resulting hot saturated solution becomes supersaturated and unstable when cooled – it can easily start to crystallise while giving off stored heat. Another well-known supersaturated solution that we often use in our everyday lives is honey. However, for the sodium acetate to crystallise faster than honey and give off all the heat at the same time, a piece of metal has to be placed in it, which then can be bent until it produces a characteristic ‘click’. This movement and the sound it generates activate the process. The solution quickly becomes cloudy, crystals start appearing, and the bag reaches a temperature of approximately 50°C. Used hand warmer can be re-activated by placing it in hot water for some time.
Petrol hand warmers
Another interesting way to warm your hands is to take advantage of catalytic oxidation. Such heaters usually burn purified petrol on a special catalyst coated with a precious metal: platinum, rhodium. When heated to a high temperature, petrol vapour burns flamelessly on the surface of this metal, emitting considerable heat. Using a catalyst and controlling oxygen access allow quite safe use of such heaters. A disadvantage of kind of a heating device is the sometimes noticeable petrol smell and potentially dangerous combustion products – such as carbon monoxide. It should not be used in closed spaces and sleeping bags.
How should it be used?
You can put the heater in large gloves, preferably ones with only one finger. Alternatively, it can be hung around the neck at the level of the solar plexus to warm the whole body.
Thusly prepared, you can shovel snow from your car with the engine off and a smile on your face – the heater is cheaper than a fine.