KEEPING COSY - THE MAGIC OF FOOT WARMERS

Truthfully telling, I had never even thought about foot warmers before, but thank goodness my partner did. The first couple of time in the woods, I would come home telling stories of how cold it was out there, saying 'my welly boots are not cutting it'. I suppose my partner just wanted me to stop talking about cold feet, as he brought me foot warmers the next day. The following week I place the insoles into my shoes and they were magical! How was this happening? My feet stayed warm the whole day! I had to know how these thin insoles worked. So I started investigating and this is what I found... 

Ingredients:  

• Air tight plastic wrapper  
• Pouch holding the mixture 
• The mixture including, iron powder, sodium, activated charcoal, water 
• Oxygen  

Opening the wrapper:  

The foot warmer is kept in an air tight plastic wrapper so that oxygen does not come into contact with it. The reason for this is, opening the wrapper, activates an exothermic oxidisation reaction, in this instance meaning, air diffuses into the pouch and the oxygen in the air reacts with the iron powder in the insole, producing heat energy which transfers to its surroundings, in this case, from the insoles to my feet. The other product produced from this reaction is rust. 

The speed of reaction is dependent on...  

The amount of iron and oxygen available to react with one another. If there is less oxygen then, less heat will be produced and the speed of the reaction will be less. This is probably why the warning on the packet explains to wear shoes that air cannot easily get into, such as walking boots and not running trainer, as the heat produced by the reaction may become to hot.  

The surface area of the iron because when powdered, the oxygen is able to come into contact with more molecules of iron, so the reaction time is faster and a higher temperature is produced.  

The sodium within the mixture which acts as a catalyse, speeding up the reaction up so that the insoles start to warm up more quickly.  

The activated charcoal within the mixture, adsorbs odours and evenly spreads out the heat produced. This means that the heat energy produced does not transfer away from the insole to quickly, but causes the insole to stay warmer for longer. 

References: 

http://dspace.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4062/16509287.pdf?sequence=

http://www.wired.com/2014/12/whats-inside-hot-hands/  

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemreac/energychangesrev1.shtml

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/exothermic/exothermic_endothermicrev1.shtml

http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/chemical_economics/reaction3rev1.shtml
http://www.fao.org/docrep/x5328e/x5328e0b.htm#10.1.7. adsorption capacity