Hot Water Bottles

Hot water bottles 'eh? We all know what they are, and what they do. They warm us up! If its a freezing night, you can snuggle up with one of these to keep you toasty throughout the night. However the question which not many people consider is: HOW DOES IT WORK?


It's atually rather simple, and it's all thanks to heat transfer! Basicly, you fill up your hot water bottle with nearly boiling water. Then through the power of conduction, the heat is able to transfer through the plastic on the hot water bottle and into your skin. This is because heat always travels from hot to cold. The particles in the water vibrate and heat up causing the surrounding particles in the material to do the same. As it's conduction it has to be in direct contact with the heat. So when you touch the hot water bottle, the heat from the hot water bottle transfers from the hot water bottle to your hand through conduction. As your hands are cooler then the hot water bottle, and heat always travels form hot to cold.

Some people put their hot water bottles in covers, usually made out of wool ( see picture below ) this is beacase wool and other colthing materials are good at insulating heat. Therefore, by insulating the heat, the hot water bottle would stay hotter for longer. However, this is a double edged sword, as if you have it in a cover. As it is being insulateda lot of the heat is being trapped by the wool, not as much heat is able to transfer through to our skin, therefore it will not feel as warm and toasty.


Although it may be more effective to make the hot water bottle out of metal, as it is an extemely good conductor of heat. However, they are made for a marketing purpose, so therefore have to be safe and have no risk of burning you. Therefore they use rubber; it can conduct heat, but not as effectively as a metal.


However, pouring the boiling water into the hot water bottle is proving to be a health and safety point. So, there is an alternative, using a gel pad. These you can put into the microwave to heat them up, meaning the heat is transferred through the energy beams and absorbed by the gel (exothermic reaction), which emits the heat again (endothermic reaction), and because the gel emits the heat back out, no boiling water is needed. These gel pads are also more flexible, making them more comfortable to put on an aching joint. But still, nothing can beat a nice, warm, hot water bottle on a cold winters night, eh?

A hot water bottle with a cover in to insulate the heat
A hot water bottle with a cover in to insulate the heat

A hot water bottle
A hot water bottle




By Elizabeth O'Brien & Tayla Morhall