Faulty flask?????

Agapanthus

I'm with tandraig on this. I always used to take a flask of boiling water to work - that is until I got a job where they had a water boiler in the office!

Quite apart from anything else, if the flask's only had boiling water in it, you don't have to wash the d**n thing up!

seraphina

Right, here goes...

A particular flask has a fixed set of insulating properties which don't change no matter what you put in it.

But, every material has a property associated with it called "specific heat capacity" (don't be put off by the name), which is basically, the amount of heat energy required to raise the temperature by a fixed amount. So it takes a certain amount of energy (for example, the amout of gas you'd need to burn, or the amount of electricity you'd need to run your kettle) to raise the temperature of 1 kilo of water by 1 degrees C. It takes a different amount of energy to raise the temperature of 1 kilo of chicken, or thick soup, or anything else also by 1 degrees C.

The specific heat capacity of water is very high. So it takes a large amount of energy (electricity/gas, what have you) to increase its temperature. However soups, chicken and other food stuffs have lower specific heat capacities - it takes less energy to heat them up. So it takes less energy to make soup cook at, say 85 degrees C than it does to heat water at the same temperature.

So, what does that mean for your flask? Well, no matter what we put in our flask, it has the same insulating properties. So just as it would be quicker to heat up soup (because it has a lower heat capacity) than water, it's also quicker to cool soup than water. And because our flask has the same insulating properties, if we put soup in at 100 degrees C, and in an identical flask we put water at 100 degrees C then the soup will cool quicker, due to it's lower heat capacity.

Another way to look at it is as a resevoir of heat. Apart from the inefficiencies in cooking ( think of the heat that goes to warm the air around the cooker rather than warming up your water or soup), the water is capable of "absorbing" more energy whilst it warms up, than soup (water has the higher heat capacity). So it has "stored" more energy (as more energy went in to making it warm) than soup, therefore it has more to get rid of which takes longer for whatever rate the flask cools at.

It's also worth thinking about milk. Milk has a much lower heat capacity than water, which is why it seems to boil so quickly on the stove!


Hope this helps!

lizzyshep

Thanks for the explanation, as a science teacher I should have been able to work that one out for myself!

allydowd

minniepoos wrote: »

It's one of the stainless steel ones, so doesn't come apart.

I see what's happened:

In a traditional Thermos-style flask there are two layers of insulation. There is a vacuum between them. As heat cannot travel (well) through a vacuum the food/drink is protected from the outside air temperature.

In a single layer stainless flask there is no vacuum to prevent the heat escaping.

Therefore a single layer flask is less effective than a double layer flask. All you can really do is add some insulation to it.

MFewings

Dh has one of the 15year Thermos Steel flasks and he fills it with boiling water at 5am as he leaves for work and when he gets home at 6.30pm any leftover water/coffee is still very hot.

Flask did cost £20 but seems value for money.