Question #8c83b

1 Answer
Apr 24, 2017

The food can be heated above #100°C# due to the increased vapour pressure and will be sooner ready.
The vapour pressure makes the boiling point higher.


To explain this problem, we have to introduce vapour pressure.
If you take a sealed container and fill half with water, this water can evaporate into the air forming a tiny bit of steam. This happens even under #100°C#. This steam exerts a pressure on the water in the container and is called the vapour pressure.

Molecules move on average faster when the temperature is higher, due to a larger translational kinetic energy. The speeds of these molecules differ, one can be slower, one can move faster. Now the fastest molecules can escape the water at room temperature and form this vapour pressure.

Now we make a big fire under this sealed container, then the molecules will have a higher temperature and move faster. More of these molecules are able to escape the water. Therefore the vapour pressure increases: more molecules in the steam!

The vapour pressure itself exerts a force on the water, making it harder for other molecules to evaporate. The molecules must gain more energy to escape the liquid, therefore the temperature must increase. But we see now, that this vapour pressure has increased the boiling point! We need more energy to make the molecules really want to evaporate fast.

The high-pressure cooker is a sealed container and with some fire in our kitchen, we can increase the temperature of the water inside. Therefore we know now, that the boiling point will rise.

Why is an increase in boiling point important to cook?
If the boiling point is increased inside the cooker, we can cook our food at a higher temperature than the #100°C#, because now the water doesn't evaporate at #100°C#. If the food is cooked at a higher temperature, the process goes faster.