Snap, crackle & pop -  If children are familiar with making their own popcorn, this is a good example of what happens when a gas is heated and expands. In fact, the water inside the corn kernel evaporates in the heat of the cooking process and the water vapour (a gas) produced expands greatly and breaks the hard shell (pericarp) of the kernel and the starchy tissue inside billows outwards.

Figure 16 - Popcorn - can be made in a microwave - watch out for hot steam hazard - burning risk

The same principle applies in the case of a micro-waved egg. If you try to microwave an egg in its shell it will explode! The water inside will become water vapour and expands rapidly, the pressure being great enough to break the shell. Even if the egg is de-shelled it is necessary to prick the yolk when cooking an unbeaten egg in a microwave, because that too may explode.

Figure 17 - Don't try this a home, or school!
 

Here is another interesting “foodie” observation concerning water vapour. You may have noticed that some bought cookies or home-made biscuits become soft if left out in the air. It is the sweeteners in the cookie which can cause this. Cookies made with sugars with a high proportion of sucrose, for example granulated sugar, will stay crisp longer than cookies made to a similar recipe, but where the sweetening substance, for instance honey, has a high proportion of fructose. Fructose is hygroscopic, meaning it absorbs water from the air.

Figure 18 - Mmmm - cookies

If you have baking facilities, your pupils can try comparing how cookies made with different types of sweetener retain their crispness over a few days if left exposed to the air. (Recipe supplied on application!)

Riding on a cushion of air