A househusband is cooking beef stew for his family in a pan that is (a) uncovered, (b) covered with a light lid, and (c) covered with a heavy lid. For which case will the cooking time be the shortest? Why?

Cooking will be shortest when the househusband covers the beef stew with the heavy lid.

Step-by-step explanation:

The explanation for this is that water boils at 100°C at normal atmospheric pressure & it remains at that temperature until all the water evaporates as steam; the temperature can then go higher in steam form.

So, if he leaves the pan uncovered the pressure above the beef stew will be the atmospheric pressure and the temperature of the stew will not pass the 100°C mark and the food cooks at 100°C.

If he places a light lid on top, the pressure will build up & the temperature at which water remains liquid is a bit higher, but eventually the steam will push the lid to escape thereby releasing pressure & reducing the temperature.

If he puts a heavy lid on the pan the pressure can build up the highest and the temperature of the water in the stew will stay the highest ensuring the fastest cooking.

Basically, the heavier the lid, greater the pressure in the pan. As the pressure gets greater, the saturation temperature at which water starts boiling will be higher. And higher boiling temperatures mean shorter cooking times.