1.5 Phase changes  (Page 8/20)

Freeze-dried foods have been dehydrated in a vacuum. During the process, the food freezes and must be heated to facilitate dehydration. Explain both how the vacuum speeds up dehydration and why the food freezes as a result.

In a physics classroom demonstration, an instructor inflates a balloon by mouth and then cools it in liquid nitrogen. When cold, the shrunken balloon has a small amount of light blue liquid in it, as well as some snow-like crystals. As it warms up, the liquid boils, and part of the crystals sublime, with some crystals lingering for a while and then producing a liquid. Identify the blue liquid and the two solids in the cold balloon. Justify your identifications using data from [link] .

How much heat transfer (in kilocalories) is required to thaw a 0.450-kg package of frozen vegetables originally at $0\text{°}\text{C}$ if their heat of fusion is the same as that of water?

A bag containing $0\text{°}\text{C}$ ice is much more effective in absorbing energy than one containing the same amount of $0\text{°}\text{C}$ water. (a) How much heat transfer is necessary to raise the temperature of 0.800 kg of water from $0\text{°}\text{C}$ to $30.0\text{°}\text{C}$ ? (b) How much heat transfer is required to first melt 0.800 kg of $0\text{°}\text{C}$ ice and then raise its temperature? (c) Explain how your answer supports the contention that the ice is more effective.

(a) How much heat transfer is required to raise the temperature of a 0.750-kg aluminum pot containing 2.50 kg of water from $30.0\text{°}\text{C}$ to the boiling point and then boil away 0.750 kg of water? (b) How long does this take if the rate of heat transfer is 500 W?

Condensation on a glass of ice water causes the ice to melt faster than it would otherwise. If 8.00 g of vapor condense on a glass containing both water and 200 g of ice, how many grams of the ice will melt as a result? Assume no other heat transfer occurs. Use $L_v$ for water at $37\text{°}\text{C}$ as a better approximation than $L_v$ for water at $100\text{°}\text{C}$ .)

On a trip, you notice that a 3.50-kg bag of ice lasts an average of one day in your cooler. What is the average power in watts entering the ice if it starts at $0\text{°}\text{C}$ and completely melts to $0\text{°}\text{C}$ water in exactly one day?

On a certain dry sunny day, a swimming pool’s temperature would rise by $1.50\text{°}\text{C}$ if not for evaporation. What fraction of the water must evaporate to carry away precisely enough energy to keep the temperature constant?