If five pounds of water are at 50 degrees F, how many BTUs must be removed to freeze it completely?

To determine the number of BTUs that must be removed to freeze five pounds of water initially at 50 degrees Fahrenheit, we need to understand the concept of specific heat and the latent heat of fusion.

First, we need to lower the temperature of the water from 50 degrees F to 32 degrees F, which is the freezing point of water. The amount of BTUs required to cool the water can be calculated using the specific heat of water, which is approximately 1 BTU per pound per degree Fahrenheit.

The temperature change required is:

32°F - 50°F = -18°F

Next, you calculate the BTUs needed to cool the five pounds of water:

5 pounds x 18 degrees F = 90 BTUs

Now that the water is at freezing point, we need to freeze the water. The latent heat of fusion for water is approximately 144 BTUs per pound.

For five pounds of water, you would calculate:

5 pounds x 144 BTUs/pound = 720 BTUs

Finally, you combine the amount of BTUs required to cool the water to the freezing point and the BTUs required to freeze it:

90 BTUs (to cool) + 720 BTUs (to freeze