How many joules are required to boil 150.0 grams of water?

Expert Answers
gsenviro eNotes educator| Certified Educator

To boil a given quantity of water, we need to supply some heat to it. The amount of heat needed to increase the temperature of any substance is dependent on the mass of the substance, its specific heat capacity and the change in temperature. The specific heat capacity of a substance is defined as the amount of heat needed to raise the temperature of 1 g of the substance by 1 degrees C. The specific heat capacity of water is 4.184 J/g/K.

Let's say we have to find the amount of energy needed to raise the temperature of 150 g of water 75 degrees Celsius. For the equation, I'll say water at 25 degrees Celsius must be heated to 100 degrees Celsius, the boiling point of water. To convert degrees Celsius to degrees Kelvin, add 273 to the Celsius value. 25 degrees Celsius is 298 degrees Kelvin; 100 degrees Celsius, 373 degrees Kelvin.

This can be calculated as:

energy needed = mass of water x specific heat capacity of water x temperature change 

= 150 g x 4.184 J/g/K x (373 - 298) K = 47,070 J.

Thus, 47,070 J are needed to increase the temperature of 150 g of water from 25 degrees C to its boiling point of 100 degrees C. 

One can follow a similar procedure to determine the energy needed to heat water from one temperature to another. Note that phase change is not taking place in the current case. If the water were to be converted to steam, we will also need to include the heat energy needed for phase change.

Hope this helps.

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