Let’s consider a more complex problem:
How much heat is required to change 10.0 g of ice at -14oC into steam at 151oC?
Since we now know that there are different formulas for heat associated with temperature changes and heat associated with phase changes, we need to look carefully at what will happen as heat is applied.
When the ice is heated, what happens first?
It is tempting to answer that it will melt, but that’s not right. The ice isn’t warm enough to melt. It must first warm up to 0 oC. After that, it can melt. During the process of melting, the temperature will not change resulting in 10.0 g of liquid water at 0 oC. If we continue to heat it, the water will warm up to the boiling point (100 oC). The water will then boil (with no change in temperature). Additional heat will then be needed to warm the steam up to 151 oC.
In simpler terms, this is a 5 step process:
1: ice warming from -14 oC to ice at 0 oC
2: melting ice
3: water warming from 0 oC to 100 oC
4: boiling water
5: steam warming from 100 oC to 151 oC
Or, if you prefer pictures…
Each step requires heat and each step must be calculated separately. The heat required for the entire process will then be the heat of the five steps added together. More...