The diagram shows a heating curve for a substance that begins as a solid. A graph titled heating curve of Substance It shows heat added in joules on the x-axis, and temperature in degrees celsius on the y-axis on a scale of 0 to 120. The graph has a diagonal line from 30 degrees up to 60 degrees. A horizontal line segment at 60 degrees. A diagonal line segment from 60 degrees up to 90 degrees. A horizontal line segment at 90 degrees and then another diagonal segment.
How many changes of state are shown?

0
1
2
3

Conventional hot-water heaters consist of a tank of water maintained at a fixed temperature. the hot water is to be used when needed. the drawback is that energy is wasted because the tank loses heat when it is not in use, and you can run out of hot water if you use too much. some utility companies are encouraging the use of on-demand water heaters (also known as flash heaters), which consist of heating units to heat the water as you use it. no water tank is involved, so no heat is wasted. a typical household shower flow rate is 2.50 gal/min (9.46 l/min ) with the tap water being heated from 51.0 ˚f (10.6 ˚c) to 120 ˚f (48.9 ˚c) by the on-demand heater.what rate of heat input (either electrical of from gas) is required to operate such a unit, assuming that all the heat goes into the water?