Maria and carla were hiking when maria tripped, fell, and sprained her ankle. carla took out the first-aid kit and unwrapped a cold pack. carla popped a vial that was inside the cold pack. the two chemicals inside the pack mixed together and the pack began to get icy cold. carla applied the pack to maria's ankle. what kind of reaction occurred in the cold pack? what evidence supports your answer? question 11 options: a. it is a physical change because the substance in the vial dissolved in the pack's liquid. b. it is a chemical change because two different chemicals are mixed together. c. it is a chemical change because heat energy is absorbed and the pack gets colder. d. it is a physical change as evidenced by a drop in temperature.

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?