A type of cold pack has two separate sections inside. One section contains a liquid and the other section contains a powder. When the cold pack is squeezed, the section that holds the liquid breaks. When the liquid combines with the powder, the pack becomes very cold. Which is the best explanation why the temperature of the pack decreases when the liquid and powder combine?

You exhale co2 which is produced during cellular respiration.  co2 combines with the water in your blood's plasma to make up one half of the body's most important buffer pair, carbonic acid.  the more physical activity you engage in, the more co2 your body is producing.  you can see this by putting some of the cabbage indicator in a glass and then blowing bubbles into it through a straw.  can you see a change in the color of the indicator?