For the reaction C + 2 H2 --> CH4 how many moles of hydrogen are required to produce 10 moles of methane, CH4?

When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?