The lowering piece of burning magnesium in a carbon dioxide gas jar is most likely referring to an experimental setup used to demonstrate the reaction of magnesium with carbon dioxide. This setup typically involves a glass jar or test tube filled with carbon dioxide gas (CO2) and a small piece of burning magnesium that is slowly lowered into the gas.
When magnesium metal reacts with carbon dioxide, it undergoes a combustion reaction, producing magnesium oxide (MgO) and carbon dioxide gas. The burning of magnesium is highly exothermic, producing a bright white light and releasing a significant amount of heat.
By lowering the burning magnesium into the carbon dioxide gas jar, you can observe the reaction between magnesium and carbon dioxide. The magnesium will continue to burn in the presence of carbon dioxide, forming magnesium oxide and releasing carbon dioxide gas. The white light emitted by the burning magnesium is quite intense and can be used to illustrate the exothermic nature of the reaction.