Final answer:
When magnesium is oxidized, it affects the measured volume of hydrogen gas produced in a reaction, decreasing it.
Step-by-step explanation:
When magnesium is oxidized, its metal surface turns black and dull instead of silver and shiny. This oxidation reaction occurs when magnesium reacts with oxygen from the air or other substances.
The oxidation of magnesium would have an effect on the measured volume of hydrogen gas in a reaction. In a typical reaction between magnesium and hydrochloric acid, magnesium is oxidized to form magnesium chloride, while hydrogen gas is produced. However, if the magnesium ribbon is already oxidized, it means that some of the magnesium has already reacted with oxygen and formed magnesium oxide. This reduces the amount of magnesium available for the reaction with hydrochloric acid, resulting in a lower volume of hydrogen gas being produced.
Therefore, the error of having an oxidized magnesium ribbon would lead to a decrease in the measured volume of hydrogen gas produced in the reaction.