Which gas in air does the magnesium in a flare react with? What kind of reaction is happening when magnesium reacts with this gas? What is the...

The gas in air that reacts with magnesium is oxygen. The type of reaction that takes place is called synthesis or combination. In this type of reaction there are two or more reactants and one product. Here's the equation for the reaction:

`2 Mg + O_2 -gt 2MgO `

There are two foms of evidence that a chemical reaction is taking place. One is the release of heat and very bright light. The other is the formation of a new substance: White, powdery magnesium oxide forms as the magnesium burns.

Here are the word equations for the reaction of magnesium, lead and potassium with the same gas, oxygen:

Solid magnesium burns with oxygen gas to produce magnesium oxide

Hot molten lead reacts with oxygen gas to produce lead(II)oxide 

Potassium metal burns with oxygen to produce potassium peroxide