What is the colour of ash produced by burning magnesium ribbon in air?

White powder

The correct answer is option 2. White powder.

When magnesium ribbon is burned in air, it undergoes a chemical reaction with oxygen to form magnesium oxide \((MgO)\). The reaction can be represented by the equation:

\[ 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \]

During this reaction, magnesium atoms combine with oxygen molecules to form magnesium oxide molecules. Magnesium oxide is a white, powdery substance. This powdery residue is what remains after the magnesium ribbon has burned completely.

The white color of magnesium oxide ash is due to its chemical composition and structure. Magnesium oxide is a compound consisting of magnesium cations \((Mg^{2+})\) and oxygen anions \((O^{2-})\) arranged in a crystal lattice structure. This arrangement scatters light in such a way that it appears white to the human eye.

Therefore, when magnesium ribbon is burned in air, the ash produced is a white powder, primarily composed of magnesium oxide.