Chapter 1 · Question 2

Why should a magnesium ribbon be cleaned with sandpaper before burning in air? Write the balanced chemical equation for the burning of magnesium ribbon.

Answer Revealed

Direct Answer:

Magnesium ribbon is cleaned before burning because it develops a thin layer of magnesium oxide (MgO) on its surface when exposed to air. This oxide layer prevents the magnesium from making direct contact with oxygen, making ignition difficult. Cleaning with sandpaper removes this layer so the fresh metal surface can react freely. Balanced equation:

2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)} + \text{Heat}

Simple Explanation

When magnesium sits in air, it develops a dull coating of magnesium oxide that acts like a shield, blocking oxygen. Rubbing it with sandpaper scrubs off that shield, exposing fresh magnesium metal that burns easily when heated. The reaction is

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

— two atoms of magnesium combine with one oxygen molecule to make two units of magnesium oxide, and a lot of heat is released.

Exam-Ready Structure

The need to clean magnesium ribbon before burning is directly linked to its reactivity with atmospheric oxygen: 1. When magnesium is exposed to air, it forms a thin layer of magnesium oxide (MgO) on its surface. 2. This oxide layer acts as a barrier — it prevents the underlying magnesium metal from coming into direct contact with oxygen. 3. Cleaning with sandpaper physically removes the oxide layer, exposing the shiny, reactive magnesium metal underneath. 4. Once the fresh metal is exposed and heated, it reacts vigorously with oxygen:

2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}

5. The reaction is both a combination reaction (two reactants → one product) and an exothermic reaction (heat is released, producing a dazzling white flame). The white ash collected in the watch-glass is magnesium oxide powder. This experiment must be performed with teacher supervision, wearing eye protection, and the burning ribbon should be held away from the eyes.

Key Points

Magnesium ribbon forms a thin, dull layer of magnesium oxide ( $\text{MgO}$ ) on air exposure
The oxide layer prevents direct contact of magnesium with oxygen, hindering ignition
Sandpaper/cleaning removes the oxide layer to expose fresh reactive magnesium metal
Balanced equation: $2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)} + \text{Heat}$
This is both a combination reaction and an exothermic reaction

Why should a magnesium ribbon be cleaned with sandpaper before burning in air? Write the balanced chemical equation for the burning of magnesium ribbon.