Combustion of element

Combustion (burning) is a fast reaction of a substance with oxygen that releases energy (it is exothermic). When an element burns in oxygen, the product is the oxide of that element.

The key to this whole topic is one rule:

• Metals → basic oxides (some dissolve to give alkaline solutions).
• Non-metals → acidic oxides (they dissolve in water to form acids).
• The one important exception is hydrogen, whose oxide is water — neutral.

So if an exam gives you an element and asks about the oxide, your first move is: is it a metal or a non-metal? That tells you whether the oxide is basic or acidic before you write anything else.

Magnesium is a reactive metal, so it gives a basic oxide.

What you see. A strip of magnesium ribbon, once lit, burns with an intense, dazzling bright white flame (so bright it can damage your eyes — never look straight at it). It leaves behind a white powder/ash of magnesium oxide.

Word equation: magnesium + oxygen → magnesium oxide

Symbol equation:

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

The oxide. Magnesium oxide (MgO) is a white solid and a basic oxide. Because it is slightly soluble, it reacts with water to form magnesium hydroxide, an alkali (pH above 7):

$\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(aq/s)}$

Being basic, MgO neutralises acids — this is why magnesium oxide and magnesium hydroxide are used in indigestion remedies ("milk of magnesia").

Memory hook: metal magnesium → metal oxide → it's a BASE.

Hydrogen is the special case: it is a non-metal, but its oxide is water, which is neutral (neither acidic nor basic).

What you see/hear. Hydrogen burns very readily. The classic test is the squeaky pop: hold a lighted splint to a tube of hydrogen and it ignites with a sharp "pop". The product is water — you can often see tiny droplets condense on a cool surface held in the flame.

Word equation: hydrogen + oxygen → water

Symbol equation:

$2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}$

The oxide. Water (H₂O) is neutral — pH 7. This makes hydrogen a clean fuel: burning it produces only water and no polluting acidic gases or carbon dioxide.

Carbon and sulfur are non-metals, so both give acidic oxides.

Carbon. Carbon (e.g. charcoal) glows red and burns in plenty of oxygen to form carbon dioxide:

$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}$

• Carbon dioxide is an acidic oxide. It turns limewater milky/cloudy (the standard test for CO₂).
• It dissolves a little in water to form the weak acid carbonic acid:

$\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{CO}_3\text{(aq)}$

Sulfur. Sulfur burns with a distinctive blue flame to form sulfur dioxide, a colourless gas with a sharp, choking smell:

$\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$

• Sulfur dioxide is an acidic oxide. It dissolves in water to form the acid sulfurous acid:

$\text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_3\text{(aq)}$

• This is why burning sulfur-containing fuels causes acid rain (a link examiners love to make).

Use this one table for any "burning an element" question:

Element	Type	Word equation	Symbol equation	Oxide	Acid/base
Magnesium	metal	magnesium + oxygen → magnesium oxide	2Mg + O₂ → 2MgO	MgO (white solid)	basic
Hydrogen	non-metal*	hydrogen + oxygen → water	2H₂ + O₂ → 2H₂O	H₂O (water)	neutral
Carbon	non-metal	carbon + oxygen → carbon dioxide	C + O₂ → CO₂	CO₂ (gas)	acidic
Sulfur	non-metal	sulfur + oxygen → sulfur dioxide	S + O₂ → SO₂	SO₂ (gas)	acidic

*Hydrogen is a non-metal but its oxide, water, is the neutral exception.

The observations to memorise (these are the high-value identification marks):

• Magnesium → bright white flame + white ash.
• Hydrogen → squeaky pop + water droplets.
• Carbon → glows + gas that turns limewater milky.
• Sulfur → blue flame + choking gas.

Acid–base behaviour links:

• A basic oxide (like MgO) neutralises acids.
• An acidic oxide (like CO₂, SO₂) neutralises alkalis and turns damp blue litmus red when dissolved.