Chemical Properties of Non-Metals and Oxides | Class 10

Quick answer: Non-metals tend to gain electrons, so their chemistry is the opposite of metals. They burn in oxygen to form acidic oxides (such as $\mathrm{SO_2}$ and $\mathrm{CO_2}$) or neutral oxides (such as $\mathrm{CO}$ and $\mathrm{N_2O}$). They do not displace hydrogen from dilute acids, because they cannot give electrons to $\mathrm{H^+}$ ions.

Electronic nature of non-metals

Non-metal atoms usually have 4, 5, 6 or 7 electrons in their outermost shell:

• They complete their octet by gaining or sharing electrons (carbon only shares).
• When they gain electrons they form negative ions, for example $\mathrm{Cl + e^- \to Cl^-}$.
• Because they take electrons from other substances, non-metals tend to act as oxidising agents, while metals tend to act as reducing agents.

Reaction with oxygen: acidic and neutral oxides

When non-metals burn in air or oxygen they form covalent oxides. These fall into two groups.

Acidic oxides

Sulphur dioxide. Sulphur burns in oxygen with a blue flame:

$\mathrm{S(s) + O_2(g) \longrightarrow SO_2(g)}$

In water, sulphur dioxide forms sulphurous acid, which turns blue litmus red:

$\mathrm{SO_2(g) + H_2O(l) \longrightarrow H_2SO_3(aq)}$

Carbon dioxide forms carbonic acid, a weak acid:

$\mathrm{CO_2(g) + H_2O(l) \longrightarrow H_2CO_3(aq)}$

Phosphorus pentoxide. Its actual formula is $\mathrm{P_4O_{10}}$ (written $\mathrm{P_2O_5}$ as the simplest ratio). With water it forms phosphoric acid:

$\mathrm{P_4O_{10}(s) + 6H_2O(l) \longrightarrow 4H_3PO_4(aq)}$

Neutral oxides

Some non-metal oxides show neither acidic nor basic properties. They do not change the colour of litmus and do not react with acids or bases to form salts. Common examples:

• Carbon monoxide, $\mathrm{CO}$
• Nitrous oxide, $\mathrm{N_2O}$ (laughing gas)
• Nitric oxide, $\mathrm{NO}$
• Water, $\mathrm{H_2O}$

Remember: Metal oxides are basic (a few, like $\mathrm{Al_2O_3}$ and $\mathrm{ZnO}$, are amphoteric). Non-metal oxides are acidic or neutral.

Why don't non-metals react with dilute acids?

When a metal such as zinc reacts with dilute hydrochloric acid, hydrogen gas is given off:

$\mathrm{Zn(s) + 2HCl(aq) \longrightarrow ZnCl_2(aq) + H_2(g)}$

To make hydrogen gas, the $\mathrm{H^+}$ ions from the acid must gain electrons:

$\mathrm{2H^+ + 2e^- \longrightarrow H_2}$

1. A metal gives electrons easily, so it can supply these electrons to $\mathrm{H^+}$ ions.
2. A non-metal tends to accept electrons, so it cannot supply electrons to $\mathrm{H^+}$ ions.
3. So non-metals such as carbon and sulphur do not displace hydrogen from dilute acids.

Note on water: Most non-metals do not react with water under ordinary conditions. White phosphorus is even stored under water because it catches fire in air but does not react with water. There are exceptions at higher levels (for example, chlorine reacts slightly with water and red-hot carbon reacts with steam), but these are not displacement of hydrogen in the Class 10 sense.

Displacement among non-metals (extension)

Syllabus note: NCERT Class 10 teaches displacement reactions for metals. The halogen displacement below is useful extra knowledge (it is studied in detail in higher classes) and is not a core Class 10 NCERT topic.

A more reactive halogen displaces a less reactive one from its salt solution. The order of reactivity is:

$\mathrm{F_2 > Cl_2 > Br_2 > I_2}$

Chlorine and potassium bromide. The colourless solution turns orange-brown as bromine is formed:

$\mathrm{Cl_2(g) + 2KBr(aq) \longrightarrow 2KCl(aq) + Br_2(aq)}$

Chlorine and potassium iodide. The solution turns brown as iodine is formed:

$\mathrm{Cl_2(g) + 2KI(aq) \longrightarrow 2KCl(aq) + I_2(aq)}$

In both cases chlorine takes electrons from the halide ions, for example $\mathrm{Cl_2 + 2Br^- \to 2Cl^- + Br_2}$.

Summary table: metals vs non-metals

Exam tips

• If asked to "name two neutral oxides", CO and N₂O are the safest answers.
• For "why don't non-metals give hydrogen with acids", write the equation $\mathrm{2H^+ + 2e^- \to H_2}$ and explain that non-metals cannot supply the electrons.
• To test the nature of an oxide, dissolve it in water and test with litmus: acidic turns blue litmus red, basic turns red litmus blue.
• Write $\mathrm{P_4O_{10}}$ or $\mathrm{P_2O_5}$ consistently, and balance the equation for the form you choose.

Common mistakes

• Writing that carbon reacts with dilute HCl to give hydrogen. Non-metals do not displace hydrogen from dilute acids.
• Calling all non-metal oxides acidic. CO, N₂O and NO are neutral.
• Calling aqueous iodine "violet". Iodine vapour is violet; iodine in solution looks brown.
• Saying non-metals "never" react with water. Most do not under ordinary conditions, but there are exceptions.

Frequently Asked Questions

What type of oxides do non-metals form?

Non-metals form covalent oxides that are either acidic, such as CO₂ and SO₂, or neutral, such as CO and N₂O.

Name two neutral non-metallic oxides.

Carbon monoxide (CO) and nitrous oxide (N₂O). Nitric oxide (NO) and water (H₂O) are also neutral.

Why do non-metals not displace hydrogen from dilute acids?

Hydrogen gas is formed only when H⁺ ions gain electrons. Non-metals tend to accept electrons rather than give them, so they cannot reduce H⁺ ions to hydrogen gas.

How can you tell if an oxide is acidic?

Dissolve it in water and test the solution with blue litmus. An acidic oxide forms an acid, which turns blue litmus red.

What happens when chlorine is passed through potassium bromide solution?

Chlorine displaces bromine because it is more reactive. The solution turns orange-brown: Cl₂ + 2KBr → 2KCl + Br₂.