Metals, Non-metals and Reactivity

Why Metals and Non-metals Matter in CDS

Elements are broadly classified into metals, non-metals and a small in-between group called metalloids (such as silicon, germanium and arsenic) that show properties of both. Roughly three-quarters of all known elements are metals, so this is one of the most heavily weighted chapters in the chemistry section of the syllabus.

In the CDS and OTA papers the questions are almost always direct recall or simple reasoning — which metal is liquid at room temperature, which non-metal conducts electricity, what gas is released when zinc meets a dilute acid, why one metal can displace another. None of them need algebra, so even a few focused hours here give a very high marks-per-minute return compared with calculation-heavy topics.

The chapter also connects directly to everyday defence and engineering contexts — the steel in a rifle barrel, the aluminium in an aircraft body, the copper in field wiring — so understanding it builds genuine general awareness as well, not just exam tricks.

Physical Properties: Metals vs Non-metals

Most physical differences come from how tightly the atoms are packed and how freely electrons move. Learn these contrasting pairs:

• Lustre: metals are shiny (metallic lustre); non-metals are usually dull (exception: iodine and graphite are shiny).
• Malleability & ductility: metals can be hammered into sheets and drawn into wires; non-metals are brittle and break.
• Conductivity: metals conduct heat and electricity well; non-metals are insulators (exception: graphite, a form of carbon, conducts electricity).
• State: most metals are solid; non-metals occur as solids, liquids or gases.
• Sonority: metals are sonorous (ring when struck); non-metals are not.
• Hardness and density: most metals are hard and dense (exceptions: sodium and potassium are so soft they can be cut with a knife), while non-metals are generally soft or brittle.
• Melting and boiling points: metals usually have high melting points because of strong metallic bonding, whereas most non-metals melt and boil at low temperatures.

None of these rules is absolute, and the CDS examiner often tests precisely the cases where a property breaks down. That is why the exceptions below deserve more memory space than the general rules themselves.

Chemical Behaviour and Oxides

The deepest difference is chemical: metals tend to lose electrons and form positive ions, so they are electropositive; non-metals tend to gain electrons and form negative ions, so they are electronegative.

This shows up clearly in the oxides they form:

• Metal oxides are basic — e.g. sodium oxide (Na₂O) and magnesium oxide (MgO) turn red litmus blue. Some, like Al₂O₃ and ZnO, are amphoteric (react with both acids and bases).
• Non-metal oxides are acidic — e.g. carbon dioxide (CO₂) and sulphur dioxide (SO₂) turn blue litmus red and form acids in water.

The Reactivity Series

This is the heart of the chapter. The reactivity series (or activity series) arranges metals in decreasing order of how readily they lose electrons and react:

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

Hydrogen is included as a reference line. Metals above hydrogen displace it from dilute acids; metals below hydrogen (Cu, Hg, Ag, Au) do not.

• The higher a metal sits, the more vigorously it reacts and the harder it is to extract.
• Potassium and sodium are so reactive they are stored under kerosene to keep them away from air and moisture.
• Gold and platinum are at the bottom — so unreactive (“noble”) that they occur free in nature and never tarnish, which is why they are used in jewellery and electrical contacts.
• Reactivity in the series matches how easily a metal loses its outer electrons: the metals at the top form positive ions most readily, which is why they also act as the strongest reducing agents.

It helps to picture the series as a single ruler that you read in different directions for different questions: top means most reactive, hardest to extract, stored most carefully and able to displace everything below it; bottom means least reactive, found free in nature and unable to displace anything above it.

Reaction with Oxygen and Water

Reactivity decides how a metal behaves with air and water:

With oxygen

• Sodium and potassium catch fire even at room temperature — hence kerosene storage.
• Magnesium burns with a dazzling white light to give MgO.
• Iron does not burn but rusts slowly; copper forms a green coating; gold and silver hardly react.

With water

• K, Na react violently with cold water, releasing hydrogen that catches fire.
• Ca, Mg react with cold/hot water more gently.
• Al, Zn, Fe react only with steam, forming oxide + hydrogen.
• Cu, Ag, Au do not react with water at all.

Reaction with Acids and Salt Solutions

Metal + dilute acid → salt + hydrogen gas. For example, zinc with dilute sulphuric acid:

Zn + H₂SO₄ → ZnSO₄ + H₂↑

The hydrogen released gives a “pop” sound with a burning splint — a favourite lab-identification question. Only metals above hydrogen in the series do this; copper does not react with dilute HCl or H₂SO₄.

Displacement from salt solutions

A more reactive metal pushes out a less reactive metal from its salt solution:

Fe + CuSO₄ → FeSO₄ + Cu

Here iron (more reactive) displaces copper, and the blue solution slowly turns pale green while a reddish-brown copper deposit forms on the iron.

Worked Example: Predicting Displacement

This single line of reasoning — “is X above or below the displaced metal?” — solves almost every displacement question in the exam.

Corrosion, Rusting and Prevention

Corrosion is the slow eating-away of a metal surface by air, moisture and chemicals. The most common example is the rusting of iron, which needs both oxygen and water (moisture).

Rust is hydrated iron(III) oxide, written roughly as Fe₂O₃·xH₂O. Other metals corrode too: silver turns black (silver sulphide), copper turns green (basic copper carbonate).

Ways to prevent rusting

• Painting, oiling or greasing — keeps out air and water.
• Galvanisation — coating iron with a layer of zinc, which corrodes in place of iron.
• Tin or chromium plating and alloying (e.g. making stainless steel by adding chromium and nickel).

Alloys and Extraction of Metals

An alloy is a homogeneous mixture of a metal with one or more metals or non-metals, made to improve strength, resistance to corrosion or appearance. Common CDS-tested alloys:

• Brass = copper + zinc.
• Bronze = copper + tin.
• Steel = iron + carbon; stainless steel = iron + chromium + nickel.
• Solder = lead + tin (low melting point, used for joining wires).
• Amalgam = any alloy that contains mercury.

Extraction and the reactivity series

Position in the series decides the extraction method:

• Top (K, Na, Ca, Mg, Al): extracted by electrolysis of molten ore.
• Middle (Zn, Fe, Pb, Cu): extracted by reduction of the oxide with carbon.
• Bottom (Ag, Au, Pt): found free (native) in nature, needing little or no chemical extraction.

Previous-Year Style Question

Quick Revision

Drill the reactivity series until you can recite it instantly — it unlocks displacement, extraction, corrosion and storage questions all at once, making this one of the highest-scoring chapters in CDS chemistry.