CDS (II) 2020 General Knowledge · Question 113 of 120

Correct — D, Silver. The reactivity series ranks metals by how readily they give up electrons, and the readiness to combine with oxygen follows the same order. Magnesium sits high in the series and burns in air with a brilliant white flame to magnesium oxide. Lead and tin sit lower but still combine with oxygen on heating — lead gives litharge, PbO, and tin gives stannic oxide, SnO₂. Silver sits near the very bottom, above only mercury, platinum and gold, and it does not react with air even at red heat. That is precisely why silver, like gold, occurs in nature in the native metallic state and why a silver article does not rust or corrode away. Its familiar blackening in air is a different reaction altogether — an attack by traces of hydrogen sulphide giving silver sulphide, not an oxide.

• (a)Magnesium — The most reactive metal on the list. A magnesium ribbon burns in air with an intensely bright flame to leave a white ash of magnesium oxide, the standard school demonstration of a metal-oxygen reaction.
• (b)Lead — Lead is unreactive enough to be used for roofing and pipes, but heated in air it does form an oxide — lead(II) oxide, litharge, and on further heating red lead, Pb₃O₄. The dull grey film on a fresh lead surface is oxide.
• (c)Tin — Tin is corrosion-resistant, which is why steel cans are tin-plated, but the protection itself comes from an oxide layer. Heated in air, tin combines with oxygen to give tin(IV) oxide, SnO₂, which occurs naturally as the ore cassiterite.

The activity series arranges metals in decreasing order of reactivity, running potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver and gold. Position in the series predicts three things at once: how vigorously a metal reacts with oxygen, whether it reacts with water or with acids, and whether it can displace another metal from that metal's salt solution. The least reactive metals — silver, gold and platinum — are called noble because they resist attack and are found uncombined in nature.

A negative stem means one option has to fail a test that the other three pass, so the reliable move is to rank the four metals in the activity series and take the lowest. Magnesium is high, lead and tin are middling, silver is near the bottom, so silver is the answer. One honest complication is worth stating: silver oxide, Ag₂O, is a real compound and appears in silver-oxide button cells, so a candidate who reads the stem as 'silver has no oxide' will hesitate. The compound is made indirectly, by adding an alkali to a solution of a silver salt, and it decomposes back to metal and oxygen above about 160 degrees Celsius. What silver does not do is combine with atmospheric oxygen directly, which is what the stem is testing and what the key marks.

• Silver does not react with air or oxygen even at red heat, which is why it is found in the native metallic state.
• Magnesium burns in air with a bright white flame to give magnesium oxide.
• Lead heated in air gives lead(II) oxide, litharge, and tin gives tin(IV) oxide, the composition of the ore cassiterite.
• The activity series runs K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au — with silver near the bottom, above only mercury and gold.
• Silver oxide exists and is used in button cells, but it is prepared from silver salts and alkali, not by direct combination with air.

• Confusing silver's blackening in air with oxidation; the black film is silver sulphide.
• Concluding that because silver oxide exists, silver must combine with atmospheric oxygen. It does not.
• Assuming corrosion resistance means no oxide at all; tin and aluminium resist corrosion precisely because a thin oxide layer forms and protects them.

As a negative-stem reactivity item, as an ordering item on the activity series, or through corrosion questions on which metals survive exposure.

• practice — not a real PYQ

  Which one of the following metals is found in nature chiefly in the free or native state?

  • (a)Sodium
  • (b)Aluminium
  • (c)Gold
  • (d)Zinc

  Answer(c) Gold — its position at the bottom of the activity series means it resists attack by air, water and most acids, so it survives uncombined.

• practice — not a real PYQ

  The white ash left after a magnesium ribbon burns in air is

  • (a)magnesium carbonate
  • (b)magnesium oxide
  • (c)magnesium nitride only
  • (d)magnesium hydroxide

  Answer(b) magnesium oxide — the metal combines with atmospheric oxygen, and the oxide later reacts slowly with water to give magnesium hydroxide.