CDS (II) 2020 General Knowledge · Question 98 of 120

Correct — D, Bromine and mercury. Of the hundred-odd elements, only two are liquid at ordinary room temperature and one atmosphere, and they sit at opposite ends of the periodic table. Mercury is the metal: its melting point is about minus 39 degrees Celsius, low enough that it pours at room temperature, which is why it filled thermometers and barometers for centuries. Bromine is the non-metal, the only halogen and the only non-metal that is liquid under these conditions, melting at about minus 7 degrees Celsius and boiling at about 59, so at 25 degrees it is a dense, fuming liquid. Every other element on Earth's surface is either a solid or a gas at 25 degrees, and each of the three wrong options works by pairing bromine or mercury with something that is not.

• (a)Bromine and fluorine — Bromine is right, fluorine is not. Fluorine is the lightest halogen and is a gas at room temperature, boiling at about minus 188 degrees Celsius. The halogen group runs gas, gas, liquid, solid down the column — fluorine and chlorine gases, bromine liquid, iodine solid.
• (b)Mercury and rubidium — The best trap on the list. Rubidium is an alkali metal so soft and low-melting that it liquefies at about 39 degrees Celsius — a warm day, but not room temperature. Caesium at about 28 degrees and gallium at about 30 degrees are the same near-miss; all three melt in the hand, none is liquid at 25 degrees.
• (c)Bromine and thallium — Thallium is a soft, heavy, highly toxic post-transition metal, but it is a solid, melting only at about 304 degrees Celsius. Its softness makes it feel like a candidate; softness and low melting point are not the same property.

Whether an element is solid, liquid or gas at a given temperature is decided by how strongly its particles hold together. Mercury's metallic bonding is unusually weak because its outer electrons are held tightly by relativistic effects on the heavy nucleus and so are reluctant to pool into the shared electron sea that makes other metals strong; the result is a metal that flows. Bromine exists as discrete Br2 molecules held together only by weak dispersion forces, but the molecules are heavy enough that those forces exceed what chlorine and fluorine can manage, putting bromine's boiling point just above room temperature.

The way into this item is to fix the two answers first and then treat every option as a test of the second element. Bromine and mercury are the only two liquids; caesium, gallium, rubidium and francium melt just above room temperature and are the standard near-misses. That is where the examiner sets the trap, and it is worth being precise about the numbers rather than the feel: rubidium melts at about 39 degrees Celsius, so it is a solid in an ordinary room and a liquid in a hot one. The wording of the stem, 'room temperature and standard pressure', is what makes the distinction decidable at all.

• Only two elements are liquid at ordinary room temperature and one atmosphere: mercury, a metal, and bromine, a non-metal.
• Mercury melts at about minus 39 degrees Celsius; bromine melts at about minus 7 and boils at about 59.
• Caesium (about 28 degrees Celsius), gallium (about 30) and rubidium (about 39) melt just above room temperature and are the usual distractors.
• Down the halogen group the physical state changes in order: fluorine and chlorine are gases, bromine is a liquid, iodine and astatine are solids.
• Bromine is the only non-metal that is liquid at room temperature.

• Counting gallium, caesium or rubidium as liquids because they melt only a little above room temperature.
• Assuming any halogen adjacent to bromine behaves like it — fluorine and chlorine are gases, iodine is a solid.
• Reading softness as low melting point; thallium is soft and melts at about 304 degrees Celsius.

Almost always as a pair-identification item, occasionally reversed into 'which one of the following is a gas or solid at room temperature' with bromine or mercury as the odd one out.

• practice — not a real PYQ

  Which one of the following elements is a solid at room temperature?

  • (a)Bromine
  • (b)Mercury
  • (c)Iodine
  • (d)Chlorine

  Answer(c) Iodine — a violet-black solid that sublimes on gentle warming. Bromine and mercury are liquids and chlorine is a gas.

• practice — not a real PYQ

  Which one of the following metals has a melting point low enough for it to melt in the palm of the hand?

  • (a)Aluminium
  • (b)Gallium
  • (c)Iron
  • (d)Copper

  Answer(b) Gallium — it melts at about 30 degrees Celsius, just above room temperature and below body temperature.