EPFO EO/AO 2023 General Ability Test · Question 53 of 120

Correct — B, (b) All compounds containing hydrogen are acidic. The booklet prints the 'not' of the stem in bold italic, so the item asks for the statement that is FALSE, and this universal claim is the one that collapses on the first counterexample. Containing hydrogen is not what makes a compound an acid. What makes a compound an acid is its ability to release hydrogen ions into aqueous solution, and that happens only where the hydrogen is bonded to an atom electronegative enough to keep the shared electrons and let the hydrogen leave as a bare proton. Where it is not, the compound behaves quite differently. Methane and the hydrocarbons generally are neutral. Ammonia is hydrogen-rich and BASIC — in water it accepts a proton and leaves hydroxide ions behind. Glucose contains twelve hydrogen atoms per molecule and is neutral. Sodium hydride reacts with water to give sodium hydroxide, which is an alkali. Water itself, two hydrogens to the molecule, is neutral and behaves as an acid or a base according to what it meets. One counterexample would be enough to defeat a statement beginning 'all'; there are dozens. The three remaining statements are true, and a candidate who recognises where they come from can answer the question in seconds — all three are propositions from the school chemistry treatment of acids, bases and salts, reproduced almost in their textbook wording. The useful lesson beneath the item is about the shape of the claim rather than the chemistry. A statement that quantifies over everything — 'all', 'every', 'always', 'never' — asks to be tested against the awkward case, and in a negative-ask question it is the first place to look. Here the awkward cases are the commonest compounds in the syllabus.

• (a)Hydrogen ions cannot exist alone. — TRUE, so it cannot answer a 'not' stem. A hydrogen ion is a hydrogen atom stripped of its only electron — that is, a bare proton, with a charge density enormously higher than any other cation because there are no electrons around it at all. It cannot persist by itself in water: it attaches immediately to a water molecule to form the hydronium ion. This is why the ions of an acid in solution are written as hydrated hydrogen ions or as hydronium ions rather than as a free proton.
• (c)Separation of H+ ions from HCl molecules cannot occur in the absence of water. — TRUE, and it is the point the classic school demonstration is designed to make. Dry hydrogen chloride gas does not turn dry blue litmus paper red; introduce moisture and the paper turns at once. The gas is covalent, and it takes water to pull the molecule apart, the released proton being stabilised by attachment to a water molecule. The corollary is the one worth carrying: acidic behaviour is a property of the aqueous solution, not of the pure substance, which is also why anhydrous acids can be handled in ways their solutions cannot. In the booklet the plus sign of the hydrogen ion is printed as a superscript.
• (d)Water soluble bases are known as alkalis. — TRUE by definition. All alkalis are bases, but not all bases are alkalis — the term is reserved for those bases that dissolve in water and so can furnish hydroxide ions in solution. Sodium hydroxide and potassium hydroxide are alkalis; calcium hydroxide is sparingly soluble and is usually counted as one; copper oxide and iron oxide are bases that are not alkalis, being insoluble. Since the alkalis are chiefly the hydroxides of the alkali metals, the name and the group name share an origin.

The oldest workable definition, due to Arrhenius, is the one school chemistry uses: an acid is a substance that furnishes hydrogen ions in aqueous solution and a base is one that furnishes hydroxide ions. The Bronsted-Lowry definition generalises it by making an acid a proton donor and a base a proton acceptor, which explains why ammonia is a base although it contains no hydroxide, and why water can act as either. Neither definition says anything about the mere presence of hydrogen in the formula. Strength is a separate axis from concentration: a strong acid such as hydrochloric, sulphuric or nitric ionises almost completely in water, while a weak acid such as acetic or carbonic ionises only partly, and either can be made dilute or concentrated. The pH scale measures the hydrogen ion concentration on a logarithmic scale from zero to fourteen, with seven neutral at ordinary temperature, values below it acidic and above it alkaline; a change of one unit therefore means a tenfold change in the concentration. The everyday consequences are the examinable ones — the acid in the stomach and the antacid that neutralises it, the pH range in which tooth enamel begins to corrode, the acidity of rain, and the liming of acidic soils.

The chemistry items in the general science block of this paper stay close to the school syllabus and are frequently lifted, in substance, from its statements. That has a practical implication for preparation: the fastest route through this block is to have read the school chapters on acids and bases, the periodic table and metals and non-metals, rather than to have studied chemistry at a higher level. The habit rewarded here is a logical one that costs no chemistry at all — look first at the statement that claims something about everything, because a universal claim is the easiest to break.

• An acid furnishes hydrogen ions in aqueous solution; a base furnishes hydroxide ions.
• Containing hydrogen does not make a compound acidic — methane is neutral, ammonia is basic, glucose is neutral.
• A hydrogen ion is a bare proton and cannot exist alone in water; it combines with a water molecule to form the hydronium ion.
• Hydrogen chloride ionises only in the presence of water — dry hydrogen chloride gas does not turn dry litmus red.
• Bases soluble in water are called alkalis; all alkalis are bases but not all bases are alkalis.
• Strong and weak refer to the degree of ionisation; concentrated and dilute refer to the amount of water present.
• The pH scale runs from zero to fourteen, with seven neutral at ordinary temperature, and each unit is a tenfold change in hydrogen ion concentration.
• Under the Bronsted-Lowry definition an acid is a proton donor and a base a proton acceptor, which is why ammonia counts as a base.

• Reading the formula for hydrogen and concluding the compound is an acid, which fails on ammonia and on every hydrocarbon.
• Missing the bold-italic 'not' and marking the first statement that reads as true.
• Treating strong and concentrated as synonyms.
• Assuming every base is an alkali, when only the soluble ones are.

Chemistry in EO/AO papers turns up as acids and bases, the periodic table and its trends, common gases and their properties, alloys and everyday chemicals. The negative-statement format used here is the Commission's favourite for the subject, and the false option is usually either a universal claim or a swap of two related terms, so those two shapes are worth hunting for before the chemistry is even considered.

No directly related past PYQ was found.

• practice — not a real PYQ

  An aqueous solution turns red litmus blue. Its pH is likely to be :

  • (a)1
  • (b)4
  • (c)5
  • (d)10

  Answer(d) 10

• practice — not a real PYQ

  Which one of the following is a base but NOT an alkali ?

  • (a)Sodium hydroxide
  • (b)Potassium hydroxide
  • (c)Copper oxide
  • (d)Ammonium hydroxide

  Answer(c) Copper oxide