Study notes

Acid-Base Chemistry

What makes a solution acidic or alkaline, the pH scale read from Universal Indicator, strong versus weak acids, the reactions of acids, and classifying oxides.

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Acids and alkalis are defined by the ions they produce in water

An acid is a substance that produces hydrogen ions, $H^{+}$ , when dissolved in water. An alkali is a soluble base that produces hydroxide ions, $O H^{-}$ , when dissolved in water. It is the presence of these ions in aqueous solution, not the substance itself, that gives a solution its acidic or alkaline behaviour.

Universal Indicator shows acidity, neutrality and alkalinity by colour

Universal Indicator turns a range of colours depending on the solution it is added to: red through orange to yellow for acidic solutions, green for neutral, and blue through to purple for alkaline solutions. The colour is compared visually against a reference chart or scale to describe how acidic or alkaline a solution is.

The pH scale is read, never calculated, at this level

The pH scale runs from 0 to 14 and describes relative acidity, alkalinity or neutrality: pH below 7 is acidic, pH exactly 7 is neutral, and pH above 7 is alkaline. A lower pH means more strongly acidic and a higher pH means more strongly alkaline. At this level, pH is read off Universal Indicator's colour against the pH scale, it is never calculated from a hydrogen ion concentration.

Strong and weak acids differ in how completely they ionise

A strong acid, such as hydrochloric acid, ionises almost completely in water, releasing a large proportion of the $H^{+}$ ions it could possibly release. A weak acid, such as ethanoic acid, only partially ionises in water, so a much smaller proportion of $H^{+}$ ions is released. This is a qualitative distinction about the EXTENT of ionisation, not about how concentrated or dilute the solution is.

Acids react with reactive metals to give a salt and hydrogen

A reactive metal reacts with a dilute acid to form a salt and hydrogen gas, e.g. $M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$ . Bubbling seen at the metal's surface is hydrogen gas being given off, and the gas can be confirmed with a lit splint, which produces a squeaky pop.

Acids react with bases and carbonates to give a salt

An acid reacts with a base (including alkalis) to form a salt and water only, e.g. $2 H C l (a q) + C u O (s) \to C u C l_{2} (a q) + H_{2} O (l)$ . An acid reacts with a carbonate to form a salt, water, and carbon dioxide gas, e.g. $2 H C l (a q) + N a_{2} C O_{3} (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$ , and the carbon dioxide gas turns limewater cloudy.

Neutralisation is the reaction of $H^+$ with $OH^-$

Neutralisation is the reaction between an acid and an alkali (or base), in which hydrogen ions react with hydroxide ions to form water: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ . The salt produced comes from the negative ion of the acid combining with the positive ion of the base.

Oxides can be classified as acidic, basic, amphoteric or neutral

A basic oxide is a metal oxide that reacts with acids to form a salt and water only, e.g. copper(II) oxide. An acidic oxide is a non-metal oxide that reacts with alkalis to form a salt and water only, e.g. carbon dioxide, and typically dissolves in water to form an acidic solution. An amphoteric oxide can react with BOTH acids and alkalis, e.g. aluminium oxide and zinc oxide. A neutral oxide reacts with neither acids nor alkalis, e.g. carbon monoxide and water.

Worked examples

A colourless solution turns Universal Indicator dark red. A student says "the pH of this solution is exactly 1, calculated from its hydrogen ion concentration." Explain what is wrong with this statement.

Dark red on Universal Indicator does correctly indicate a strongly acidic solution, so that part of the observation is fine.
However, at this level, pH is determined by READING the colour of Universal Indicator against the pH scale, it is never calculated from a hydrogen ion concentration value, that calculation is explicitly beyond this syllabus.
So while it is correct to say the solution's dark red colour corresponds to a low pH value (strongly acidic, roughly in the 0 to 2 range read from the colour chart), it is incorrect to claim an exact pH of 1 was CALCULATED.
The correct statement is that the pH is read as a low, strongly acidic value from the Universal Indicator colour, not calculated.

Solution A is dilute hydrochloric acid and Solution B is dilute ethanoic acid, both at the same concentration. Solution A produces bubbles much more vigorously than Solution B when each is added to magnesium ribbon. Explain this difference in terms of strong and weak acids.

Both solutions contain the same concentration of acid dissolved in water, so concentration alone cannot explain the difference in reaction rate.
Hydrochloric acid is a strong acid: it ionises almost completely in water, releasing a high proportion of $H^{+}$ ions into solution.
Ethanoic acid is a weak acid: it only partially ionises in water, so at the same overall concentration, far fewer $H^{+}$ ions are actually present in solution.
Since it is the concentration of $H^{+}$ ions available to react with the magnesium that drives the reaction, Solution A (more $H^{+}$ ions from more complete ionisation) reacts faster and bubbles more vigorously than Solution B.

A student adds excess copper(II) oxide powder to warm dilute sulfuric acid until no more dissolves, filters off the leftover solid, and is left with a blue solution. Write the word equation for this reaction and explain why using excess copper(II) oxide, rather than exact amounts, is a sensible method.

Copper(II) oxide is a basic oxide, and a base reacts with an acid to form a salt and water only.
Word equation: copper(II) oxide + sulfuric acid $\to$ copper(II) sulfate + water, matching the pattern $C u O (s) + H_{2} S O_{4} (a q) \to C u S O_{4} (a q) + H_{2} O (l)$ .
Using excess (more than enough) copper(II) oxide ensures that ALL of the sulfuric acid is used up and neutralised, since the acid is the reactant being consumed to make the desired salt.
Filtering removes the unreacted, leftover solid copper(II) oxide, leaving behind a solution containing only pure copper(II) sulfate and water, with no leftover acid to contaminate the product, which is exactly why excess base is a standard method for this type of neutralisation.

Three oxides are tested: oxide X reacts with dilute hydrochloric acid but not with sodium hydroxide solution; oxide Y reacts with both dilute hydrochloric acid and sodium hydroxide solution; oxide Z reacts with neither. Classify X, Y and Z, and explain your reasoning.

Oxide X reacts with an acid only, forming a salt and water; an oxide that reacts with acids but not alkalis is a basic oxide (typically a metal oxide).
Oxide Y reacts with BOTH an acid and an alkali; an oxide able to react with both is classified as amphoteric, such as aluminium oxide or zinc oxide.
Oxide Z reacts with neither an acid nor an alkali; an oxide that reacts with neither is classified as a neutral oxide, such as carbon monoxide.
So X is a basic oxide, Y is an amphoteric oxide, and Z is a neutral oxide, based purely on which of the two reagents each oxide reacted with.

Mind map

Acid-Base Chemistry

Definitions

acid: produces H+ in water · alkali: produces OH- in water

pH scale

read from Universal Indicator colour · never calculated from [H+] · below 7 acidic, 7 neutral, above 7 alkaline

Strong vs weak acids

strong: almost complete ionisation · weak: partial ionisation · not about concentration

Reactions of acids

acid + metal -> salt + hydrogen · acid + base -> salt + water · acid + carbonate -> salt + water + CO2

Neutralisation

H+ + OH- -> H2O

Classifying oxides

basic: reacts with acids · acidic: reacts with alkalis · amphoteric: reacts with both · neutral: reacts with neither

Practise this topic →

Acid-Base Chemistry

Learn it step by step

Acids and alkalis are defined by the ions they produce in water

Universal Indicator shows acidity, neutrality and alkalinity by colour

The pH scale is read, never calculated, at this level

Strong and weak acids differ in how completely they ionise

Acids react with reactive metals to give a salt and hydrogen

Acids react with bases and carbonates to give a salt

Neutralisation is the reaction of \(H^+\) with \(OH^-\)

Oxides can be classified as acidic, basic, amphoteric or neutral

Worked examples

Mind map