Acids, Bases & Salt Preparation

Acids and Alkalis

Acids and alkalis are two families of chemicals that sit at opposite ends of the pH scale. What makes an acid acidic is the ion it releases when dissolved in water.

An acid is a substance that produces hydrogen ions, $H^+$, when dissolved in water. Common laboratory acids include hydrochloric acid ($HCl$), sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$).

A base is a substance that neutralises an acid. A base that is soluble in water is called an alkali, and it produces hydroxide ions, $OH^-$, in solution. Examples include sodium hydroxide ($NaOH$) and potassium hydroxide ($KOH$). Ammonia solution is also alkaline.

Key terms Acid — produces $H^+$ ions in aqueous solution.

Base — neutralises an acid (often a metal oxide or hydroxide).

Alkali — a soluble base; produces $OH^-$ ions in aqueous solution.

The pH Scale and Indicators

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.

pH less than 7 → acidic (the lower the number, the more acidic)
pH equal to 7 → neutral (e.g. pure water)
pH greater than 7 → alkaline (the higher the number, the more alkaline)

Indicators are dyes that change colour depending on pH. You must know these four:

Universal indicator is special: it gives a range of colours and so estimates the actual pH value, not just acid-or-alkali. Litmus, methyl orange and phenolphthalein only tell you which side of neutral you are on.

Exam tip Phenolphthalein is the classic titration indicator: a sharp change between colourless (acid) and pink (alkali). Remember "phenolphthalein is pink in alkali".

Neutralisation

When an acid reacts with a base, the $H^+$ ions from the acid combine with the $OH^-$ ions from the alkali to make water. This is neutralisation:

$$H^+ + OH^- \rightarrow H_2O$$

This ionic equation is the heart of every acid–alkali reaction. The other ions (the metal ion and the acid's negative ion) stay in solution and form the salt.

Reactions of Acids

Acids react in three important ways. The general patterns are worth memorising because they let you predict products and write equations for any acid.

Only fairly reactive metals (such as magnesium, zinc and iron) react with dilute acids to give hydrogen. The gas can be tested: a lighted splint gives a squeaky pop.

For carbonates, the carbon dioxide produced turns limewater milky/cloudy — a quick test for $CO_2$.

Worked example Write the equation for zinc oxide reacting with nitric acid.

Metal oxide + acid → salt + water. Nitric acid gives nitrate salts, so:

$ZnO + 2HNO_3 \rightarrow Zn(NO_3)_2 + H_2O$

Naming Salts

The name of the salt comes from two parts: the metal (from the base) and the acid used.

So copper oxide + sulfuric acid gives copper sulfate; magnesium + hydrochloric acid gives magnesium chloride.

Solubility of Salts

The method you use to prepare a salt depends on whether it is soluble or insoluble in water. Learn these rules.

Preparing a Soluble Salt from an Insoluble Base

To make a soluble salt (e.g. copper sulfate) from an acid and an insoluble base or metal:

1. Warm the dilute acid, then add the insoluble base (e.g. copper oxide) a little at a time.
2. Stir until excess solid remains — this makes sure all the acid has reacted.
3. Filter to remove the unreacted excess solid.
4. Heat the filtered solution gently to evaporate some water, until it is saturated (the crystallisation point).
5. Leave to cool so crystals form, then filter and dry the crystals between filter papers.

Using excess insoluble base is the trick that guarantees no leftover acid contaminates your salt.

Preparing a Soluble Salt by Titration

If the base is itself soluble (a soluble base / alkali, e.g. sodium hydroxide), you cannot add excess and filter — there is no solid left to remove. Instead you use titration to find the exact volume of acid needed.

1. Use a pipette to put a measured volume of alkali into a conical flask; add a few drops of indicator (e.g. phenolphthalein).
2. Add acid from a burette, swirling, until the indicator just changes colour (pink → colourless). Record the volume — this is the end point.
3. Repeat without indicator, using the same volumes, so the salt is not contaminated by the dye.
4. Evaporate and crystallise the resulting salt solution as before.

Preparing an Insoluble Salt by Precipitation

Insoluble salts (e.g. barium sulfate, silver chloride, lead iodide) are made by precipitation: mix two soluble salt solutions that contain the right ions.

For example, to make insoluble lead sulfate, mix solutions of lead nitrate and sodium sulfate:

$$Pb(NO_3)_2 + Na_2SO_4 \rightarrow PbSO_4 + 2NaNO_3$$

The solid precipitate is then filtered, washed with distilled water and dried.

Watch out Choose two soluble starting salts whose unwanted ions stay in solution. Mixing the wrong pair can give a soluble product and no precipitate at all.

Tests for Acids and Alkalis

A solution that turns blue litmus red (or methyl orange red) is acidic.
A solution that turns red litmus blue (or phenolphthalein pink) is alkaline.
A few drops of universal indicator gives the approximate pH from the colour.
Adding a carbonate (e.g. sodium carbonate) to an unknown: fizzing that produces a gas which turns limewater milky confirms an acid.

Real world Neutralisation is everywhere: indigestion tablets (bases) cancel excess stomach acid, lime is spread on acidic farm soil to raise its pH, and toothpaste neutralises the acids that cause tooth decay.