Salt preparation by acid–alkali titration

An acid reacts with an alkali to form a salt and water. Both reactants are in solution, so any excess acid or alkali remains mixed with the salt solution and cannot be removed by filtration. Titration finds the volumes needed to react completely, without leaving an excess of either reactant.

Finding the reacting volumes

1. Measure a fixed volume of one solution into a flask and add a few drops of an indicator.
2. Add the other solution from a burette, swirling the flask. Add it drop by drop near the end of the titration.
3. Stop when the indicator changes colour. This is the end-point. Record the volume added.

The end-point shows when the solutions have reacted in the required proportions. The measured volumes can then be used to prepare the salt without leaving an excess of acid or alkali.

Preparing the salt crystals

Repeat the reaction using the measured volumes, but without the indicator. This produces a salt solution that is not contaminated by indicator.

Gently evaporate some of the water to make the solution more concentrated. Leave it to cool so that salt crystals form. Separate the crystals from the remaining solution and dry them.

Salt preparation using excess insoluble reactants

A soluble salt can be prepared by reacting an acid with an insoluble solid. The solid is added in excess, so that all the acid is used up. The remaining solid can then be filtered off, leaving a salt solution.

The products depend on the solid used:

• A suitable metal reacts with the acid to produce a salt and hydrogen.
• An insoluble base reacts with the acid to produce a salt and water.
• An insoluble carbonate reacts with the acid to produce a salt, water and carbon dioxide.

Preparing the salt solution

Add the insoluble solid to the acid a little at a time, stirring as it reacts. Continue until some solid remains unreacted even after stirring. This indicates that the acid has been used up: there is no acid left to react with the extra solid.

Filter the mixture. The excess solid stays on the filter paper, while the salt solution passes through as the filtrate.

Obtaining salt crystals

Gently evaporate some water from the filtrate to make the salt solution more concentrated. Then leave it to cool. As it cools, salt crystals form from the solution.

Separate the crystals from the remaining liquid by filtration, then dry them, for example between sheets of filter paper.

Solubility rules for salts

Solubility rules predict whether a salt will dissolve in water. A soluble salt dissolves; an insoluble salt dissolves so little that it is classified as insoluble. To classify a salt, identify its ions and check both the general rule and any exceptions.

All sodium, potassium and ammonium salts are soluble. The rules for other groups are:

• Nitrates: all are soluble.
• Chlorides: soluble, except silver chloride and lead chloride.
• Sulfates: soluble, except barium sulfate, calcium sulfate and lead sulfate.
• Carbonates: insoluble, except sodium, potassium and ammonium carbonates.
• Hydroxides: insoluble, except sodium, potassium and ammonium hydroxides. Calcium hydroxide is partially soluble.

For example, sodium carbonate is soluble: although carbonates are generally insoluble, sodium carbonate is an exception. Barium sulfate is insoluble: although sulfates are generally soluble, barium sulfate is an exception. Using only the general rule would give the wrong classification in both cases.

Hydrated and anhydrous substances

A hydrated substance has water chemically combined with it. The water is part of the substance, rather than simply making its surface wet.

An anhydrous substance contains no water. A substance is described as hydrated or anhydrous according to whether water is chemically combined with it or absent from it.