3.16 - Making salts with soluble reactants
Some soluble salts are made by neutralising an acid with a soluble reactant, usually an alkali. Because both reactants are dissolved, the method has to avoid leaving any excess acid or alkali in the final solution. The central idea is simple but easy to miss: titration is used to find the correct reacting proportions, so the final solution contains only the salt dissolved in water.
Why solubility changes the method
A soluble reactant dissolves in water to form a solution. In this lesson, the important soluble reactants are soluble bases called alkalis, such as sodium hydroxide solution or potassium hydroxide solution. They react with acids in neutralisation reactions.
Alkali
An alkali is a soluble base. In water, alkalis form hydroxide ions, OH-.
The general reaction is:
acid + alkali -> salt + water
For example:
hydrochloric acid + sodium hydroxide -> sodium chloride + water
The salt made depends on the ions in the acid and the soluble reactant. Hydrochloric acid makes chlorides, nitric acid makes nitrates, and sulfuric acid makes sulfates.
This method is different from using an insoluble solid such as copper oxide. If an insoluble reactant is added in excess, the leftover solid can be removed by filtration. If a soluble reactant is added in excess, the extra reactant stays dissolved in the same solution as the salt, so filtration will not remove it.
Why titration is needed
A titration is used when the acid and the other reactant are both in solution. It lets you find the volume of one solution that just neutralises a measured volume of the other solution.
Titration
A titration is a technique in which one solution is added carefully to another solution to find the volume needed for a reaction to reach its end point.
For this salt-preparation reasoning, titration matters because it finds the point where neither reactant is in excess. An indicator can show when neutralisation has just happened. The exact method, including the full sequence with a burette and pipette, is the focus of the next titration lesson; here the key point is why the measuring step is needed.
Compare the two salt-preparation routes:
| Starting reactants | How excess is avoided or removed | Why this works |
|---|---|---|
| acid + insoluble base, oxide or carbonate | add excess solid, then filter off the leftover solid | the excess reactant is insoluble, so it can be separated from the solution |
| acid + soluble alkali | use titration to find the neutralising volume, then mix correct proportions | any excess soluble reactant would stay dissolved, so it must not be added |
The common mistake is to say "add excess alkali and filter it off". That method does not work because an alkali is soluble. There would be no solid excess to trap in filter paper.
Titration is needed because a dissolved excess reactant cannot be filtered out; the amount added must be right in the first place.
Now check that you can turn that reason into a linked explanation.
Continue this lesson.
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