Salts

Below you will find my attempt at writing a GCSE resource topic on SALTS for students and teachers.

Ø  Salts

You have already been introduced to acids and alkalis, but what actually happens when an acid and an alkali react together?

The general word equation for this is: acid + alkali --> salt + water

This reaction is also known as neutralisation as the alkali has neutralised the acid by removing the H+ ions and turning them into water:

H⁺(aq) + OH^-(aq) --> H₂O(l)

Lets look at an example for the reaction between an acid and alkali:

Hydrochloric acid + sodium hydroxide --> sodium chloride + water

           HCl(aq)        +          NaOH(aq)   -->        NaCl(aq)      + H₂O(l)

You will notice that the salt, sodium chloride is dissolved in water (aq); later on in this chapter we will talk about how to get the salt from its solution.

Q. Can you remember what the difference is between an alkali and a base?

Ø  Preparation of soluble salts

Below are some general methods of preparation of a salt that can be made from acids:

1.     Acid + metal  -->salt + hydrogen

E.g. hydrochloric acid + magnesium --> magnesium chloride + water

               2HCl(aq)       +       Mg(s)   -->      MgCl₂(aq)           + H₂O(l)

Most metals react with acid giving off hydrogen gas; however the metal copper does not react with acid.

       Q. Which acid and metal would you start with to make aluminium sulphate?

       Q. Do you remember how to test for whether hydrogen gas has been given off?

2.     Acid + metal carbonate -->salt + carbon dioxide + water

E.g. copper carbonate + hydrochloric acid  --> copper chloride + carbon dioxide + water

                CuCO₃(s)    +          2HCl(aq)       -->       CuCl₂(aq)      +        CO₂(g)       + H₂O(l)

       Q. How could you test for the gas given off?

       Calcium carbonate, CaCO₃, is also a common carbonate which can be found in chalk, limestone and marble.

NOTE! Metal oxides and metal hydroxides DO NOT dissolve in water.

Ø  Naming salts

When naming a salt, it has two parts, like naming a person. The first part comes from the metal in the base or the metal itself, and the second part is from the acid used.

Look at the common examples in the table below:

Acid                                                  Its salts	Example
Hydrochloric acid, HCl    -->            chlorides	sodium chloride, NaCl
Sulphuric acid, H2SO4        -->            sulphates	magnesium sulphate, MgSO4
Nitric acid, HNO3                -->            nitrates	potassium nitrate, KNO3

Ø  Preparing salts

You now know how to make a salt by reacting an acid with an alkali. But an alkali is just a base that dissolves in water.

Q. Do you think bases will react with acids, the same way that acids react with alkalis?

Experiment 1: Making a salt from an acid and a soluble base

1.     Put 25 cm³ of sodium hydroxide in a small beaker. Gradually add hydrochloric acid to the base to make a neutral solution.

How could you test to see when all the acid has reacted?

2.     Put your solution into an evaporating dish and slowly heat till you see crystals. The solution now has to be left for a few days to crystallise.

Tip! Heating the solution slowly will leave you with larger sized crystals.

What is the name of the salt formed?

You will remember that metal oxides and metal hydroxides do not dissolve in water; so when making a salt from either of these two compounds, a couple of steps have to be added to the start of the experiment above:

1.     Add the insoluble base to the acid until no more will dissolve or react. Some of the base will be left behind – this is called excess.

2.     Filter the mixture into a separate beaker to remove the excess base.

3.     Evaporate the water left behind in the filtrate, leaving the salt behind.

Ø  Preparation of insoluble salts

When preparing an insoluble salt, this involves mixing two solutions of two soluble salts. The insoluble salt is then precipitated – a precipitation reaction.

E.g. The insoluble salt, lead carbonate can be prepared by reacting a solution of soluble lead salt (lead nitrate) and a solution of soluble carbonate (sodium carbonate).

Lead nitrate + sodium carbonate   --> lead carbonate + sodium nitrate

    Pb(NO₃)₂(aq)  +     Na₂CO₃(aq)      -->      PbCO₃(s)    +   2NaNO₃(aq)

      We can also show what happens by writing an ionic equation:

                  Pb²⁺(aq) + CO₃^2-(aq)    -->    PbCO₃(s)

     From this we can now see which ions stick together – the partners that make up the individual compounds have swapped partners and formed a precipitate.

The nitrate ions, NO₃^-(aq) and the sodium ions, Na²⁺(aq) have not disappeared; they are still in the solution and do not change so they don’t appear in the ionic equation. They are known as spectator ions.

Remember! When writing chemical or ionic equations, especially in the exam, always remember to include state symbols. Marks can sometimes be lost if they are not included, unless told otherwise.

·       Solid (s)

·       Liquid (l)

·       Gas (g)

·       Dissolved in water (aq)

·       Questions

1.     Copy and complete:

Acid + base (or alkali)   -->   ................ + water

Acid + metal   -->  salt + ................

Acid + carbonate   -->    salt + .............. + ...............

When an acid reacts with a base (or alkali) to ‘cancel’ each other out, what type of reaction do we call this?

2.     Copy and complete the following word equations:

a)     sodium hydroxide + hydrochloric acid   -->  ....................... + water

b)     magnesium + sulphuric acid   --> ............................. + water

c)     ......................... + nitric acid   --> copper nitrate + water

d)     calcium oxide + ........................   --> calcium chloride + water

e)     copper .............. + hydrochloric acid   --> copper ................. + carbon dioxide + water

3.     Explain the difference in procedure when using a soluble base or an insoluble base to make a salt.

4.     Copy and complete the following chemical equation by filling in the missing state symbols:

Na (s) + H₂O (  )   --> NaOH (  ) + H₂(  )