Preparation of salts

Salts are ionic substances which are formed as a reaction between an acid and a metal or metal compound.

Reactions of acids with metals

Acids react with metals to produce a salt in solution and hydrogen gas:

(One way of remembering this is M.A.S.H)

The more reactive the metal, the quicker the reaction. Metals that are very reactive such as sodium, react with dilute acids and produce hydrogen very quickly, whereas less reactive metals such as copper do not react with dilute acids. The speed of reaction can be observed as the rate at which bubbles of hydrogen are released. The presence of hydrogen can be confirmed using the lit splint test where a positive result is indicated by a squeaky pop sound.

Salts always have two-part names. The first part of the name comes from the metal and the second part of the name comes from the type of acid used. If the acid used is nitric acid, the salt will be a nitrate. If the acid is hydrochloric acid, the salt will be a chloride. If the acid is sulphuric acid, the salt will be a sulphate.

Hydrochloric acid reacts with magnesium to produce magnesium chloride (MgCl₂) solution and hydrogen gas:

Sulphuric acid reacts with magnesium to produce magnesium sulphate (MgSO₄) solution and hydrogen gas:

Nitric acid reacts with magnesium to produce magnesium nitrate (Mg(NO₃)₂) solution and hydrogen gas:

Reactions of acids with metal hydroxides

Metal oxides and metal hydroxides are bases. Those metal oxides and metal hydroxides which dissolve in water are known as alkalis. Bases which are not soluble in water will also react with acids.

Bases such as metal oxides and metal hydroxides, all react with acids to form a salt and water. For example, hydrochloric acid and sodium hydroxide react in a neutralisation reaction to produce the salt sodium chloride (NaCl) and water (H₂O):

Hydrochloric acid is an ionic compound made up of hydrogen ions (H⁺) and chloride ions (Cl^–). Sodium hydroxide is an ionic substance made up of sodium ions (Na⁺) and hydroxide ions (OH^–). Sodium chloride is also an ionic compound and is made up of sodium ions and chloride ions (Cl^–). If we were to write out the full ionic equation for this reaction it would be:

As we can see from the equation, the Na⁺ and Cl^– ions appear on both sides of the equation and do not change. They are known as spectator ions as they do not take part in the chemical change that occurs.

We can therefore cancel these out from the overall ionic equation leaving us with:

Here, the reaction is between hydroxide ions and hydrogen ions in solution where water is produced.  This is known as the ionic equation of neutralisation and can be used to represent the changes that occur during any acid-base neutralisation reaction.

Sulphuric acid neutralises sodium hydroxide producing sodium sulphate (Na₂SO₄) and water:

Nitric acid neutralises calcium hydroxide to produce calcium nitrate (Ca(NO₃)₂) and water:

Reactions of acids with metal carbonates

Metal carbonates react with dilute acids to produce a salt, water and carbon dioxide gas. Hydrochloric acid reacts with sodium carbonate (Na₂CO₃) to produce sodium chloride, water and carbon dioxide gas (CO₂), as shown by the equation:

Sulphuric acid reacts with sodium carbonate to produce sodium sulphate and carbon dioxide gas:

Rules for predicting the solubility of ionic compounds in water

Some ionic compounds are soluble in water whereas others are insoluble. You must know the general rules about solubilities and use them to predict whether an ionic compound will be soluble if added to water. The general solubility rules are summarised in the table below:

Making insoluble salts from soluble reactants

Insoluble salts are those which will not dissolve in solution. When these salts are produced in a chemical reaction they will be observed as a precipitate suspended in the solution. A precipitate is a solid which is suspended in the solution. Hydroxides except for sodium, potassium and calcium hydroxides are all insoluble. Carbonates except for sodium, potassium and ammonium carbonates are all insoluble. Silver chloride and lead(II) chloride are insoluble. Lead(II) sulphate, barium sulphate and calcium sulphate are all insoluble.

To produce an insoluble salt such as those mentioned above, two soluble reactants are added together in a precipitation reaction where the insoluble salt is produced as a precipitate in the solution.

For example, when a colourless solution of lead(II) nitrate Pb(NO₃)₂ and a colourless solution of sodium chloride are added together, a white precipitate of lead(II) chloride is produced. The equation for this reaction is:

The solid precipitate produced will not be dry or pure. In order to obtain a pure, dry sample of the salt, the precipitate must be removed and processed through a series of steps.  You must be able to describe an experiment which can be used to prepare a pure, dry sample of an insoluble salt.

Making a pure, dry sample of silver chloride

Silver chloride can be produced by adding colourless silver nitrate solution to colourless silver chloride solution, as shown by the equation:

The method for this experiment is outlined below:

1. Add a small amount of silver nitrate solution into the test tube using a pipette.
2. Add an equal amount of sodium chloride solution to the test tube using a separate pipette. Place the bung into the top to seal the tube and gently shake the tube to mix the two solutions together.
3. A white precipitate should be formed. The test tube will now contain the silver chloride as a white precipitate and a colourless solution of sodium nitrate. Remove the silver chloride precipitate by pouring the contents of the test tube through filter paper placed in a filter funnel into a small beaker. The impure silver chloride precipitate will remain on the filter paper whilst the colourless sodium nitrate flows through into the beaker.
4. Rinse the test tube thoroughly with distilled water and pour the washings through the filter paper to make sure that none of the precipitate formed remains in the tube.
5. The silver chloride precipitate may still contain traces of the sodium nitrate salt, making it impure. To remove these soluble impurities, the sample of silver chloride must be washed gently using distilled water. The sodium nitrate impurities will dissolve into the distilled water and will run off with the water. The silver chloride does not dissolve into the water so will remain as a solid on the filter paper. The silver chloride should now be pure.
6. To dry the silver chloride, it should be very gently scraped onto a fresh piece of filter paper. Make sure that you don’t scrape too hard as this will remove the paper as well and will therefore contaminate the sample. The silver chloride can then be left to dry on the filter paper in a warm place. To speed up the drying process, the sample can be gently pressed between two pieces of filter paper to absorb the excess water. The salts produced through this method tend to be powdery in appearance.

Making soluble salts

A soluble salt is one which will dissolve in solution. These salts do not therefore form precipitates in solution and must be separated from the reaction mixture using a combination of filtration and evaporation. Soluble salts include:

• Sodium hydroxide, potassium hydroxide and calcium hydroxide
• Most common sulphates (except lead(II), barium and calcium sulphates)
• Sodium carbonate, potassium carbonate and ammonium carbonate
• Most common chlorides (except lead(II) and silver chlorides)

Soluble salts can be made by reacting acids with insoluble reactants or soluble reactants such as alkalis.

Making soluble salts using insoluble reactants

Insoluble reactants which can be added to acids to make salts include metals, metal oxides and metal hydroxides. The general method used to obtain a pure, dry sample of a soluble salt using insoluble reactants is as follows:

1. Add a small amount of the acid into a boiling tube.
2. Add the metal, metal oxide or metal hydroxide solid to the tube using a spatula. The reactant will dissolve into the acid.
3. Place a bung into the tube and shake it gently to dissolve the solid.
4. Continue to add the solid until no more solid will dissolve into the acid. You will know when this point is reached as you will see the solid start to fall to the bottom of the tube.
5. Pour the contents of the tube through a filter paper placed in a filter funnel into a small beaker. This will remove the impurities from the undissolved metal, metal oxide or metal hydroxide.
6. Rinse the tube using distilled water and pour the washings through the filter paper to make sure that you have collected all of the salt formed.
7. The beaker should now contain a solution of the salt in water.

To obtain a pure, dry sample of the salt, the water will need to be removed. This is done through the process of evaporation. The solution should be heated using a Bunsen burner to drive off most off the water and the vessel can be left to dry in a warm place so the remainder of the water can evaporate off slowly. As the water is removed, the crystals of the salt are left behind. This process is known as crystallisation.

For example, magnesium can be added to sulphuric acid to make the soluble salt magnesium sulphate and hydrogen gas, as shown by the equation:

The magnesium and sulphuric acid are placed into a test tube and the magnesium dissolves into the sulphuric acid. Any unreacted magnesium is filtered off and the magnesium sulphate solution is heated until most of the water is removed. The rest is left in a warm place to allow all of the water to evaporate slowly and crystals of magnesium sulphate are formed.

Making soluble salts using soluble reactants

With soluble bases such as sodium hydroxide and potassium hydroxide, it is not possible to know when the reaction has reached completion. You have to add exactly the right volume of alkali to neutralise the acid to produce the salt. An indicator must be used to show when the neutralisation point has been achieved.

Once the acid has been neutralised, and you know the volume of alkali needed to achieve this, you can repeat the experiment using exactly the same volumes of acid and alkali but this time without the addition of the indicator to avoid the salt becoming contaminated.

Once the alkali has neutralised the acid, salt and water are produced. The pure, dry salt is obtained by crystallisation where the solution is heated to evaporate off the water leaving behind the pure, dry crystals of the salt.