C2.6 Acids, Bases and Salts

  • Created by: Fiona S
  • Created on: 21-03-15 17:12


Acids turn blue Litmus paper pink/red. 
           turn Universal Indicator red/orange/yellow depending how strong acid is.

Alkalis turn red Litmus paper blue
           turn Universal Indicator blue/
purple depending how strong the alkali is.

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The pH Scale


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When dissolved in water it releases hydrogen(H+) ions. Some compounds will release their H+ very easily when they dissolve in water. We say they fully disassociate(split up).


      HCl --> H+ + Cl-
      H2SO4 --> 2H+ + SO4-
      HNO3 --> H+ + NO3-

The examples are all strong acids that release all their hydrogen.

When a weak acid is dissolved in water only some of the molecules disassociate to release their H+.

CH3COOH <--> H+ + CH3OO-

All acids taste sour. Concentrated acids are corrosive. More dilute acids are irritants.

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Concentration is a measure of how many particles there are in a set volume. It is not limited to the strength of an acid/alkali.

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A base is a chemical that can neutralise an acid. Alkalis are bases that dissolve in water to release hydroxide ions (OH-).

All alkalis are bases but not all bases are alkalis.

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Dissolve in water to release OH-


      NaOH --> Na+ + OH-
      KOH --> K+ + OH-
      Ca(OH)2 --> Ca+ + OH-

Ammonia solution is a weak alkali NH3(aq)

All alkalis dissolve grease and feel soapy to the touch. NEVER touch an alkaline solution on purpose. Many of them are corrosive.

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acid + alkali --> water

H+ + OH- --> H2O

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Metal Salts contain positive ions(metal) and negative ions(non-metal part). They are generally made when an acid reacts with a metal or metal compounds.

Soluble Salts will dissolve in water as soon as they are formed and to retrieve them we need to evaporate off the water and crystallise the salt.

The metal part of a metal salt comes from the metal/metal oxide etc. The non-metal part comes from the acid.

Acid + Metal(Fe and above in the Reactivity Series) --> Metal Salt + Hydrogen

Acid + Metal Oxide --> Metal Salt + Water

Acid + Metal Hydroxide --> Metal Salt + Water

Acid + Metal Carbonate --> Metal Salt + Carbon Dioxide + Water

When we react a metal, metal oxide or metal carbonate with an acid we have to use excess of the solid to ensure all the acid has reacted. The excess solid will need to be filtered from the salt solution.

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Reacting Metals with Acids

Reacting metal with acid is an easy way of making soluble salts. However, metals at the top of the reactivity series react violently with acids. Metals below hydrogen in the reactivity series DO NOT react with acids.


1. Place acid in a test tube
2. Slowly add the metal. Effervescence signals production of H2(g)
3. When no more effervescence is produce, test solution with blue litmus paper. If it turns red add more metal.
4. Add metal until blue litmus paper stays blue (all acid has reacted)
5. Filter off excess metal
6. Evaporate water from salt solution
7. Allow salt to crystallise

We can't use this method to make copper salts so we use copper oxide or copper carbonate instead.

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Making insoluble salts

When we mix 2 soluble salts together the ions separate. Some rejoin and are soluble salts. Some combinations of positive and negative ions precipitate out as they are insoluble salts. Their state symbol is (s) rather than (aq).

Potassium bromide + Silver nitrate --> Silver bromide + Potassium nitrate

KBr(aq) + AgNO3(aq) --> AuBr(s) + KNO3(aq)

This reaction where you mix two soluble salts to make an insoluble salt is called a precipitation reaction.

Insoluble salts form a precipitate. We can remove this by filtration. The salt needs to be washed with distilled water. This is done by washing the salt through filter paper a few times with distilled water. Finally leave the salt to dry.

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Making soluble salt from metal hydroxide and acid

Metal hydroxides are soluble in water so we find them as solutions. Both our acid and our metal hydroxide(alkali) are clear, colourless solutions. There is no visible change when they react. We cannot use an excess of one reactant as them reactant cannot be filtered so would be mixed with our metal salts.

1. Measure out 25cm3 Sulphuric Acid into a measuring cylinder and pour into a conical flask

2. Put Sodium Hydroxide into the burette using a funnel. Check if the tap is closed beforehand. Let some run through to clear the burette.

3. Add 1cm3 at a time and swirl it so they are mixed and test on blue litmus paper

4. Repeat until the blue litmus paper stops turning red and stays blue

5. Evaporate off the water using a bunsen burner, an evaporating dish and a tripod

6. Then leave it to crystallise

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An alkali is a salt that dissolves in water to release hydroxide ions. Ammonia is NH3 yet it is an alkali. It is a weak alkali with a pH of 9 or 10 depending upon concentration.

When ammonia dissolves in water it reacts with the water to form ammonium hydroxide.

Ammonia + Water --> Ammonium  Hydroxide

NH3 + H2O --> NH4OH(aq)

NH4OH <--> NH4+ + OH-

Ammonia + hydrochloric Acid --> Ammonium Chloride + Water

Ammonium Salt are used as fertilisers

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Making Copper Sulphate

1. Pour boiling water from the kettle into the beaker

2. Add 15ml of 1.4M sulphuric(IV) acid to the boiling tube

3. Weigh 1.8-2.0g of copper(II) oxide into a weighing boat

4. Add half of copper(II) oxide to the warm acid

5. Now add the other half

6. Mix and leave for a few minutes

7. Filter the suspension into the evaporating basin

8. Leave your solution on a sunny window sill for you copper sulphate to crystallise

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