4. Chemical changes

• pH scale


  Goes from 0 to 14
• pH


  Measure of how acidic or alkaline a solution is
• The lower the pH of a solution, the more acidic it is
• The higher the pH of a solution, the more alkaline it is
• A substance (or pure water) has pH 7
• Indicator

  A dye that changes colour depending on whether it's above or below a certain pH
• Wide range indicators


  • Contain a mixture of dyes that means they gradually change colour over a broad range of pH
  • Useful for estimating the pH of a solution
• Universal indicator


  Can turn a variety of colours to indicate a narrow range of pH values
• Universal indicator is made from a mixture of different indicators
• During a titration between an alkali and an acid, you want to see a sudden colour change at the end-point
• Single indicators used for titrations

  • Litmus
  • Phenolphthalein
  • Methyl orange
• Strong acids


  Ionise completely in water, all acid particles dissociate to release H+ ions
• Weak acids

  Do not fully ionise in solution, only a small proportion of acid particles dissociate to release H+ ions
• Neutralisation reaction between acids and bases

  Acid + Base → Salt + Water
• When an acid neutralises a base (or vice versa), the products are neutral, i.e. they have a pH of 7
• Titration


  A method of analysing the concentrations of solutions
• Titration procedure

  1. Add a set volume of the alkali to a conical flask

  2. Use a burette to add the acid of known concentration

  3. The indicator changes colour when all the alkali has been neutralized

  4. Record the final volume of acid in the burette to calculate the volume used
• To increase the accuracy of a titration, you need several consistent readings and calculate a mean, ignoring any anomalous results
• Universal indicator is used to estimate the pH of a solution, but for a titration you need to use a single indicator that shows a sudden colour change at the end-point
• Strong acids are more reactive than weak acids of the same concentration, because they have a higher concentration of H+ ions
• pH


  Measure of the concentration of hydrogen ions in the solution
• For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10
• You can have a dilute (not very concentrated) but strong acid, or a concentrated weak acid
• Metal oxides and metal hydroxides are bases that react with acids in neutralisation reactions
• Reaction of acids with metal oxides or metal hydroxides


  1. Acid + Metal Oxide → Salt + Water

  2. Acid + Metal Hydroxide → Salt + Water
• pH

  Measure of acidity or basicity of a solution
• pH will decrease with increasing solute concentration


  Regardless of whether it's a strong or weak acid
• Acids are acidic because of H+ ions
• Strong acids are strong because they let go of all their H+ ions at the drop of a hat
• If pH is 6, the pH of the new solution will increase
• Reactions of Acids


  1. Neutralisation

  2. Metal Oxides and Metal Hydroxides are Bases

  3. Acid + Metal Oxide → Salt + Water

  4. Acid + Metal Hydroxide → Salt + Water
• Metal Oxides and Metal Hydroxides


  • Copper oxide + Potassium hydroxide → Copper chloride + Potassium sulfate
  • Hydrochloric acid + Sodium carbonate → Sodium chloride + Water + Carbon dioxide
• To work out the formula of an ionic compound, you need to balance the charges of the positive and negative ions so the overall charge of a compound is neutral
• Making Soluble Salts Using an Insoluble Base

  1. Pick the right acid and insoluble base

  2. Gently warm the dilute acid

  3. Add the insoluble base to the acid a bit at a time

  4. Filter out the excess solid

  5. Evaporate some of the water to get pure, solid crystals of the salt
• Calcium carbonate is added to filter out impurities
• Reactivity Series


  Lists metals in order of their reactivity towards other substances
• Reactivity of Metals


  • Determined by how easily they lose electrons to form positive ions
  • The higher up the reactivity series a metal is, the more easily they form positive ions
• How Metals React With Acids

  1. Acid + Metal → Salt + Hydrogen

  2. Speed of reaction indicates reactivity

  3. Very reactive metals react explosively, less reactive metals react less violently
• Magnesium reacts with cold dilute acid


  Produces lots of bubbles
• Zinc and iron react with dilute acid


  But more strongly if you heat them up