C4 - Chemical Changes

• The pH scale is a measure of how acidic or alkaline a solution is.
• A neutral substance has pH 7.
• The lower the pH the more acidic it is.
• The higher the pH the more alkaline it is.
• Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7.
• Indicator - dye that changes colour depending on whether it's above or below a certain pH
• Wide range indicators - contains a mixture of dyes meaning they'll gradually change colour over a broad pH of a solution. Useful for estimating the pH of a solution.
• pH probe - attached to a pH meter used to measure a pH electronically. Probe is placed in the solution being measured. pH is given as a numerical value meaning it's more accurate than an indicator.
• Acids form H+ ions in water.
• A base is a substance with a pH greater than 7
• Bases are substances that will neutralise acids
• Acids produce hydrogen when you add water to them
• Alkalis produce hydroxide when you add water
• An alkali is a base that dissolves in water to form a solution with a greater pH than 7.
• Alkalis form OH- ions in water
• Neutralisation = acid + base -> salt + water
• Acid and Alkali Neutralisation:

  H+ + OH- -> H20

  Hydrogen + Hydroxide -> Water
• Strong acids ionise completely in water. (sulfuric, hydrochloric acid) All solid particles dissociate to release H+ ions.
• Weak acids do not fully ionise in a solution (citric acid). Only a small proportion dissociate to release H+ ions
• The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid
• The pH of an acid or alkali is a measure of the concentration of H+ ions in the solution.
• For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10.
• Acid strength tells you what proportion of the acid molecules ionise in the water
• Concentration means how much acid there is in a certain volume of water.
• The larger the amount of acid there is in a certain volume of liquid the more concentrated the acid is.
• Some metal oxides and metal hydroxides dissolve in water they are alkalis.
• Bases that don't dissolve in water take part in neutralisation reactions with acids.
• All metal oxides and metal hydroxides react with acids to form salt and water.
• Acid + Metal Oxide -> Salt + Water
• Acid + Metal Hydroxide -> Salt + Water
• Acids and Metal Carbonates produce Carbon dioxide
• Acid + Metal Carbonate -> Salt + Water + Carbon Dioxide
• Redox reactions – one substance is reduced and another substance is oxidised
• Acids are neutralised by metal carbonates to produce salts, water and carbon dioxide
• Crystallisation

  1. Gently warm the dilute acid using a bunsen burner
  2. Turn it off then add the insoluble base to the acid slowly until no more reactants - excess will sink to the bottom of the flask
  3. Filter out excess solid to get salt solution
  4. Gently heat the solution using a water bath and leave the solution to cool to get solid crystals
  5. Filter out and dry
• In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water.
• Oxidation is the loss of electrons and reduction is the gain of electrons.
• The reactivity series lists metals in order of their reactivity towards other substances.
• Their reactivity is determined by how easily they lose electrons forming positive ions. The higher up the reactivity series the more easily they for positive ions.
• When metals react with water or acid they lose electrons to form positive ions. Higher the metal the more easily it reacts with water or acid.