Chemical Changes

• Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.
• When metals react with other substances the metal atoms form positive ions.
• The reactivity of a metal is related to its tendency to form positive ions.
• Metals can be arranged in order of their reactivity in a reactivity series. The metals potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper can be put in order of their reactivity from their reactions with water and dilute acids.
• The non-metals hydrogen and carbon are often included in the reactivity series.
• A more reactive metal can displace a less reactive metal from a compound.
• The reactions of metals with water and acids are limited to room temperature and do not include reactions with steam.
• Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal.
• Metals less reactive than carbon can be extracted from their oxides by reduction with carbon.
• Reduction involves the loss of oxygen.
• Oxidation is the loss of electrons and reduction is the gain of electrons.
• Acids react with some metals to produce salts and hydrogen.
• Acids are neutralised by alkalis (eg soluble metal hydroxides) and bases (eg insoluble metal hydroxides and metal oxides) to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide.
• The particular salt produced in any reaction between an acid and a base or alkali depends on:

  • the acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates)
  • the positive ions in the base, alkali or carbonate.
• Soluble salts can be made from acids by reacting them with solid

  insoluble substances, such as metals, metal oxides, hydroxides or

  carbonates. The solid is added to the acid until no more reacts and

  the excess solid is filtered off to produce a solution of the salt.
• Salt solutions can be crystallised to produce solid salts.
• Acids produce hydrogen ions (H+ ) in aqueous solutions.
• Aqueous solutions of alkalis contain hydroxide ions (OH– ).
• The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.
• A solution with pH 7 is neutral. Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7.
• In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water.
• A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.
• A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.
• For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
• As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.