Chemical changes

• Indicators and the pH scale

  The pH scale measures the acidity or alkalinity of a solution. The pH of a solution can be measured using a pH probe, or estimated using universal indicator and a colour chart.

  Figure caption, Universal indicator colours

  Universal indicator is one example of an acid-alkali indicator. Indicators show whether a solution is acidic, neutral (pH 7) or alkaline. The table shows the colours for litmus paper.

  IndicatorRed litmus paperAcidicStays redNeutralStays redAlkalineTurns blueIndicatorBlue litmus paperAcidicTurns redNeutralStays blueAlkalineStays blue
• Sulfuric acid + Iron → Iron(II) sulfate + Hydrogen
• Reaction between sulfuric acid and iron
• A base is any substance that reacts with an acid to form a salt and water only. This means that metal oxides and metal hydroxides are bases.

  Bases that are soluble in water are called alkalis and they dissolve in water to form alkaline solutions. For example:

  • copper oxide is a base, but it is not an alkali because it is insoluble in water
  • sodium hydroxide is a base, and it dissolves in water so it is also an alkali
• An acid is any substance that reacts with a base to produce salt and water.