Acid-Base Ions in Solution

Specification Point 3.1:
  • Recall that acids in solution are sources of hydrogen ions and alkalis in solution are sources of hydroxide ions.
  • When acids are mixed with water, they will lose electrons to form positively charged hydrogen ions (H+).
  • The presence of H+ ions in the aqueous solution is what makes it acidic.
  • Acids dissociate (ionise) to produce the H+ ions e.g. HCl:

HCl → H+ + Cl

  • When bases are mixed with water, they gain electrons to form negative hydroxide ions (OH).
  • The presence of the OH ions in the aqueous solution is what makes it alkaline.
  • Bases dissociate (ionise) to produce the OH ions e.g. NaOH:

NaOH → Na+ + OH

The pH Scale

Specification Point 3.2:
  • Recall that a neutral solution has a pH of 7 and that acidic solutions have lower pH values and alkaline solutions higher pH values.
  • The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is.
  • It goes from 1 – 14 (extremely acidic substances can have values of below 1).
  • All acids have pH values below 7, all alkalis have pH values of above 7.
  • The lower the pH then the more acidic the solution is.
  • The higher the pH then the more alkaline the solution is.
  • A solution of pH 7 is described as being neutral e.g. water.

Black & White pH Scale, Edexcel GCSE Chemistry

The pH scale showing acidity, neutrality and alkalinity

Universal indicator

  • Universal indicator is a mixture of different indicators which is used to measure the pH.
  • A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours.

Universal-Indicator-pH-Scale, Edexcel GCSE Chemistry

The pH scale with the Universal Indicator colours which can be used to determine the pH of a solution


Specification Point 3.3:
  • Recall the effect of acids and alkalis on indicators, including litmus, methyl orange and phenolphthalein.
  • An indicator is a type of dye that changes colour depending on the pH of the solution in which it is added.
  • Indicators have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached.
  • Only a few drops of indicator are needed and it is usually added using a teat pipette. 
  • Universal indicator is useful for determining the pH of an unknown solution.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.