Specification Point 2.28:
Describe the Use of Litmus, Phenolphthalein and Methyl Orange to distinguish between Acidic and Alkaline Solutions

Litmus

  • Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions and turns purple in neutral solutions.
  • Litmus paper is usually more reliable and comes as red litmus paper and blue litmus paper.
Use of Litmus, Edexcel IGCSE Chemistry

Phenolphthalein & Methyl Orange

  • Phenolphthalein changes colour sharply at about pH 8 and is commonly used in titrations.
  • Methyl Orange is a solution and changes into a range of warm colours.
Specification Point 2.29:
Understand how to use the pH scale, from 0-14, can be used to classify solutions as strongly acidic (0-3), weakly acidic (4-6), neutral (7), weakly alkaline (8-10) and strongly alkaline (11-14)

How to Use the pH Scale

pH Scale: Some acids are weaker than others. It is the same with alkalis. The strength of an acid or alkali is shown using the pH scale which goes from 0 to 14.

On this scale,

  • An acidic solution has a pH number less than 7.
  • An alkaline solution has a pH number greater than 7.
  • A neutral solution has a pH number of exactly 7.

The smaller the number the stronger the acid. The larger the number the stronger the alkaline.

pH scale, Edexcel IGCSE Chemistry
Specification Point 2.30:
Describe the use of universal indicator to measure the approximate pH value of an Aqueous Solution

Use of Universal Indicator

Universal Indicator: Supplied as a solution or as a paper and has a range of colours to specifically identify the pH of a solution.

Use of Universal Indicator to Measure pH of Aqueous Solution:

  • If using universal indicator solution, a few drops of this solution is added to the aqueous solution and colour change is observed and matched to specific pH using the pH scale.
  • If using universal indicator paper, this paper is dipped into the aqueous solution and colour change is observed and matched to specific pH using the pH scale.

Universal Indicator and pH Scale:

The colour change of the universal indicator is matched to a specific colour on the pH scale, which tells the approximate pH.

pH scale, Edexcel IGCSE Chemistry

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.