IGCSEScienceEdexcelScience (Double Award) (Modular)Chemistry Unit 1Revision Notes2. Inorganic Chemistry: Part 1Acids, bases & salt preparationsBases

Bases (Edexcel IGCSE Science (Double Award) (Modular)): Revision Note

Exam code: 4XSD1

Stewart Hird

Written by: Stewart Hird

Reviewed by: Philippa Platt

Updated on

Edexcel Science (Double Award) (Modular): Chemistry Unit 1Edexcel Chemistry (Modular): Unit 1Edexcel Science (Double Award): ChemistryEdexcel Chemistry

Bases

What makes a base act like a base?

  • Bases are substances which can neutralise an acid, forming a salt and water

  • The term base and alkali are not the same

  • A base which is water-soluble is referred to as an alkali

    • So, all alkalis are bases, but not all bases are alkalis

  • Alkalis have pH values of above 7

  • In basic (alkaline) conditions red litmus paper turns blue

  • Bases are usually oxides, hydroxides or carbonates of metals

  • The presence of the OH- ions is what makes the aqueous solution an alkali

  • One unusual base is ammonia solution

    • When ammonia reacts with water it produces hydroxide ions

Some common alkalis and the ions they contain

Name of alkali

Formula

Ions formed in water

Sodium hydroxide

NaOH

Na+ + OH-

Potassium hydroxide

KOH

K+ + OH-

Aqueous ammonia

NH3 (+ H2O)

NH4+ + OH-

Examiner Tips and Tricks

Aqueous ammonia and ammonium hydroxide are the same thing. When ammonia gas dissolves in water it forms ammonium hydroxide. Be careful to use the correct terminology: ammonia is the gas, NH3, ammonium is the ion present in ammonium compounds, NH4+

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener