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

Bases (Edexcel IGCSE Chemistry (Modular))

Revision Note

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Stewart Hird

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 Tip

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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