Acids, Bases and Salts

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Soluble Salts from Metals

Metals react with dilute acid to form a metal salt and hydogen.

Salt is a word used to describe any metal compound made from a reaction between a metal and an acid.

Some metals react with acid more vigorously than others:

  • silver - no reaction
  • zinc - fairly reasonable reaction
  • magnesium - vigorous reaction
  • potassium - very violent and dangerous reaction
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Soluble Salts from Insoluble Bases

Bases - oxides & hydroxide of metals. Alkalis are soluble bases.

The oxides & hydroxides of transisition metals are insoluble. Their salts are prepared by:

1. The metal oxide of hydroxide is added to an acid until no more will react

2. The excess metal oxide or hydroxide is then filtered, leaving a solution of the salt

3. The salt solution is then evaporated

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Insoluble Salts & Salts of Alkali Metals

Insoluble salts can be made by mixing appropriate solutions of ions so a precipitate is formed. Precipitation can be used to remove unwanted ions.

Compounds of alkali metals, called salts can be made by reacting soltutions of their hydroxides with a particular acid. The salt produced depends on the metal in the alkali and the acid used.

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Neutralisation

Acids and alkalis are chemical opposites:

  • acids contain hydrogen ions
  • alkalis contain hydroxide ions

If they are added together in the correct amounts the can 'neutralise' each other. When an acid reacts with an alkali, the ions react together to produce water.

This type of reaction is called neutralisation because the solution that remains has a pH of 7, showing it is neutral.

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Neutralising Ammonia

Ammonia - alkaline gas that dissolves in water 2 make an alkaline solution. Used in production of fertilisers 2 increase nitrogen content of soil. Ammonia neutralises nitric acid - produce ammonium nitrate. The aqueous ammonium nitrate is then evaporated to dryness.

Ammonium nitrate, a fertiliser rich in nitrogen. Nitrogen based fertilisers r important chemicals bc increase yield of crops. Nitrates can be problems if they find their way into streams, rivers or ground water.

Ammonium nitrate can be neutralised with acids to produce ammonium salts.

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Indicators and pH Scale

Indicators are dyes that change colour depending on whether they are in acidic or alkaline solutions

Universal indicator - mixture of dyes that show range of colours to indicate how acidic or alkaline a substance is

The pH scale is a measure of the acidity or alkalinity of an aqueus solution, across a 14-point scale. When substances dissolve in water, they dissociate into their individual ions:

  • hydoxide ions, make solutions alkaline
  • hydrogen ions, make solutions acidic
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