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Acids

  • Acids form a class of chemical substances which contain hydrogen ions in aqueous solution, H+ (aq), as the only positive ion.

  • Acids are usually classified into mineral or organic acids:

    1. Mineral Acids

      • generally much stronger

      • most do not occur naturally

      • usually have simpler molecules

      • Examples: Sulphuric Acid, Nitric Acid, Hydrochloric Acid, Phoshoric Acid and Carbonic Acid

    2. Organic Acids

      • naturally occurring

      • found in vegetables, fruit and other foodstuffs

      • usually weaker and less corrosive

      • Examples: Ethanoic Acid, Citric Acid, Lactic Acid, Tartaric Acid and Acetic Acid

  • Acids are substances that form hydrogen ions when dissolved in water.

  • A hydrogen ion is actually a proton. Therefore, acids are called the proton donors.

Basicity of Acids

  • Basicity of an acid is the number of hydrogen ions which can be produced by one molecule of the acid.

  • There are 3 common types of Basicity of an acid

    1. monoprotic: acids that produced 1 H+ ion from each acid molecule.

    2. diprotic: acids that produced 2 H+ ion from each acid molecule.

    3. triprotic: acids that produced 3 H+ ion from each acid molecule.

Physical Properties of Acids

  • Acids have the following physical properties:

  • Tastes sour

  • Turns moist blue litmus to red

  • pH value < 7

  • Can conduct electricity

  • Corrosive

Chemical Properties of Acids

  • Acid + Reactive Metal → Salt + Hydrogen gas

  • Acid + Metal Carbonate → Salt + Water + Carbon Dioxide gas

  • Acid + Base oxide → Salt + Water

  • Acid + Alkali → Salt + Water

Role of Water to Show Properties of Acid

  • The presence of water is essential for the formation of hydrogen ions and it is only the presence of these ions which causes acidity.

  • Without water, an acid won’t show the properties of acid.

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Example

  • Without water, the molecules of ethanoic acid do not disassociate to form hydrogen ions.

  • Without hydrogen ions, ethanoic acid does not shows acidity

  • With the presence of water, the molecules of ethanoic acid disassociate and form hydrogen ions.

  • With the presence of hydrogen ions, ethanoic acid shows acidity 

Strong Acids and Weak Acids

  • Acids are chemical substances that ionize/dissociate in the presence of water to produce hydrogen ions (or hydroxonium ions).

  • The strength of an acid depends on the degree of ionization/dissociation of the acid.

  • Strong acids are acids that ionise completely to form hydrogen ions in water.

  • Examples:

    • Sulphuric acid

    • Hydrochloric acid

    • Nitric acid

  • Weak acids are acids that partly ionise to form hydrogen ions in water.

  • Examples:

    • Ethanoic acid

    • Phosphoric acid

    • Citric acid

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