Acids and Alkalis (u1)

• acids turn a ‘shade’ of red: deep red to yellow.
• alkalis turn a ‘shade’ of blue/green: dark green to purple.
• neutral solutions turn light green.
• Acids are any substance with a pH below 7.
• Examples of household acids include vinegar, fizzy drinks and lemon juice.
• Standard lab acids include hydrochloric (HCl), nitric (HNO3), and sulphuric (H2SO4).
• acids conduct electricity meaning they must contain ions!
• Acids contain positive hydrogen ions in solution: H+(aq).
• Formation of an acid: soluble non-metal oxides + water= acid
• Alkalis are any substance with a pH above 7.
• Examples of household alaklis include bleach, sodium bicarbonate and toothpaste.
• Standard lab acids include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2), also known as limewater.
• Alkalis will generally contain the negative hydroxide ions in solution: OH-(aq).
• Formation of an alkali: soluble metal oxides + water= alkali
• Ammonia solution, NH3(aq) is an important alkali, that does not contain a metal. It is formed dissolving ammonia gas in water.
• Ammonia is used to make fertilisers, explosives, drugs, hair dyes etc.
• Pure water (H2O) contains no dissolved ions yet slightly conducts electricity. The ions present in pure water must come from the water molecules. Some water molecules must ionise.
• Water and other neutral solutions have a pH of 7 because the number of hydrogen ions (H+) equals the number of hydroxide ions (OH-).
• Acidic solutions contain more hydrogen ions than hydroxide ions.

  Alkaline solutions contain more hydroxide ions than hydrogen ions.
• acid pH < 7 [H+] > [OH-]

  neutral pH = 7 [H+] = [OH-]

  alkali pH > 7 [H+] < [OH-]
• Dilution: When we dilute acids and alkalis, we are effectively adding more water molecules (only a tiny proportion of which dissociate into ions). This means that the concentration of these ions in the acids and alkalis decrease.
• Diluting acids decreases the concentration of H+(aq) ions. This means that the pH increases towards 7.
• Diluting acids decreases the concentration of H+(aq) ions. This means that the pH increases towards 7.