acids

• acids in solution are sources of hydrogen ions (H+), which are protons.
• alkalis in solution are sources of hydroxide ions (OH-). they are bases that are soluble in water
• a neutral solution has a pH of 7
• an acidic solution is a solution that has a pH below 7
• an alkaline solution has a pH above 7
• litmus: blue litmus turns red in acidic solutions and red litmus turns blue in alkaline solutions (only indicates whether an acid or alkali is present)
• methyl orange: red in acid, yellow when neutral and yellow in alkali, so can only tell if solution is acid
• phenolphthalein: colourless in acid and neutral, magenta/ pink in alkali so can only tell if solution is alkaline
• the higher the concentration of hydrogen ions in an acidic solution, the lower the pH
• the higher the concentration of hydroxide ions in an alkaline solution, the higher the pH
• as hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1
• general methods to measure pH:

  pH meter/probe (most accurate)

  universal indicator

  litmus solution (only works to tell acidity or alkalinity)
• Core Practical: Investigate the change in pH on adding powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid

  method:

  add​ ​20cm^3 dilute​ hydrochloric acid to​ ​the​ ​beaker​ ​and​ ​measure​ ​pH

  add​ ​weighed​ ​mass​ ​of​ ​calcium​ ​oxide​ ​and​ ​stir​ ​then​ ​record​ ​pH

  keep​ ​adding​ ​weighed​ ​masses​ ​of​ ​calcium​ ​oxide until​ ​there​ ​is​ ​no​ ​more change​ ​to​ ​the​ ​pH
• a dilute solution contains a relatively small amount of dissolved solute compared to the volume of solution
• a concentrated solution contains a relatively large amount of dissolved solute compared to the volume of solution
• a strong​ ​acid​ ​fully​ ​dissociates​ ​in​ ​aqueous​ ​solution​ ​so all hydrogen ions are released (dissociation​ ​is​ ​where​ ​an​ ​acid breaks​ ​down​ ​to​ ​release​ ​hydrogen​​ ​ions​ ​in​ ​solution)
• a weak​ ​acid​​ ​partially​ ​dissociates​ ​in​ ​aqueous​ ​solution so fewer hydrogen ions released than in the acid
• a base is any substance that reacts with an acid to form a salt and water only
• alkalis are soluble bases
• acid + metal -> salt + hydrogen gas
• acid + metal oxide (insoluble base) -> salt + water
• acid + metal hydroxide (base/alkali) -> salt + water
• acid + metal carbonate (not base) -> salt + water + carbon-dioxide
• Test​ ​for​ ​hydrogen:

  o Use​ ​a​ ​burning​ ​splint​ ​held​ ​at​ ​the​ ​open​ ​end​ ​of​ ​a​ ​test​ ​tube​ ​of​ ​the​ ​gas

  ▪ Creates​ ​a​ ​‘squeaky​ ​pop’​ ​sound if hydrogen is present
• Test​ ​for​ ​carbon​ ​dioxide:

  o Bubble​ ​the​ ​gas through​ ​the​ ​limewater​ ​(calcium​ ​hydroxide​ ​solution)​ ​

  it will​ ​turn​ ​milky​ ​(cloudy) if carbon-dioxide is present
• a neutralisation reaction is a reaction between an acid and a base
• an acid-alkali neutralisation is a reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH–) from the alkali to form water

  H+ + OH- -> H2O (IONIC EQUATION FOR NEUTRALISATION)
• if soluble salts are prepared from an acid and an insoluble reactant:

  • excess of reactant is added to ensure volume of acid reacts completely
  • the excess reactant is removed by filtration so you are just left with salt and water
  • the solution remaining is only salt and water after the reaction because all of your acid has reacted (e.g copper oxide + sulphuric acid makes salt and water)
• if soluble salts are prepared from an acid and a soluble reactant:

  • titration must be used because both reactants are liquids/soluble
  • you need to determine the volumes of acid and alkali that must be mixed to obtain a solution containing only salt and water
  • the acid and soluble reactant are then mixed in the correct proportions so that only salt and water is formed
  • the solution remaining, after reaction, is only salt and water because the exact amount of acid has been added to the soluble reactant to neutralise it
• Core Practical: Investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide including the use of a water bath

  • Pour sulfuric acid in a conical flask & warm in a water bath.
  • Add copper oxide powder (using spatula) to the acid & stir with glass rod to speed up reaction
  • Add copper oxide until it is in excess to ensure all acid has reacted
  • Filter mixture to remove excess copper oxide.
  • Pour the filtrate into an evaporating basin.
  • Heat the copper sulfate solution with a bunsen burner to evaporate the water, leaving behind solid copper sulfate crystals
• substances that are soluble in water:

  • all sodium, potassium ammonium salts
  • all nitrates
  • most chlorides
  • most sulfates
• substances that are insoluble in water:

  • silver chloride
  • lead chloride
  • lead sulfate
  • barium sulfate
  • calcium sulfate
  • most carbonates (except sodium, ammonium, potassium)
  • most hydroxides (except sodium, ammonium, potassium)
• sulfuric acid = H2SO4 (aq)
• nitric acid = HNO3 (aq)
• phosphoric acid = H3PO4 (aq)
• hydrochloric acid = HCl
• sodium hydroxide = NaOH
• colour of copper sulfate is blue
• in a reaction, fizzing (effervescence) occurs whenever a gas is released