Topic 3

• Ions that make aqueous solutions acidic

  Hydrogen ions (H+)
• Ions that make aqueous solutions alkaline

  Hydroxide ions (OH-)
• pH scale


  Ranges from pH 0 to pH 14 and measures the acidity or alkalinity of a solution
• pH ranges


  • Acid - Less than pH 7 (pH 1 is strongest)
  • Neutral - pH 7
  • Alkali - Greater than pH 7 (pH 14 is strongest)
• Methods to measure pH

  • Universal indicator
  • pH probe
• Blue litmus paper in acid

  Turns red
• Blue litmus paper in alkali

  Stays blue
• Red litmus paper in acid

  Stays red
• Red litmus paper in alkali

  Turns blue
• Problems with using universal indicator

  • Colour of solution matched to pH colour chart is subjective
  • Doesn't provide an exact pH value
• Neutralisation reaction


  A reaction between an acid and a base where H+ ions from the acid react with OH- ions from the alkali to form water
• Ionic equation for neutralisation

  H+(aq) + OH-(aq) → H2O(l)
• Concentrated and dilute acids


  Concentrated acids have more moles of acid per unit volume of water than dilute acids. Concentration is not the same as strength, which refers to whether the acid has completely dissociated in water or not.
• An acid only partially dissociates in water
• Base


  Any substance that reacts with an acid to form salt and water only
• Acid reacting with a metal

  Salt + hydrogen
• Acid reacting with a metal oxide


  Salt + water
• Acid reacting with a metal hydroxide

  Salt + water
• Acid reacting with a metal carbonate


  Salt + water + carbon dioxide
• Why are metal oxides normally bases rather than alkalis?
• Reaction of acid with metal hydroxide

  Acid + metal hydroxide → salt + water
• Reaction of acid with metal carbonate

  Acid + metal carbonate → salt + water + carbon dioxide
• Metal oxides


  Normally bases rather than alkalis because they are insoluble, unlike alkalis which are soluble
• Salt formed from magnesium and sulfuric acid

  Magnesium sulfate
• Salt formed from zinc oxide and nitric acid


  Zinc nitrate
• Salt formed from calcium carbonate and hydrochloric acid

  Calcium chloride
• Chemical test for hydrogen


  Insert a lit splint into a test tube of gas. A 'squeaky pop' will be heard if hydrogen is present.
• Chemical test for carbon dioxide

  Bubble the gas through limewater (calcium hydroxide). Limewater turns cloudy if carbon dioxide is present.
• Reason for adding excess insoluble reactant when preparing a soluble salt

  To ensure all the acid reacts
• Removing excess insoluble reactant when preparing a soluble salt


  By filtration. It is removed to leave a pure solution of the salt.
• Method for preparing a salt from an acid and a soluble reactant

  Titration. Since both the reactants are soluble, a titration allows you to combine the reactants exactly and avoid adding an excess of either reactant as this would be hard to remove.
• Steps for producing a pure dry salt from an acid and alkali

  1. Complete a titration to find the volume of acid that reacts exactly with a set volume of alkali.

  2. Use the results from the titration to mix the acid and alkali in the correct proportions.

  3. Evaporate the water from the solution, leaving pure dry salt crystals.
• How to carry out an acid-alkali titration

  1. Use a pipette to add a measured volume of acid to the conical flask then add a few drops of indicator. Place on a white tile.

  2. Fill the burette with the alkali, noting the initial volume.

  3. Add the alkali to the conical flask. First complete a rough trial to find the end point (the point at which the indicator first changes colour).

  4. Repeat, adding the alkali drop by drop near the end point and swirling the flask constantly to mix.

  5. Record the final volume in the burette. Repeat until you have concordant results.
• The two exceptions to most common chlorides being soluble are silver chloride and lead chloride, which are insoluble.
• All nitrates are soluble.
• All common sodium, potassium and ammonium salts are soluble.
• The three exceptions to most common sulfates being soluble are lead sulfate, calcium sulfate and barium sulfate, which are insoluble.
• The three exceptions to most common carbonates and hydroxides being insoluble are the carbonate / hydroxides of sodium, potassium and ammonium, which are soluble.
• Salt produced when lead reacts with sulfuric acid

  Lead sulfate. A precipitate will form because lead sulfate is insoluble.
• Preparing a pure, dry sample of an insoluble salt

  1. Mix the two solutions required to form the salt.

  2. Filter the mixture using filter paper.

  3. The residue on the filter paper is the insoluble salt.

  4. Wash the salt with distilled water and leave to dry.