Chemical Changes

• 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
• Acid X has a pH of 1
• Alkali Y has a pH of 8.5
• If pH decreases by one unit
• 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
• Alkalis are insoluble bases
• 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?
• What are the products when an acid reacts with a metal hydroxide?

  Acid + metal hydroxide → salt + water
• What are the products when an acid reacts with a metal carbonate?

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

  Normally bases rather than alkalis because they are insoluble, while alkalis are soluble
• Magnesium sulfate


  The name of the salt formed from magnesium and sulfuric acid
• Zinc nitrate

  The name of the salt formed from zinc oxide and nitric acid
• Calcium chloride


  The name of the salt formed from calcium carbonate and hydrochloric acid
• Describe the chemical test for hydrogen

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

  Bubble the gas through limewater (calcium hydroxide). Limewater turns cloudy if carbon dioxide is present.
• Excess of the insoluble reactant

  Added when preparing a soluble salt from an acid and an insoluble reactant, to ensure all the acid reacts
• How and why is the excess reactant removed?

  By filtration, to leave a pure solution of the salt
• What method must be used to prepare a salt from an acid and a soluble reactant?

  Titration, since both reactants are soluble and this allows combining them exactly to avoid excess
• Filtration


  The method that could be used to prepare a sample of soluble copper sulfate from insoluble copper oxide and sulfuric acid
• What 3 steps are required when producing a pure dry salt from an acid and alkali?


  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.
• Describe how to carry out an acid-alkali titration

  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.
• Silver chloride and lead chloride are insoluble
• All nitrates are soluble