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)
• What can be used 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 the solution is matched to a pH colour chart, which is quite subjective as people may disagree with which colour the solution matches
  • It doesn't provide an exact pH value
• Acid X has a pH of 1. There is a high concentration of hydrogen ions in the acid, making it a strong acid. The lower the pH of the acid, the higher the concentration of H+ ions.
• Alkali Y has a pH of 8.5. There is a low concentration of hydroxide ions in the alkali, making it a weak alkali. The lower the pH of the alkali, the lower the concentration of OH- ions.
• If pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10.
• 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 a neutralisation reaction

  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. The concentration of an acid is not the same as strength, which refers to whether the acid has completely dissociated in water or not.
• An acid that only partially dissociates in water is a weak acid.
• Base


  Any substance that reacts with an acid to form salt and water only
• Alkalis are soluble bases, not insoluble bases.
• Acid reacting with a metal

  Acid + metal → salt + hydrogen
• Acid reacting with a metal oxide


  Acid + metal oxide → salt + water
• Acid reacting with a metal hydroxide

  Acid + metal hydroxide → salt + water
• Acid reacting with a metal carbonate

  Acid + metal carbonate → salt + water + carbon dioxide
• Metal oxides are normally bases rather than alkalis because they are normally insoluble.
• Products when an acid reacts with a metal hydroxide

  1. Salt

  2. Water
• Products when an acid reacts with a metal carbonate

  1. Salt

  2. Water

  3. Carbon dioxide
• Metal oxides


  Normally bases rather than alkalis because they are insoluble
• 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

  1. Insert a lit splint into a test tube of gas

  2. A 'squeaky pop' will be heard if hydrogen is present
• Chemical test for carbon dioxide

  1. Bubble the gas through limewater (calcium hydroxide)

  2. 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


  1. By filtration

  2. 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
• Exceptions to common chlorides being soluble

  • Silver chloride
  • Lead chloride
• All nitrates are soluble
• All common sodium, potassium and ammonium salts are soluble