chemical changes chem

• Acids


  Sources of hydrogen ions in solution
• Alkalis


  Sources of hydroxide ions in solution
• Neutral solution


  pH of 7
• Acidic solutions


  Have lower pH values than 7
• Alkaline solutions


  Have higher pH values than 7
• pH scale


  Measures the acidity or alkalinity of a solution, ranges from 0 to 14
• pH values


  • pH 7 is neutral
  • pH < 7 is acidic
  • pH > 7 is alkaline
• Phenolphthalein


  • Alkaline = pink, Acidic = colourless
• Methyl orange


  • Alkaline = yellow, Acidic = red
• Litmus


  • Litmus solution: Alkaline = blue, Acidic = red

  Litmus paper: Blue litmus goes red in acidic, stays blue in alkaline. Red litmus goes blue in alkaline, stays red in acidic.
• Higher concentration of hydrogen ions in an acidic solution

  Lower the pH
• Higher concentration of hydroxide ions in an alkaline solution

  Higher the pH
• As hydrogen ion concentration increases by a factor of 10, the pH decreases by 1
• Investigating change in pH on adding calcium hydroxide or calcium oxide to dilute hydrochloric acid

  Add dilute HCl, measure pH

  Add weighed mass of calcium hydroxide, stir and record pH

  Keep adding weighed masses until no more change to pH

  Draw line graph with mass added on horizontal axis and pH on vertical axis, draw line of best fit
• Concentrated


  Larger amount of substance in a given volume of solution
• Dilute


  Lesser amount of substance in a given volume of solution
• Strong acid


  Fully dissociates in aqueous solution
• Weak acid


  Partially dissociates in aqueous solution
• Stronger an acid, greater the dissociation, more H+ ions released, lower the pH
• Base


  Substance that reacts with an acid to form a salt and water only
• Alkali


  Soluble base
• General reactions of aqueous acid solutions with metals, metal oxides, metal hydroxides, and metal carbonates


  Acid + metal → salt + hydrogen gas

  Acid + metal oxide → salt + water

  Acid + metal hydroxide → salt + water

  Acid + metal carbonate → salt + water + carbon dioxide
• Metal oxides are normally bases, metal hydroxides are bases/alkalis if insoluble/soluble
• Naming salts


  First part is the name of the metal, second part comes from the acid (chloride, nitrate, sulfate)
• Chemical test for hydrogen

  Use a burning splint held at the open end of a test tube - creates a 'squeaky pop' sound
• Chemical test for carbon dioxide

  Bubble the gas through limewater (calcium hydroxide solution) - it will turn milky (cloudy)
• Neutralisation reaction


  Reaction between an acid and a base
• Acid-alkali neutralisation


  Hydrogen ions (H+) from the acid react with hydroxide ions (OH-) from the alkali to form water
• For any neutralisation reaction with an acid and an alkali, the ionic equation is: H+(aq) + OH-(aq) → H2O(l)
• If soluble salts are prepared from an acid and an insoluble reactant: excess of the reactant is added, the excess reactant is removed, and the solution remaining is only salt and water
• Acid-alkali neutralisation


  Reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH-) from the alkali to form water
• Ionic equation for any neutralisation reaction with an acid and an alkali

  H+(aq) + OH-(aq) -> H2O(l)
• Preparing pure, dry hydrated copper sulfate crystals from copper oxide


  1. Add excess copper oxide (insoluble) to sulfuric acid

  2. Filter off any unreacted copper oxide

  3. Evaporate the water by placing the solution in a water bath
• Carrying out an acid-alkali titration to prepare a pure, dry salt


  1. Wash burette using acid and water

  2. Fill burette to 100cm3 with acid

  3. Use 25cm3 pipette to add 25cm3 of alkali into a conical flask

  4. Add indicator to the conical flask

  5. Add acid from burette to alkali until end-point is reached

  6. The titre is the difference between the first and second burette readings

  7. Repeat the experiment for more precise results

  8. Warm the salt solution to evaporate the water and form crystals
• Solubility rules for common types of substances in water

  • All common sodium, potassium and ammonium salts are soluble
  • All nitrates are soluble
  • Common chlorides are soluble except those of silver and lead
  • Common sulfates are soluble except those of lead, barium and calcium
  • Common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium
• Predicting if a precipitate will form when solutions are mixed

  1. Work out the products of the reaction

  2. Use the solubility rules to determine if any salts formed are soluble or insoluble

  3. Any insoluble salts will form as a precipitate
• Preparing a pure, dry sample of an insoluble salt

  1. Mix the two solutions needed to form the salt

  2. Filter the mixture using filter paper, leaving the insoluble salt

  3. Wash the salt using distilled water

  4. Dry the salt, e.g. in an oven
• The reactants are the substances that take part in a chemical change, while products are new substances formed during the process.
• In an exothermic reaction, energy is released as heat or light, while in an endothermic reaction, energy is absorbed from its surroundings.
• A balanced equation shows how many atoms of each element are involved on both sides of the equation.