C4 - CHEMICAL CHANGES

• pH scale

  Measure of how acidic or alkaline a solution is, ranging from 0 to 14 with low numbers being most acidic and high numbers being most alkaline
• Neutral pH is 7, which is neither acidic nor alkaline
• pH indicators

  • Chemical dyes that change color depending on the pH
  • Wide range indicators like universal indicator change color across a wide range of pH
• pH probe and meter

  Electronically measures pH precisely, more reliable than human judgment of color changes
• Acid


  Substance that forms aqueous solutions with a pH less than 7, releases hydrogen ions in water
• Base

  Substance with a pH greater than 7
• Alkali

  A base that dissolves in water to form a solution with a pH greater than 7, forms hydroxide ions in water
• Neutralization reaction

  1. Acid + base → salt + water

  2. H+ from acid + OH- from base → H2O
• Common acids

  • Hydrochloric acid
  • Sulfuric acid
  • Nitric acid
• Common bases

  • Sodium hydroxide
  • Calcium carbonate
• Universal indicator is made up of a several different indicator solutions. This means that it undergoes a smooth colour change over a wide range of pH values. 

  It becomes more red in colour in acidic solutions, but more bluey-purple in alkaline solutions.
• OH- ions are responsible for making an alkaline pH when dissolved in solution
• Strong acid

  Acid that ionizes completely, releasing all hydrogen ions
• Weak acid

  Acid that only partially ionizes, releasing some hydrogen ions
• pH is related to the concentration of hydrogen ions in a solution
• An acid is a substance that forms aqueous solutions with a pH less than 7
• Acid ionization

  Acid splits or dissociates into its ions in aqueous solution
• Strong acids

  • Ionize completely, releasing all hydrogen ions
• Weak acids

  • Only partially ionize, releasing some hydrogen ions
• Reversible ionization of weak acids

  Equilibrium between undissociated and dissociated forms
• Concentration of hydrogen ions

  Determines pH
• Decrease in pH by 1

  Increases hydrogen ion concentration by a factor of 10
• Strong acid at any concentration

  Has lower pH than weak acid at same concentration
• Concentration measures amount of acid per unit volume, not degree of ionization
• Types of bases

  • Metal oxides
  • Metal hydroxides
  • Metal carbonates
• Neutralization reactions

  Reactions between acids and bases
• Making soluble salts using an insoluble base

  1. Place dilute acid in beaker and heat gently

  2. Add insoluble base (e.g. copper oxide) a little at a time

  3. Excess base indicates acid has been neutralized

  4. Filter out excess base

  5. Evaporate solution to form crystals

  6. Filter out crystals and dry
• Acid reacts with metal oxide or hydroxide


  Forms salt and water
• Acid + metal oxide--> salt + water

  • Hydrochloric acid + sodium oxide -> Sodium chloride + water
• Acid + metal hydroxide---> salt + water

  • Sulfuric acid + potassium hydroxide -> Potassium sulfate + water
• Acid reacts with metal carbonate

  Forms salt, water and carbon dioxide
• Acid + metal carbonate

  • Nitric acid + calcium carbonate -> Calcium nitrate + water + carbon dioxide
• Salts formed from acid-base reactions can be soluble or insoluble in water
• To make a soluble salt, react an acid with an insoluble base (metal oxide, hydroxide or carbonate)
• Excess insoluble base indicates the acid has been neutralized
• Filter out excess insoluble base to leave the dissolved soluble salt
• Evaporate the solution to form solid salt crystals
• Filter out the salt crystals and dry them
• During a neutralisation reaction, the positive hydrogen ions from the acid react with the negative hydroxide ions from the alkali to form molecules of water
• Acid + metal carbonate ➔ salt + water + carbon dioxide