LEARNING OBJECTIVES
What you will be able to do
- describe acid and base reactions and indicator colours
- use pH and hydrogen-ion concentration
- distinguish strong and weak acids
- classify acidic, basic and amphoteric oxides
- apply solubility rules
- prepare soluble and insoluble salts
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Use ions, indicators, solubility and preparation methods to understand acids, bases and salts.
Acid–base chemistry is driven by proton transfer and the reaction of H⁺ with OH⁻. Indicator colours show acidity, but equations explain what changes.
The correct salt-preparation method depends on whether the reactants and product are soluble.
SECTION 01
Properties, indicators and pH
Acids react with metals to form salt and hydrogen, with bases to form salt and water, and with carbonates to form salt, water and carbon dioxide.
Bases are metal oxides or hydroxides; an alkali is a soluble base. Bases neutralise acids, while alkalis also react with ammonium salts to release ammonia.
Acids turn blue litmus red, keep thymolphthalein colourless and turn methyl orange red. Alkalis turn red litmus blue, thymolphthalein blue and methyl orange yellow.
DETAILED EXPLANATION
- Lower pH means greater hydrogen-ion concentration.
- Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).
SECTION 02
Proton transfer and acid strength
A Brønsted–Lowry acid donates a proton and a base accepts a proton.
A strong acid completely dissociates in water. A weak acid only partially dissociates, creating an equilibrium.
Strength is not concentration: strength describes degree of dissociation, while concentration describes amount per volume.
SECTION 03
Oxides
Many non-metal oxides, including carbon dioxide and sulfur dioxide, are acidic. Many metal oxides, including calcium oxide and copper(II) oxide, are basic.
Amphoteric oxides react with both acids and bases to form salt and water. Aluminium oxide and zinc oxide are the required examples.
DETAILED EXPLANATION
- Basic oxide + acid → salt + water.
- Amphoteric does not mean neutral.
SECTION 04
Solubility rules
All sodium, potassium, ammonium and nitrate salts are soluble. Most chlorides are soluble except silver and lead chlorides.
Most sulfates are soluble except barium, calcium and lead sulfates. Most carbonates and hydroxides are insoluble, with sodium, potassium and ammonium exceptions; calcium hydroxide is only slightly soluble.
SECTION 05
Preparing salts
Prepare a soluble salt from an acid and excess metal, insoluble base or insoluble carbonate by warming, adding excess, filtering, concentrating and crystallising.
Use titration when both reactants are soluble because excess reagent cannot be filtered off. Repeat without indicator using the measured volumes before crystallising.
Prepare an insoluble salt by mixing two suitable soluble solutions, then filter, wash and dry the precipitate.
DETAILED EXPLANATION
- An anhydrous substance contains no water.
- Water of crystallisation is shown in formulae such as CuSO₄·5H₂O and CoCl₂·6H₂O.
Choosing a method for copper(II) sulfate
- Copper(II) sulfate is soluble.
- Copper(II) oxide is an insoluble base.
- Warm dilute sulfuric acid and add excess copper(II) oxide.
- Filter off excess solid, concentrate the filtrate, cool, then dry the crystals.
Answer: Use the excess-insoluble-base method.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Acids provide H⁺ and alkalis provide OH⁻ in water.
- Strong acids dissociate completely; weak acids partially.
- Amphoteric oxides react with acids and bases.
- Solubility determines the preparation method.
- Crystallisation requires concentration followed by cooling.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- describe acid and base reactions and indicator colours
- use pH and hydrogen-ion concentration
- distinguish strong and weak acids
- classify acidic, basic and amphoteric oxides
- apply solubility rules
- prepare soluble and insoluble salts