Chemical Changes

• The pH scale is a measure of how acidic or alkaline a solution is. A neutral substance has a pH of 7
• An acid is a substance with a pH less than 7. An alkali is a substance with a pH more than 7.
• The higher the concentration of hydrogen ions in a solution, the more acidic it is, so the lower its pH will be.
• A base is a substance that reacts with an acid to produce a salt and water.
• All alkalis form OH- ions or hydroxide ions in water.
• An indicator is a dye that changes colour depending on whether it's above or below a certain pH.
• Litmus paper is red in acidic solutions, purple in neutral and blue in alkaline.
• Methyl orange is red in acidic solutions, yellow in neutral and alkaline solutions.
• Phenolphthalein is colourless in acidic or neutral solutions and pink in alkaline solutions.
• The reaction between an acid and a base is called neutralisation.
• All acids can ionise (or disassociate) in solution - that means splitting up to produce a hydrogen ion
• Strong acids ionise almost completely in water
• Weak acids only partially ionise in water
• The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium. Since only a few of acid particles release H+ ions, the equilibrium lies well to the left.
• Acid strength tells you what proportion of the acid molecules ionise in water.
• Concentration measures how much acid there is in a litre of water. Concentration is basically how watered down your acid is.
• A salt is formed during a neutralisation reaction. Salts are ionic compounds.
• Hydrochloric acid produces chloride salts, sulfuric acid produces sulfate salts and nitric acid produces nitrate salts.
• You can test for hydrogen using a lighted splint. Hydrogen makes a "squeaky pop" with a lighted splint. The noise comes from the hydrogen burning with the oxygen in the air to form water.
• You can test for carbon dioxide by bubbling it through limewater. If the gas is carbon dioxide, the limewater will turn cloudy.
• To make a pure, dry sample of insoluble salt, you can use precipitation reaction. You have to pick the right two soluble salts and react them together to get your insoluble salt.
• You can make a soluble salt by reacting an acid that contains the ions that you need in the salt with an insoluble base that contains the other ion you need.
• Soluble salts can be made by reacting an acid with an alkali. You have to work out exactly the right amount of alkali to neutralise the acid.
• Electrolysis is the breaking down of a substance using electricity. An electric current is passed through an electrolyte.
• In electrolysis, oxidation and reduction occurs.
• The positive ions in the electrolyte move towards the cathode and are reduced(gain electrons). The negatived ions in the electrolyte move towards the anode and are oxidised (lose electrons).
• As ions gain or lose electrons they form the uncharged substances and are discharged from the electrolyte.
• An electrochemical cell is a circuit, made up of the anode, cathode, electrolyte, a power source and the wires that connect the two electrodes.
• An ionic solid cannot be electrolysed because the ions are in fixed positions and can't move.
• Molten ionic compounds can be electrolysed because the ions can move freely and conduct electricity.
• Positive metal ions are reduced to metal atoms at the cathode
• Negative ions are oxidised to atoms or molecules at the anode.
• In aqueous solutions, as well as the ions from the ionic compound there will be hydrogen ions and hydroxide ions from the water.
• At the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced if the metal is more reactive than hydrogen. If the metal is less reactive than hydrogen then a solid layer of the pure metal will be produced instead.
• At the anode, if OH- and halide ions are present, molecules of chlorine, bromine or iodine will be formed. If no halide ions are present then oxygen will be formed.
• In electrolysis of aqueous solutions oxygen gas and a halogen is produced at the anode.
• In electrolysis of aqueous solutions a hydrogen gas and a metal is produced at the cathode