Chemical Changes

Acids

Acids are substances that, in solution, are sources of hydrogen ions (pH < 7), whilst alkalis are sources of hydroxide ions when in solution (pH > 7). Neutral solutions have a pH of 7

The higher the concentration of H+ ions in acidic solutions, the lower the pH, while the higher the OHions in alkali solutions, the higher the pH. As hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.

A concentrated solution contains a higher quantity of dissolved solute per volume, while a dilute solution has a lower amount per volume. The pH of an acid depends on the type of acid. Strong acids have lower pH values and higher proportions of the molecules dissociate into ions when dissolved in water to produce high concentrations of H+ ions. Weak acids, however, do not dissociate completely into ions and only a few H+ ions enter the solution. A dilute strong acid may be less harmful than a concentrated weak acid as there may be a higher number of H+ ions after dissociation

Metal + Acid → Salt + Hydrogen

Metal Oxide + Acid → Salt + Water Metal Hydroxide + Acid → Salt + Water

Metal Carbonate + Acid → Salt + Water + Carbon Dioxide

Hydrochloric acids react with metal compounds to produce chloride salts; sulfuric acid produces sulfates and nitric acid produces nitrates. To test for hydrogen, place a lit splint in the test tube and wait for a squeaky pop. To test for carbon dioxide, bubble the gas through limewater which should make it cloudy

Bases are substances that neutralise acids to form a salt and water; alkalis are soluble bases. Neutralisation is therefore a reaction between an acid and a base. H+ ions from the acid react with OHions from the alkali to form water.

CORE PRACTICAL: Investigating Neutralisation

A – Using a measuring cylinder, add a volume of dilute hydrochloric acid to a beaker

B – Put a piece of universal indicator paper on a white tile. Dip the end of a glass rod into the liquid, then drop on the paper. Wait, then compare colour to pH colour chart. Rinse the glass rod

C – Measure out a fixed mass of calcium hydroxide in a weighing boat. Add to the beaker and stir. Estimate and record pH of the mixture as detailed above

D – Repeat until the solution is alkaline and there is no further colour change

E – Plot a graph of pH against mass of Ca(OH)2. There should be an ‘S’ shaped graph formed showing rapid neutralisation

Preparing Salts

There are three methods for preparing salts, and the method depends on the solubility of the salt being collected.

Soluble in Water	Insoluble in water
All sodium, potassium and ammonium salts	No exceptions (at GCSE)
All nitrates	No exceptions
Most chlorides	Silver and lead chlorides
Most sulfates	Lead, barium and calcium sulfates
Sodium, potassium and ammonium carbonates	Most carbonates
Sodium, potassium and ammonium hydroxides	Most hydroxides

A precipitate is an insoluble solid formed when two soluble substances in solution are combined. If a question asks whether a precipitate will form, look at the salt formed and see whether it matches the rules above to be insoluble.

Please remember that Method 2 is a core practical so must be learnt in more detail. Method 3 appears as a core practical in Topic 5

Method	Explanation
Preparing Insoluble Salts	Mix Sodium Y and X Nitrate in a beaker A precipitate will form, of salt XY Filter the mixture. The aqueous sodium nitrate will filter through Rinse the beaker with distilled water and pour through the funnel Pour distilled water over the precipitate Remove filter paper containing filtered solid and leave to dry
Core Practical: Preparing Copper Sulfate(Preparing Soluble Salts from insoluble reactant)	Warm hydrochloric acid in water bath Add excess of copper oxide slowly to the acid Excess of the solid will sink to the bottom of the beaker to show that all the acid has reacted Continue to warm in water bath to speed up the reaction Filter to remove unreacted solid from the solution Heat in evaporating dish until crystals form on the side to evaporate some of the water and concentrate the salt solution Leave on watch-glass to dry and form hydrated, large blue crystals of pure copper sulfate
Preparing Soluble Salts from Aqueous Solutions (Titration)	Measure out set amount of alkali using a pipette to provide accurate measurements Add to conical flask with single-colour indicator e.g. methyl orange or phenolphthalein Pour acid into burette Slowly add acid to the alkali, dropwise near the end Once the colour of the indicator changes, stop the acid flow Record the volume of acid added Repeat until concordant results are achieved Take average then repeat experiment without indicator, stopping at the average result Pour the solution into an evaporating dish and heat until some water evaporates, and crystals start to form Leave to dry over a few days to form hydrated salt crystals

Electrolysis

Electrolysis is a process used to break down substances. Electrical energy from a direct current supply decomposes electrolytes, ionic compounds in their molten state or dissolved in water, and electricity can therefore be conducted.

Electrolysis is carried out using inert electrodes, usually graphite or platinum. Positively charged cations are attracted to the negatively charged cathode, where they undergo reduction, gaining electrons to be discharged as atoms or molecules. Negatively charged anions move towards the positively charged anode, where oxidation occurs, and the anions lose electrons to be discharged as atoms or molecules. The changes of ions to atoms results in chemical changes at electrodes, which can be represented using half equations. Take the example of copper chloride:

In molten solutions, there is only one anion and one cation which will form a product. However, in aqueous solutions, the water also dissociates, meaning H+ and OH– ions enter the solution. Hydrogen will be produced at the cathode if the metal is more reactive than hydrogen. Oxygen will be formed if there are no halide ions present.

	Cathode	Anode
Copper Chloride	Cu2+ reduced to brown solid copper	Cl– oxidised to form pale green chlorine gas
Sodium Chloride	H+ reduced to form hydrogen gas	Cl– oxidised to form pale green chlorine gas
Sodium Sulfate	H+ reduced to form hydrogen gas	OH– oxidised to form oxygen gas
Water acidified with H2SO4	H+ reduced to form hydrogen gas	OH– oxidised to form oxygen gas
Lead Bromide	Pb2+ reduced to form silver liquid lead	Br– oxidised to form brown bromine gas

Copper can be purified by the electrolysis of copper sulfate solution using copper electrodes. The copper atoms in the anode lose electrons to become copper ions which dissolve in the solution and move to the cathode, where they are deposited as pure copper. Impurities from the anode do not form ions and collect below the anode as sludge.

Core Practical: Electrolysis of Copper Sulfate Solution

Part 1 – Graphite Electrodes

A – Set up circuit with DC current supply and graphite electrodes with copper sulfate solution in a beaker. Turn on the current

B – Copper is less reactive than hydrogen so copper metal is produced at the cathode as a coating on the electrode. No halide ions are present so oxygen gas forms as bubbles

Part 2 – Graphite Electrodes

A – Select two clean pieces of copper foil. Measure and record the masses of the two electrodes B – Set up circuit with variable resistor and give a constant current of 0.2A. Leave for 20 minutes

C – Turn off power and remove electrodes from beaker. Wash gently with distilled water then dip in propanone. Shake off the liquid and allow for the rest to evaporate off

D – Record masses of dry electrodes then repeat with increasing current. There should be an increasing mass at the cathode and a decreasing mass at the anode as current increases