Chemical Changes | Edexcel GCSE Combined Science Chemistry, Foundation tier (1SC0)

Chemical Changes

222 questions
16 subtopics
Paper 3
Paper 3

Chemical Changes is examined in Paper 3, Chemistry 1.

It covers acids, alkalis and the pH scale, the indicators litmus, methyl orange and phenolphthalein, investigating pH change when neutralising an acid, bases and alkalis, reactions of acids, and testing for hydrogen and carbon dioxide, neutralisation and its ionic equation, preparing soluble salts from insoluble and soluble reactants, preparing pure, dry copper sulfate crystals, carrying out an acid-alkali titration, solubility rules, predicting precipitates and preparing an insoluble salt, electrolytes and what electrolysis is, movement of ions and predicting the products of molten electrolysis, electrolysis with inert electrodes, electrolysis of copper sulfate solution and investigating the electrolysis of copper sulfate.

Sample questions from Chemical Changes

Answer each one closed book first, then open the answer.

Acids, alkalis and the pH scale

Explain why hydrogen chloride only behaves as an acid once it has dissolved in water.

Show the answer
Only in solution does it release hydrogen ions, and these ions are what make a solution acidic.
Investigating pH change when neutralising an acid

Explain why the pH stops rising once excess calcium hydroxide has been added to dilute hydrochloric acid.

Show the answer
Calcium hydroxide is only slightly soluble, so once the acid is neutralised the solution soon becomes saturated and no more hydroxide ions can dissolve.
Reactions of acids, and testing for hydrogen and carbon dioxide

Name the salt formed when zinc oxide reacts with dilute sulfuric acid.

Show the answer
Zinc sulfate is formed.
Preparing soluble salts from insoluble and soluble reactants

Explain why leaving unreacted acid in the mixture would spoil a salt prepared this way.

Show the answer
The acid would stay in the solution and contaminate the crystals when the water is evaporated.
Carrying out an acid-alkali titration

Describe how a pure, dry salt is made once the volume of acid needed to neutralise the alkali is known.

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The same volumes are mixed again with no indicator, and the solution is then evaporated and crystallised.
Predicting precipitates and preparing an insoluble salt

Explain why mixing potassium sulfate solution with sodium nitrate solution produces no precipitate.

Show the answer
Both of the possible products are soluble, so no solid can form.
Movement of ions and predicting the products of molten electrolysis

Explain why cations move towards the cathode during electrolysis.

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Their positive charge is attracted to the negatively charged cathode.
Electrolysis of copper sulfate solution

State which electrode is made of impure copper when copper is purified by electrolysis.

Show the answer
The anode is made of the impure copper.

The 16 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Acids, alkalis and the pH scale	The hydrogen ions released by acids and hydroxide ions released by alkalis, the ions in sodium hydroxide and sulfuric acid solutions, and the pH values of acidic, neutral and alkaline solutions.	13
Indicators: litmus, methyl orange and phenolphthalein	The colours of litmus, methyl orange and phenolphthalein in acidic and alkaline solutions, and why litmus is less useful than phenolphthalein for finding the exact point of neutralisation.	8
Investigating pH change when neutralising an acid	The apparatus, variables and pH readings when calcium hydroxide is added to hydrochloric acid, how the pH rises and levels off, the equations, why stirring and small additions matter, and presenting the results in tables and graphs.	19
Bases and alkalis	What bases and alkalis are, metal oxides and hydroxides as bases, why a metal carbonate does not fit the definition of a base, and why every alkali is a base but insoluble bases such as copper oxide are not alkalis.	11
Reactions of acids, and testing for hydrogen and carbon dioxide	The products when acids react with metals, metal oxides, hydroxides and carbonates, naming the salts and writing their equations, choosing the acid for a nitrate, and the tests for hydrogen and carbon dioxide.	18
Neutralisation and its ionic equation	What neutralisation is, how the pH changes as alkali is added, the products of neutralising sulfuric acid, how indigestion remedies work, and the ionic equation H⁺ + OH⁻ → H₂O that explains why water always forms.	12
Preparing soluble salts from insoluble and soluble reactants	Why an excess of insoluble solid is used and filtered off, crystallising the filtrate, and why a titration and a second indicator-free mixing are needed with a soluble base.	13
Preparing pure, dry copper sulfate crystals	Reacting excess copper oxide with warm dilute sulfuric acid, the equation and colour change, filtering off the excess, concentrating over a water bath without evaporating to dryness, and crystallising and drying the crystals.	13
Carrying out an acid-alkali titration	Using a pipette, burette and phenolphthalein, adding acid drop by drop near the end point, concordant results, making a pure salt, and accuracy, precision, repeatability, random and systematic errors and the uncertainty in a mean titre.	19
Solubility rules	The solubility of sodium, potassium and ammonium salts, nitrates, chlorides, sulfates, carbonates and hydroxides, the insoluble exceptions such as silver chloride and barium sulfate, and applying the rules to particular salts.	9
Predicting precipitates and preparing an insoluble salt	Predicting which mixed solutions give a precipitate, naming silver chloride, barium sulfate, copper hydroxide and lead sulfate, and mixing, filtering, washing and drying an insoluble salt.	16
Electrolytes and what electrolysis is	What an electrolyte is and why molten or dissolved ionic compounds conduct while solids do not, electrolysis as decomposition by electrical energy, why a direct current is used, and the electrodes.	11
Movement of ions and predicting the products of molten electrolysis	The cathode and anode, cations and anions and the electrodes they move to, and predicting the metal and non-metal products when molten compounds such as zinc chloride, potassium iodide and aluminium oxide are electrolysed.	13
Electrolysis with inert electrodes	The products at each electrode when copper chloride, concentrated sodium chloride and sodium sulfate solutions, acidified water and molten lead bromide are electrolysed, why hydrogen and oxygen form instead of sodium or a sulfur gas, and the tests for chlorine and oxygen.	15
Electrolysis of copper sulfate solution	How the anode loses mass and the cathode gains it when copper sulfate solution is electrolysed with copper electrodes, why the blue colour stays, and purifying copper with an impure anode, a pure cathode and copper sulfate electrolyte.	9
Investigating the electrolysis of copper sulfate	What is seen at graphite electrodes, weighing copper electrodes before and after, how current affects the mass of copper deposited and keeping the comparison fair, hypotheses and predictions, and straight-line graphs, direct proportion and gradients.	23

Chemical Changes is 222 of the 1,206 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.