Chemical changes
Reactivity of metals and extraction, acids, alkalis and pH, neutralisation and salts, and electrolysis.
Learn
Chemical changes, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Use the reactivity series and displacement reactions
The reactivity series ranks metals from most reactive (potassium) to least (gold). A more reactive metal displaces a less reactive one from its compound - iron added to copper sulfate solution pushes out the copper. The order also predicts how vigorously a metal reacts with water or acid.
Explain how metals are extracted by reduction
A metal joined to oxygen must be reduced - have its oxygen removed - to get the pure metal. Metals less reactive than carbon, like iron, can be extracted by heating with carbon, which takes the oxygen. More reactive metals such as aluminium hold their oxygen too tightly and need electrolysis instead.
Describe the reactions of acids with metals, bases and carbonates
Acids follow reliable patterns. Acid + metal → salt + hydrogen (the gas that pops a lit splint). Acid + base (metal oxide or hydroxide) → salt + water. Acid + carbonate → salt + water + carbon dioxide (which turns limewater milky). The salt's name comes from the acid - hydrochloric gives chlorides, sulfuric gives sulfates.
Use the pH scale and describe neutralisation
The pH scale runs 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline, and indicators or a pH probe measure it. Acids release H⁺ ions and alkalis release OH⁻ ions; in neutralisation these combine to form water, moving the pH towards 7. That is why indigestion tablets (alkaline) ease an acidic stomach.
Describe how to make a soluble salt
To make a soluble salt, add an excess of an insoluble base (like a metal oxide) to warm acid until no more reacts, then filter off the leftover solid. Evaporate the filtered solution gently to leave salt crystals. Using excess base makes sure all the acid is used up, so the salt is pure.
Describe electrolysis and what forms at each electrode
Electrolysis uses electricity to split an ionic compound that is molten or dissolved, so its ions can move. Positive ions travel to the negative electrode (cathode) and gain electrons; negative ions go to the positive electrode (anode). Molten lead bromide gives lead at the cathode and bromine at the anode - this is how reactive metals like aluminium are extracted.
Chemical changes key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Oxidation
- The gain of oxygen in a reaction. (On the Higher tier it is also defined as the loss of electrons.)
- Reduction
- The loss of oxygen in a reaction. (On the Higher tier it is also defined as the gain of electrons.)
- Reactivity series
- Metals arranged in order of how vigorously they react, from potassium at the top down through zinc and iron to copper and gold.
- Displacement reaction
- A reaction in which a more reactive metal takes the place of a less reactive metal in its compound.
- Neutralisation
- The reaction between an acid and a base, producing a salt and water.
- pH scale
- A scale from 0 to 14 measuring how acidic or alkaline a solution is: below 7 is acidic, 7 is neutral, above 7 is alkaline.
- Salt
- A compound formed when the hydrogen in an acid is replaced by a metal, such as copper sulfate from sulfuric acid.
- Electrolysis
- Using an electric current to split an ionic compound into its elements, either molten or in solution.
- Anode and cathode
- The electrodes in electrolysis: the anode is the positive electrode, where negative ions go; the cathode is the negative electrode, where positive ions go.
Practice
Try a Chemical changes question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
Metal X displaces copper from copper sulfate solution, but there is no reaction when X is added to magnesium sulfate solution. What does this show about the reactivity of X?
- X is more reactive than copper but less reactive than magnesium
- X is more reactive than magnesium but less reactive than copper
- X is less reactive than both copper and magnesium
- X is more reactive than both copper and magnesium
Show the answer and the working
Answer: X is more reactive than copper but less reactive than magnesium
Displacing copper puts X above copper in the series, and not reacting with magnesium sulfate puts X below magnesium.
- A metal displaces another metal from solution only if it is more reactive.
- X displaces copper from copper sulfate, so X is above copper in the reactivity series.
- X does not displace magnesium from magnesium sulfate, so X is below magnesium.
- So the order is magnesium > X > copper.