GCSE-readyGCSE Combined Science: common mistakes
The wrong answers students pick most often in our Combined Science questions, and why they are wrong. Read these before a test, then open the lesson for anything that surprises you.
Getting started
- Getting started: cells and the light microscope
- Not “Cytoplasm”. The cytoplasm is where reactions happen, but it does not hold the genetic material.
- Not “Cell membrane”. The membrane controls what enters and leaves, not the cell's activities.
- Not “Vacuole”. The vacuole holds cell sap.
- Getting started: variables, fair tests and means
- Not “The time taken for the reaction”. This is what is measured: the dependent variable.
- Not “The volume of enzyme”. This should be kept the same: a control variable.
- Not “The pH”. pH should be kept the same: a control variable.
- Getting started: atoms, elements, compounds and formulae
- Not “Fluorine”. Fluorine is F.
- Not “O2”. O2 has only oxygen atoms, so it is an element.
- Not “He”. Helium is an element.
- Getting started: chemical reactions and conservation of mass
- Not “Atoms are destroyed”. Atoms are never destroyed in a chemical reaction.
- Getting started: forces, mass and weight
- Not “There are no forces acting”. Weight still acts; it is balanced by air resistance.
- Not “Kilogram (kg)”. The kilogram is the unit of mass.
- Not “Joule (J)”. The joule is the unit of energy.
Biology: Key concepts in biology
- Animal, plant and bacterial cells
- Not “Ribosomes”. Ribosomes make proteins.
- Not “Nucleus”. The nucleus holds the genetic material.
- Not “Cell wall”. The cell wall strengthens plant cells.
- Microscopes and magnification
- Not “It uses coloured stains”. Light microscopes also use stains. Stains do not reveal tiny structures.
- Not “The cells are alive”. Samples for electron microscopes are not alive.
- Not “It uses a brighter bulb”. Brightness does not change how much detail can be separated.
- Core practical: using a light microscope
- Not “1 × 10-6 m”. That is a micrometre.
- Not “1 × 10-9 m”. That is a nanometre.
- Not “1 × 10-3 m”. That is a millimetre.
- Enzymes
- Not “The enzyme is killed”. Enzymes are molecules. They are not alive, so they cannot be killed.
- Not “The substrate is used up”. The substrate is still there. It just no longer fits.
- Not “The enzyme moves too fast”. Faster movement speeds reactions up until the shape changes.
- Core practical: pH and enzyme activity
- Not “The enzyme is denatured”. Adding substrate does not denature the enzyme.
- Not “The substrate runs out”. There is lots of substrate at high concentrations.
Biology: Cells and control
- Mitosis and the cell cycle
- Not “Prophase”. In prophase the chromosomes condense and the nuclear membrane breaks down.
- Not “Anaphase”. In anaphase the chromatids are pulled apart.
- Not “Telophase”. In telophase new nuclei form.
- The nervous system and reflexes
- Not “Motor neurone”. Motor neurones carry impulses away from the CNS to effectors.
- Not “Relay neurone”. Relay neurones are inside the CNS.
- Not “Effector neurone”. There is no such neurone. Effectors are muscles or glands.
Biology: Genetics
- Meiosis and reproduction
- Not “It only needs one parent”. Needing only one parent is an advantage of asexual reproduction.
- Not “It is faster than asexual reproduction”. Asexual reproduction is usually faster.
- Not “Offspring are identical to the parent”. Identical offspring come from asexual reproduction.
- DNA and the genome
- Not “All the DNA in an organism”. That is the genome.
- Inheritance and Punnett squares
- Not “The alleles an organism has”. That is the genotype.
Chemistry: Key concepts in chemistry
- How the model of the atom developed
- Not “1”. That is the mass of a proton or neutron.
- Not “−1”. −1 is its relative charge, not its mass.
- Atomic structure
- Not “Atoms with different numbers of protons”. Changing the number of protons changes the element.
- Ionic, covalent and metallic bonding
- Not “Diamond”. Diamond is giant covalent.
- Not “Magnesium oxide”. Magnesium oxide is ionic.
- Not “Copper”. Copper is metallic.
- Ions, ionic formulae and ionic lattices
- Not “NH3”. NH3 is ammonia, a molecule.
- Not “NO3−”. That is nitrate.
- Not “NH4−”. The ammonium ion is positive.
Chemistry: Chemical changes
- Neutralisation and salts
- Not “It turns limewater milky”. That is the test for carbon dioxide.
- Not “It relights a glowing splint”. That is the test for oxygen.
- Not “It bleaches damp litmus paper”. That is the test for chlorine.
Physics: Key concepts of physics
- Units, prefixes and vectors
- Not “Newton”. The newton is the unit of force, including weight.
Physics: Motion and forces
- Acceleration and velocity-time graphs
- Not “Distance”. Distance is the area under a velocity-time graph.
Physics: Waves
- Wave properties
- Not “The distance between two crests”. That is the wavelength.
- Not “The number of waves per second”. That is the frequency.