The IB Diploma's Group 4, the Sciences, is the subject group where the wrong choice causes the most damage to a university application and where the damage is most commonly discovered late. A student who takes Physics HL and Biology HL without Chemistry HL because both feel relevant to a Medicine direction finds at application time that nearly every UK medical school requires Chemistry HL specifically. A student who takes Chemistry HL and Biology HL for Engineering finds that most engineering programmes expect Physics HL rather than Chemistry as the second science. A student who takes the IB's own Computer Science subject at HL but not Mathematics AA HL finds that the subject they thought was the most relevant preparation does not satisfy the actual prerequisite most CS programmes state.
This piece maps the right science HL combination for five specific degree directions, covers what each science subject involves at HL, and names the specific mistakes that show up in applications every cycle.
IBDP: The Six Science Options and What Each One Is
Before mapping combinations to degree directions, a clear picture of what each science involves at HL matters, because the right choice depends on what the subject actually demands, not just what its name suggests.
Biology HL covers cell biology, molecular biology, genetics, ecology, evolution, and human physiology across 240 teaching hours, with additional Higher Level material including more detailed treatment of DNA and protein synthesis mechanics, advanced genetics, and the human immune system. The subject is primarily conceptual and content-heavy rather than mathematically demanding: a student needs to understand, explain, and apply biological principles across a wide range of scenarios, including extended written paper responses where the ability to construct an argument from biological evidence matters as much as content knowledge. Biology HL is the science that rewards strong memory and strong writing in roughly equal measure.
Chemistry HL covers stoichiometry, atomic structure, bonding, energetics, kinetics, equilibrium, acids and bases, redox reactions, and organic chemistry, again across 240 teaching hours, with HL additional material that includes more quantitative treatments of thermochemistry, electrochemistry, and significantly more depth in organic reaction mechanisms. Of the three standard sciences, Chemistry HL is the most mathematically demanding in terms of multi-step quantitative problem-solving, and the most dependent on a secure understanding of each earlier topic before the next can be meaningfully engaged with. A student with gaps in stoichiometry from the early weeks of the course finds those gaps compounding throughout the two years.
Physics HL covers mechanics, thermal physics, waves, electricity and magnetism, circular motion, quantum physics, and nuclear physics, with HL additional material including wave phenomena, advanced electromagnetic induction, and more detailed quantum and nuclear treatment. Physics HL is the most mathematically demanding science overall, working as a direct extension of the mathematical reasoning that Mathematics AA HL develops. A student strong in Mathematics AA HL who finds the quantitative work in Physics similarly engaging is well-positioned for the combination. A student who finds Mathematics challenging and took Mathematics AI instead will find Physics HL a harder pairing than either Biology or Chemistry.