Science explained: every chemistry, physics and biology idea students search for, worked line by line and placed in its course

Science help works best one idea at a time: see the idea worked line by line, read the line students usually get wrong, then try a sibling problem cold. This section does that for 26 ideas of Grade 12 chemistry, physics and biology, each placed in its course so you know what it rests on and what it unlocks. Below, the ideas are listed in the order the courses teach them, with one sentence on what each explainer shows.

How a science explainer on this site works

Whether you came for chemistry help, physics help or biology help, every explainer here has the same four parts.

1. One worked example, computed in full. Every given is stated, every unit is carried, and every significant figure is accounted for. You can check each line yourself.
2. The common wrong line. What students usually write instead, and why it fails. Most lost marks on a science test come from a handful of these.
3. A sibling problem to try cold. A fresh problem of the same kind, with the answer under a Check heading, so you can mark yourself.
4. The course map. The ideas before and after this one, by our own concept titles, so you know what to go back to and what comes next.

Each part rests on research. Sweller and Cooper (1985) found that students who studied worked examples before solving became faster at solving similar problems than students who went straight to problems. Roediger and Karpicke (2006) found that taking a test on material produced better recall a week later than restudying it, which is why each page ends with a problem rather than a summary to reread. Rohrer and Taylor (2007) found that shuffling problem types beat practising one type at a time, which is why the night-before plan below mixes ideas.

The three subjects also reward different habits. How to study biology, chemistry and physics, each its own way covers that, and worked examples, practice problems and past exams, in the order that teaches covers where your practice should come from.

Chemistry help (SCH4U), in the order the course teaches it

The ideas below follow the SCH4U course map, with one change: the pack files stoichiometry and the gas laws last, as prerequisite review, and this list puts them first because that is where most students meet them. Students in US courses will find the same ideas in AP Chemistry.

• Stoichiometry takes one reaction through five steps: balancing, grams to moles, the mole ratio, the limiting reactant and percent yield, with the wrong line named at each step.
• The gas laws and PV = nRT shows the units that make the ideal gas law work (kelvin, kilopascals and R = 8.314), the one rearrangement you need, and the volume of a gas produced by a reaction.
• Functional groups and IUPAC naming puts the twelve groups in one table, names one alcohol in five steps with the numbering trap, and counts the isomers of C₄H₁₀O.
• Electron configuration and periodic trends works iron and its ions: the filling order, Hund's rule, why 4s empties before 3d, the chromium and copper exceptions, and the four trends.
• Lewis structures gives a counting method that always lands on the right structure, then works formal charge and resonance on the nitrate ion.
• Molecular geometry and bond angles puts every VSEPR shape in one table, with the lone-pair effect and a ten-molecule self-test; it lives on the course page as the molecular geometry explainer.
• Electronegativity, polarity and intermolecular forces runs the chain from a bond's electronegativity difference to a molecule's polarity to its boiling point, on three liquids of the same size whose boiling points are 120 degrees apart.
• Enthalpy, Hess's law and calorimetry works heat three ways (a coffee-cup calorimeter with q = mcΔT, Hess's law by adding equations, and enthalpies of formation) and names the sign error that flips an answer.
• Rate laws and the Arrhenius equation finds the reaction order and the rate constant's units from a table of initial rates, then the activation energy from two temperatures.
• Equilibrium and Le Chatelier's principle writes the Kc expression, solves an ICE table to the end, finds Ksp from a solubility, and explains which way a system shifts for each change and why.
• pH and buffers does the calculations in the order a test asks them: a strong acid by the log, a weak acid by Ka and an ICE table, a buffer by Henderson–Hasselbalch, then the titration curve.
• Redox and galvanic cells assigns oxidation numbers by rule, balances in acid by half reactions, finds a galvanic cell's voltage, and uses Faraday's law for what an electrolytic cell deposits.

Physics help (SPH4U), in the order the course teaches it

These follow the SPH4U course map.

• The kinematics equations gives the rule that picks one of the five (the quantity that is missing) and works a projectile off a cliff with units and the sign error named.
• Free-body diagrams, Newton's laws, friction and inclines turns a diagram into Newton's second law, worked on a box sliding down a 30° incline with friction.
• Centripetal force and universal gravitation shows that centripetal force is the net force toward the centre, not a new force, then works one satellite's speed, period and the value of g at its height (the course returns to this in its gravitation strand).
• Work, energy and power follows one skier down a hill and across the flat, solved by energy and then by forces, so you can see when energy is the shortcut.
• Momentum, impulse and collisions treats impulse as the change in momentum and works one crash with signs: the shared speed, the energy lost and the force.
• Electric and magnetic fields works Coulomb's law with the units converted, the electric field and potential that follow, and the right-hand rule on a proton moving through a magnetic field.
• Light and modern physics covers the double slit and its fringe spacing, a diffraction grating's angles, the photoelectric effect as an energy ledger in electronvolts, and time dilation at 0.80c.

Biology help (SBI4U), in the order the course teaches it

These follow the SBI4U course map.

• Macromolecules, protein structure and enzymes covers the four macromolecules and their bonds, the four levels of protein structure, and one enzyme's cycle traced to the rate graph with both kinds of inhibition.
• Cellular respiration traces one glucose through glycolysis, the Krebs cycle and the electron transport chain with every count, and explains why the ATP total reads 30 to 32 in some books and 36 to 38 in others.
• Photosynthesis splits it into two stages: the light reactions split water and load ATP and NADPH, and the Calvin cycle fixes three CO₂ for each G3P, with every count traced and the mirror of respiration drawn.
• DNA to protein covers DNA's structure and replication, then follows one twelve-base gene through transcription and translation to a peptide, with the template-strand error named.
• Mitosis vs meiosis traces both side by side on one human cell, with chromosome and chromatid counts at every stage and the three differences tests ask about (the inheritance unit assumes this one).
• Punnett squares and Mendelian ratios works a monohybrid cross, then a dihybrid cross to 9:3:3:1, checked with the product rule.
• Homeostasis and the nervous system traces the feedback loop on body temperature, then the action potential from −70 mV to its peak and back, with the ions in order.
• The endocrine system and the kidney covers how hormones work, the ones SBI4U asks for, and the nephron from filtrate to urine, with ADH and aldosterone placed where they act.

How to use these pages the night before a test

Three steps, whichever subject it is.

1. Find the idea in the lists above. If the test covers several, write them down as a short list first.
2. Read the worked example once, then redo it cold. Close the page and work it on paper from the question alone. Compare line by line, and mark yourself honestly; how to mark yourself honestly has the rules.
3. Do the sibling problem and check it. That is the test of whether you own the idea, not whether the example looked familiar.

Mix two or three ideas in one sitting rather than finishing one before you start the next. A test mixes them, so your practice should too.

Each subject has its own habit. Chemistry rewards running the same procedure every time. Physics rewards rebuilding each formula from its idea, so it cannot slip away. Biology rewards retelling a mechanism in order, so the vocabulary has somewhere to attach.

Every science explainer, on one screen

• Four parts on every page: a worked example, the common wrong line, a sibling problem with its answer, and the course map.
• Three course maps: SCH4U chemistry, SPH4U physics and SBI4U biology, each list in the order its course teaches.
• One routine: find the idea, redo the example cold, do the sibling, mix ideas in a sitting.

Worked examples before problems is the habit in chapter 3 of the free playbook.

Where YSR Studies does the bookkeeping

Every idea on this page is a node on a course map in the YSR Studies app, shown as Learned, Shaky or Not shown yet. A card's answer stays on the server until you have tried it, and a numerical answer is checked by code, the maths compared rather than the spelling. Five free hours every month.