https://webtoolyard.com/guides/ Guides | Toolyard Step-by-step guides for files, money, science and engineering: shrinking PDFs, opening HEIC, budgeting, dilutions, PCR primers, resistor codes and more. 2026-10-14T00:00:00Z Toolyardhttps://webtoolyard.com/ https://webtoolyard.com/guides/study-ac-circuits/AC circuits: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZIn alternating current the voltage and current oscillate sinusoidally. Inductors and capacitors oppose the current by an amount that depends on frequency, and the total opposition is the impedance. https://webtoolyard.com/guides/study-acids-bases/Acids, bases and buffers: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZThe pH scale measures acidity on a logarithmic scale. Strong acids and bases dissociate completely, weak ones only partly, and a buffer resists changes in pH. https://webtoolyard.com/guides/study-alcohols-phenols-ethers/Alcohols, phenols and ethers: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAlcohols have an –OH on a saturated carbon, phenols have it on a benzene ring, and ethers have an oxygen between two carbons. How the carbon bearing the –OH is substituted decides much of the chemistry. https://webtoolyard.com/guides/study-aldehydes-ketones/Aldehydes and ketones: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAldehydes and ketones share the carbonyl group C=O, whose carbon is attacked by nucleophiles. Aldehydes have a hydrogen on the carbonyl carbon, which makes them easy to oxidize. https://webtoolyard.com/guides/study-amines/Amines: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAmines are organic derivatives of ammonia. They are bases, and whether the nitrogen carries one, two or three carbon groups decides how they react in the classic tests. https://webtoolyard.com/guides/study-aromatic-compounds/Aromatic compounds: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAromatic compounds such as benzene are unusually stable because of a ring of delocalized pi electrons. They keep the ring intact and undergo substitution rather than addition. https://webtoolyard.com/guides/study-astronomy-calculations/Orbits and starlight: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAstronomy problems rely on a small set of scaling laws. Kepler’s third law links orbit size and period, parallax gives distance, Wien’s law gives temperature from color, and redshift gives recession speed. https://webtoolyard.com/guides/study-atomic-structure/Atomic structure and light: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAn atom has a small nucleus of protons and neutrons surrounded by electrons in shells and orbitals. Light is emitted or absorbed when electrons move between energy levels. https://webtoolyard.com/guides/study-biological-macromolecules/Biological macromolecules: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZBiological macromolecules are polymers built from small monomers by condensation reactions. Knowing which monomer, which bond and which test belongs to each class answers most questions, and a few counting rules answer the rest. https://webtoolyard.com/guides/study-biology-statistics/Statistics for biology: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZBiologists test whether observed counts fit a prediction, and they report how precisely a mean is known. The chi-square test and the standard error cover most of what is asked. https://webtoolyard.com/guides/study-biotechnology-methods/Biotechnology and molecular methods: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZMolecular methods turn DNA into numbers: qPCR gives expression changes, restriction digests give fragment sizes, gels give sizes by migration, and PCR gives copy numbers. Each has a short calculation. https://webtoolyard.com/guides/study-carboxylic-acids/Carboxylic acids and derivatives: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZCarboxylic acids contain –COOH and are weak acids. Their derivatives, acyl chlorides, anhydrides, esters and amides, differ greatly in reactivity and are made from one another by nucleophilic acyl substitution. https://webtoolyard.com/guides/study-cell-cycle-meiosis/Cell cycle and meiosis: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZCells grow, copy their DNA and divide in a regulated cycle. Mitosis makes two identical diploid cells for growth and repair, and meiosis makes four different haploid cells for sexual reproduction. https://webtoolyard.com/guides/study-cell-size-transport/Cells and transport: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZWhy cells stay small, how fast substances move through them, and how temperature affects rates are all questions of scale. Surface area, volume and distance explain most of them. https://webtoolyard.com/guides/study-cell-structure-organelles/Cell structure and organelles: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZCells are built from organelles that each do a particular job. Comparing plant, animal and bacterial cells, and calculating real sizes from micrographs, are the two skills tested most. https://webtoolyard.com/guides/study-chemical-equilibrium/Chemical equilibrium: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAt equilibrium the forward and reverse reactions go at the same rate and the concentrations stop changing. The equilibrium constant K tells how far a reaction goes, and Le Chatelier’s principle predicts how it shifts. https://webtoolyard.com/guides/study-circuits/Electric circuits: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZCircuit analysis finds the current and voltage in every part of a circuit. Ohm’s law links them for a resistor, and two rules about series and parallel connections do the rest. https://webtoolyard.com/guides/study-circular-motion/Circular motion and gravitation: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZAn object moving in a circle at constant speed is always accelerating toward the center. Gravity supplies that inward force for planets and satellites, which links circular motion to orbits. https://webtoolyard.com/guides/study-confidence-intervals-tests/Confidence intervals: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZInference draws conclusions about a population from a sample. A confidence interval gives a range of plausible values, and a test asks whether a sample result is far enough from a claimed value to call it unlikely. https://webtoolyard.com/guides/study-data-sizes-and-networks/Data size and networks: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZNetwork and file-size problems count bits and bytes. Subnet masks decide how many hosts fit, transfer time is size over speed, and the size of an image or sound follows from how many bits each sample takes. https://webtoolyard.com/guides/study-descriptive-statistics/Descriptive statistics: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZDescriptive statistics summarizes a set of values by its center and its spread. The mean, median and mode describe the center, while the range, variance, standard deviation and coefficient of variation describe the spread. https://webtoolyard.com/guides/study-dna-molecular-biology/DNA and molecular biology: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZDNA is a double helix whose base pairing sets simple numerical rules. Counting bases, lengths, codons and PCR copies are the usual calculations. https://webtoolyard.com/guides/study-earth-science-calculations/Earthquakes, plates and dating: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZEarth science problems turn a few logarithmic and linear relations into numbers. Earthquake magnitude is logarithmic, seismic wave lag gives distance, and rates of plate motion and radioactive decay give ages. https://webtoolyard.com/guides/study-ecology-energy-flow/Ecology and energy flow: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZEcology measures how many organisms there are, how diverse they are and how energy moves between feeding levels. Most questions are short calculations from field data. https://webtoolyard.com/guides/study-electrochemistry/Electrochemistry: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZElectrochemistry connects chemical reactions to electrical energy. In a galvanic cell a spontaneous redox reaction produces voltage, and in electrolysis an applied current drives a reaction. https://webtoolyard.com/guides/study-electrostatics/Electrostatics: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZElectrostatics deals with the forces and energy of charges at rest. Like charges repel and unlike charges attract, and the force falls off with the square of the distance. https://webtoolyard.com/guides/study-enzymes-proteins/Enzymes and proteins: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZEnzymes speed reactions by lowering the activation energy, and the Michaelis–Menten equation describes how the rate depends on substrate concentration. Inhibitors change the picture in a predictable way. https://webtoolyard.com/guides/study-evolution-selection/Evolution and natural selection: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZEvolution is a change in allele frequencies over generations. Natural selection acts on heritable variation in fitness, while drift, gene flow and mutation change frequencies without regard to fitness. https://webtoolyard.com/guides/study-fluids/Fluids: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZFluid mechanics covers liquids and gases at rest and in motion. Pressure grows with depth, objects float by displacing fluid, and a flowing fluid trades pressure against speed. https://webtoolyard.com/guides/study-forces/Forces and Newton’s laws: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZNewton’s laws link force to motion. The net force on an object equals its mass times its acceleration, and every force has an equal and opposite partner force acting on a different object. https://webtoolyard.com/guides/study-free-energy/Free energy and spontaneity: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZFree energy combines enthalpy and entropy into one test for spontaneity. A process is spontaneous at a given temperature when ΔG is negative, and ΔG° is directly tied to the equilibrium constant. https://webtoolyard.com/guides/study-functional-groups/Functional groups: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZA functional group is a small arrangement of atoms that gives a molecule its characteristic reactions. Learning to recognize the groups tells you what family a compound belongs to and how it will behave. https://webtoolyard.com/guides/study-gases/Gases: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZThe gas laws describe how pressure, volume, temperature and amount of gas depend on each other. For most problems the ideal gas law PV = nRT covers all of them at once. https://webtoolyard.com/guides/study-gene-expression/Gene expression and protein synthesis: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZGene expression turns the information in DNA into protein through transcription and translation. The questions test base pairing, the genetic code and the effect of mutations. https://webtoolyard.com/guides/study-genetics-crosses/Genetics and inheritance: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZGenetics predicts how alleles pass from parents to offspring. Punnett squares and probability rules handle crosses, the Hardy–Weinberg equation handles populations, and recombination frequency measures linkage. https://webtoolyard.com/guides/study-human-physiology-calculations/Human physiology: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZPhysiology questions often come down to a flow multiplied by a rate. Cardiac output, ventilation and blood pressure are the standard examples, and each has a simple formula. https://webtoolyard.com/guides/study-hydrocarbons/Hydrocarbons: alkanes, alkenes and alkynes: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZHydrocarbons contain only carbon and hydrogen. Alkanes are fairly unreactive, while alkenes and alkynes take part in addition reactions at their pi bonds. https://webtoolyard.com/guides/study-immunity-epidemiology/Immunity and epidemiology: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZThe immune system and public health share a toolkit: cells with defined jobs, and a handful of formulas for how far a disease spreads and how well a test or vaccine works. https://webtoolyard.com/guides/study-induction/Electromagnetic induction: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZA changing magnetic flux through a loop induces an emf. Faraday’s law gives its size, Lenz’s law gives its direction, and the same idea explains generators, transformers and inductors. https://webtoolyard.com/guides/study-inheritance-patterns/Patterns of inheritance: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZInheritance patterns go beyond simple dominance. ABO blood groups have three alleles, sex-linked traits depend on the sex of the child, and some alleles blend. Each is solved with a Punnett square and the multiplication rule. https://webtoolyard.com/guides/study-interest-and-loans/Interest and loans: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZInterest problems differ in whether interest is earned on the principal only or on the growing balance. Simple interest, compound interest, effective annual rate and loan payments all follow from a few formulas. https://webtoolyard.com/guides/study-investment-returns/Investment returns: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZInvestment questions ask how well money performed or whether a plan pays off. Return on investment, compound annual growth, real return, net present value and break-even each answer a different version of that question. https://webtoolyard.com/guides/study-kinematics/Kinematics: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZKinematics describes motion with position, velocity and acceleration, without asking what causes it. With constant acceleration, four equations connect displacement, time, and the initial and final velocity. https://webtoolyard.com/guides/study-magnetism/Magnetism: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZMagnetic fields are made by moving charges and act on moving charges. The force is always perpendicular to both the field and the velocity, so it changes the direction of motion but not the speed. https://webtoolyard.com/guides/study-materials/Stress, strain and structures: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZMaterials and structures are described by stress and strain. Stress is force per area, strain is relative deformation, and for a given material the ratio between them is a constant called the modulus. https://webtoolyard.com/guides/study-membranes-transport/Membranes and transport: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZMembranes control what enters and leaves a cell. Substances cross by passive processes that need no energy or by active transport that does, and water moves by osmosis down a water-potential gradient. https://webtoolyard.com/guides/study-microbial-growth-counting/Microbial growth and counting: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZBacteria are counted by diluting a culture and counting colonies, and they grow by doubling. A few formulas turn plate counts into concentrations and growth curves into generation times. https://webtoolyard.com/guides/study-modern-physics/Modern physics: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZModern physics describes light and matter at small scales. Light comes in quanta called photons, particles have wavelengths, atoms have discrete energy levels, and nuclei decay at random with fixed half-lives. https://webtoolyard.com/guides/study-molecular-lab-calculations/Molecular biology lab calculations: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZMolecular biology benches run on a small set of calculations: how much DNA is in a tube, how to dilute it, how much insert to ligate, and what temperature a primer anneals at. https://webtoolyard.com/guides/study-moles-stoichiometry/Moles and stoichiometry: study guide2026-10-14T00:00:00Z2026-10-14T00:00:00ZThe mole links the mass you weigh to the number of particles. Stoichiometry then uses the coefficients of a balanced equation to convert between amounts of reactants and products.