Some Basic Concepts of Chemistry

The One Thing

Chemistry quantifies matter and chemical change through particles, moles, measurements, and balanced equations. The mole and stoichiometry connect microscopic particles with measurable masses, volumes, and reaction yields.

Reactions, Processes and Experiments

Key Terms

• Matter: Anything that has mass and occupies space.
• Element: A pure substance made of only one type of atom.
• Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio.
• Mixture: A physical combination of two or more substances in variable proportions.
• Homogeneous mixture: A mixture with uniform composition and properties throughout.
• Heterogeneous mixture: A mixture whose composition is not uniform throughout.
• Physical change: A change in state or physical form that does not produce a new substance.
• Chemical change: A change in which one or more new substances with different properties are formed.
• Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical reaction; the total mass of reactants equals the total mass of products.
• Law of Definite Proportions: A pure compound always contains the same elements in the same fixed proportion by mass.
• Law of Multiple Proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.
• Atomic mass: The relative mass of an atom compared with one-twelfth of the mass of a carbon-12 atom.
• Molecular mass: The sum of the atomic masses of all atoms present in one molecule.
• Molar mass: The mass of one mole of a substance, expressed in grams per mole.
• Mole: The amount of substance containing 6.022 × 10^23 specified particles.
• Avogadro constant: The number of particles in one mole, equal to 6.022 × 10^23 mol^-1.
• Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
• Molecular formula: The actual number of atoms of each element in one molecule of a compound.
• Limiting reagent: The reactant that is completely consumed first and limits the amount of product formed.
• Excess reagent: The reactant that remains after the limiting reagent has been completely consumed.
• Stoichiometry: The quantitative study of the relationships between reactants and products in a chemical reaction.
• Molarity: The number of moles of solute present in one litre of solution.
• Molality: The number of moles of solute present in one kilogram of solvent.
• Significant figures: The meaningful digits in a measured quantity, including all certain digits and the first uncertain digit.
• Dimensional analysis: A method of converting units and checking whether equations are dimensionally consistent.

Easily Confused

• Element vs compound: An element contains one type of atom; a compound contains two or more elements chemically combined in a fixed ratio.
• Compound vs mixture: A compound has a fixed chemical composition; a mixture contains substances physically combined in variable proportions.
• Homogeneous vs heterogeneous mixture: A homogeneous mixture is uniform throughout; a heterogeneous mixture is not uniform throughout.
• Physical vs chemical change: A physical change does not produce a new substance; a chemical change produces one or more new substances.
• Atomic mass vs molecular mass: Atomic mass refers to the relative mass of one atom; molecular mass is the sum of the atomic masses in one molecule.
• Molar mass vs molecular mass: Molar mass is the mass of one mole in grams per mole; molecular mass is the relative mass of one molecule.
• Empirical formula vs molecular formula: The empirical formula gives the simplest whole-number ratio; the molecular formula gives the actual number of atoms in one molecule.
• Limiting reagent vs excess reagent: The limiting reagent is consumed first and controls product formation; the excess reagent remains after the reaction.
• Molarity vs molality: Molarity uses moles of solute per litre of solution; molality uses moles of solute per kilogram of solvent.
• Actual yield vs theoretical yield: Actual yield is the experimentally obtained amount; theoretical yield is the maximum amount predicted from stoichiometry.
• Coefficients vs subscripts: Coefficients balance an equation without changing substance identities; subscripts are part of a chemical formula and must not be changed when balancing.
• Decimal-place rules vs significant-figure rules: Addition and subtraction use the fewest decimal places; multiplication and division use the fewest significant figures.

What Gets Asked

• Classifying substances and changes: Questions may ask students to distinguish elements, compounds, homogeneous mixtures, heterogeneous mixtures, physical changes, and chemical changes. Marks are lost by confusing physical combinations with chemically combined substances.
• Applying chemical laws: Questions may test the law of conservation of mass, the law of definite proportions, or the law of multiple proportions, including their association with Antoine Lavoisier, Joseph Proust, and Dalton’s atomic theory. Marks are lost by using fixed-ratio reasoning for mixtures or by failing to identify the relevant law.
• Balancing equations and using stoichiometric ratios: Questions require balanced equations to calculate reactant or product quantities. Marks are lost by changing subscripts instead of coefficients or by using unbalanced coefficients as mole ratios.
• Mole and composition calculations: Questions may require $n=$ given mass / molar mass, particle number, mass percentage, empirical formula, molecular formula, molarity, molality, or mole fraction. Marks are lost through incorrect units, failure to reduce empirical ratios to whole numbers, or confusion between solution volume and solvent mass.
• Limiting-reagent and yield calculations: Questions may require identification of the limiting reagent, the remaining excess reagent, theoretical yield, actual yield, or percentage yield. Marks are lost by comparing masses rather than available moles in the stoichiometric ratio.
• Measurement and reporting: Questions may test dimensional analysis, significant figures, decimal places, and gas molar volume. Marks are lost by retaining too many decimal places, using too many significant figures, treating leading zeros as significant, or applying 22.7 L and 22.4 L without checking the stated temperature and pressure conditions.