Material World: chemical reactions, solutions, and energy transformations
This domain examines how matter reacts and how dissolved substances and energy behave. Questions ask you to balance equations, recognize reaction types, calculate concentration, and distinguish endothermic from exothermic changes.
Conservation and balancing
Atoms are conserved in a chemical reaction. Coefficients multiply whole formulas; subscripts are fixed. Balance by changing coefficients only, never subscripts. A balanced equation supports the law of conservation of mass. Common reaction types include combustion (fuel plus oxygen producing oxides), acid-base neutralization (acid plus base producing salt and water), precipitation (two solutions forming an insoluble solid), and oxidation.
Solutions and concentration
A solution is a solute dissolved in a solvent. Concentration = mass of solute / volume of solution, often in g/L. Dilution keeps the amount of solute constant: C1V1 = C2V2. pH below 7 is acidic, 7 is neutral, and above 7 is basic. Each pH unit represents roughly a tenfold change in hydrogen-ion concentration, so pH 3 is much more acidic than pH 5.
Energy
Exothermic changes release energy and usually raise the temperature; endothermic changes absorb energy and usually lower it. Activation energy starts a reaction, and a catalyst lowers activation energy without changing the overall energy released or absorbed. Bond breaking absorbs energy; bond forming releases energy.
Worked example
Dissolve 5.0 g of NaCl to make 250 mL of solution. Convert volume: 250 mL = 0.250 L. Concentration = 5.0 g / 0.250 L = 20 g/L. If this solution is diluted to 500 mL, use C1V1 = C2V2: 20 g/L × 0.250 L = C2 × 0.500 L, so C2 = 10 g/L. The amount of salt is unchanged; only the volume doubled.
Traps
- Changing subscripts instead of coefficients when balancing.
- Dividing by litres of solvent rather than litres of solution.
- Treating pH as a linear scale.
- Reversing exothermic and endothermic temperature effects.
- Thinking a catalyst is consumed or changes the final energy change.