Question 1 of 18: Ten Chemistry Definitions

Question 1: Ten Chemistry Definitions (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1.1 Saturated solution. A solution in which the dissolved solute is in dynamic equilibrium with undissolved (excess) solute at a given temperature and pressure — the solvent holds the maximum concentration of solute it can hold under those conditions; adding more solute produces no further net dissolution.

1.2 Mole fraction. The ratio of the number of moles of one component to the total number of moles of all components in a mixture, $x_i = n_i/\sum n_i$ (dimensionless, $0\le x_i\le 1$, and $\sum x_i=1$); used in gas-transfer and vapour-pressure (Raoult's Law) calculations.

1.3 Chelating agent. A ligand (e.g. EDTA) that binds a metal ion through two or more donor atoms simultaneously, forming a stable ring (chelate) complex — used analytically in EDTA hardness titration and in water treatment to sequester metals and prevent their precipitation/scaling.

1.4 ORP (oxidation–reduction potential). The electrical potential (mV, relative to a standard hydrogen electrode) of a solution that reflects its overall tendency to accept or donate electrons; used operationally to monitor disinfection (chlorine residual proxy) and anaerobic/anoxic conditions in treatment processes.

1.5 Fick's Law. The fundamental diffusion law stating that the mass flux of a species is proportional to the negative of its concentration gradient, $J=-D\dfrac{dC}{dx}$ (1st law, steady state) with diffusivity $D$; governs molecular diffusion of gases/solutes (e.g. oxygen transfer across the air–water interface, membrane transport).

1.6 NTU (nephelometric turbidity unit). The standard unit of water turbidity, measured by the intensity of light scattered at 90° by suspended particles relative to a formazin polymer reference standard; used to gauge raw-water quality and filtered-water performance in drinking-water treatment.

1.7 Equilibrium constant. The dimensionless (or unit-bearing) ratio of product to reactant activities/concentrations at chemical equilibrium for a reversible reaction $aA+bB\rightleftharpoons cC+dD$, $K=\dfrac{[C]^c[D]^d}{[A]^a[B]^b}$; a large $K$ favours products, a small $K$ favours reactants, and it is temperature-dependent.

1.8 Normality. A concentration unit expressing the number of gram-equivalent weights of solute per litre of solution ($N=$ equivalents/L $=M\times$valence); used in titrimetric methods (EDTA hardness, acid–base) because it directly reflects reacting capacity.

1.9 Temporary hardness. The portion of total water hardness attributable to calcium and magnesium bicarbonate salts, which can be removed by boiling (driving off CO₂ and precipitating CaCO₃/Mg(OH)₂) — distinct from permanent hardness (Ca/Mg sulphates, chlorides, nitrates), which boiling cannot remove.

1.10 CT concept. The disinfection design/regulatory concept that log-inactivation of a target pathogen depends on the product of disinfectant residual concentration $C$ (mg/L) and contact time $t$ (min), $CT=C\times t$; regulators specify a minimum required CT for a given organism, temperature and pH that a treatment train's contact basin must deliver.