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Working in the laboratory definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

30 terms

Measurement, uncertainty and significant figures in chemistry

Resolution: The smallest change in the quantity being measured that gives a change in the reading on the instrument.
Uncertainty: The interval within which the true value of a measurement can be expected to lie, written as ± a value in the unit of the measurement.
Parallax error: An error in reading a scale caused by viewing it from an angle instead of with the eye level with the mark or meniscus being read.
Percentage uncertainty: The absolute uncertainty of a measurement divided by the measured value, multiplied by 100.
Accuracy: A measurement or result is accurate when it is close to the true value.
Precision: Results are precise when repeated measurements are close to one another, with little spread about the mean.
Random error: An error that makes repeated readings scatter unpredictably on both sides of the true value; its effect is reduced by repeating the measurement and taking a mean.
Systematic error: An error that shifts every reading from the true value in the same direction by a similar amount; repeating the measurement neither reveals it nor reduces it.
Concordant results: Titres that agree within 0.10 cm³ of each other, from which the mean titre is calculated.
Anomalous result: A result that does not fit the pattern of the others, such as a titre outside the concordant set, and is left out of the mean.

Standard solution: A solution whose concentration is known accurately, made by dissolving a known mass of solute and making it up to a known volume.
Weighing by difference: Finding the mass of solid transferred by weighing its container before and after the transfer and subtracting.
Heating under reflux: Heating a reaction mixture with a vertical condenser fitted, so that vapours condense and return to the flask and no volatile substance is lost.
Distillation: Separating a liquid from a mixture by boiling it and condensing the vapour in a separate vessel.
Filtrate: The liquid that passes through the filter paper during filtration.
Recrystallisation: Purifying a solid by dissolving it in the minimum volume of hot solvent and letting the solution cool, so that the product crystallises and soluble impurities stay in solution.
Hazard: A substance, object or activity with the potential to cause harm.
Risk: The likelihood that harm from a hazard will occur in the way an activity is carried out, together with how severe that harm would be.

Polyatomic ion: An ion made of two or more atoms covalently bonded together and carrying an overall charge, such as SO₄²⁻ or NH₄⁺.
Water of crystallisation: Water molecules held in a fixed ratio within the crystal lattice of a hydrated salt.
Diatomic molecule: A molecule made of two atoms, as in the elements hydrogen, nitrogen, oxygen and the halogens.
Balanced equation: An equation with the same number of atoms of each element, and the same total charge, on each side.
Coefficient: The number written in front of a formula in an equation, giving how many units of that substance react or form.
State symbol: A symbol after a formula giving its physical state: (s) solid, (l) liquid, (g) gas or (aq) dissolved in water.
Aqueous: Dissolved in water, shown by the state symbol (aq) in an equation.
Precipitate: An insoluble solid formed when two solutions are mixed and react.
Spectator ion: An ion present in the reaction mixture that is unchanged by the reaction and appears identically on both sides of the equation.
Ionic equation: An equation showing only the ions and other species that take part in a reaction, with spectator ions left out.
Salt: A compound formed when the hydrogen ions of an acid are replaced by metal ions or ammonium ions.
Half-equation: An equation for the oxidation or the reduction half of a redox reaction on its own, with electrons shown to balance the charge.