Apparatus for experimental measurements

Choose measuring apparatus according to what quantity is being measured and, for volume, what material is being measured. Instruments for time, temperature and mass measure different quantities:

• A stop-watch measures elapsed time: the time between starting and stopping it.
• A thermometer measures temperature.
• A balance measures mass.

Measuring liquid volume

Different pieces of apparatus are useful depending on whether a liquid volume needs to be measured, delivered as a fixed amount or delivered in a variable amount.

• A measuring cylinder measures the volume of liquid it contains. Its scale allows different volumes to be measured.
• A volumetric pipette delivers one fixed volume of liquid. Choose a pipette with the required volume marked on it.
• A burette delivers measured, variable volumes of liquid. Its scale is read before and after delivery; the difference between the readings gives the volume delivered.

Measuring gas volume

A gas syringe measures the volume of a gas. As gas enters the syringe, it pushes the plunger out, and the gas volume can be read from the scale.

Evaluating experimental methods and apparatus

To evaluate an experimental choice, compare what it can do with what the experiment needs. A useful judgement identifies a strength or limitation, explains its effect on the result, and relates that effect to the task.

Evaluating apparatus

First consider whether the apparatus measures the required quantity. Then consider whether its capacity and scale are suitable: can it hold the required volume, and can its markings show the amount of detail needed?

For example, a fixed-volume pipette designed to deliver $$25\ \text{cm}^3$$ is useful when the same volume must be transferred repeatedly. Its fixed volume is a limitation if several different volumes are needed. A measuring cylinder can measure a range of volumes, but its scale may give less precise readings than the pipette.

A gas syringe can measure the volume of gas collected during a reaction. However, it must have enough capacity for the gas produced, and its scale must be fine enough to show the changes in volume the experiment needs to observe.

Evaluating method steps

A method must make it possible to obtain the intended result reliably: repeated trials should give similar results when the conditions are kept the same. Examine whether each step allows the required measurement to be made and whether any step could cause material to be lost or measurements to vary.

For example, if gas production begins before the collection apparatus is sealed, some gas may escape. The measured volume would then be less than the volume produced, and the amount lost could vary between trials. A method that seals the apparatus before the reaction starts avoids this limitation.

A choice is therefore not simply “good” or “bad”. Its value depends on whether its features meet the needs of the particular experiment.

Solutions and saturation

A solution is a mixture in which one or more substances are dissolved in another substance. The substance that dissolves is the solute; the substance that dissolves it is the solvent. For example, when sugar dissolves in water, sugar is the solute and water is the solvent.

Saturation of a solution

A solvent can dissolve only a certain maximum concentration of a particular solute at a specified temperature. A solution at that limit is saturated. If more of the same solute is added under those conditions, it will not dissolve and will remain separate from the solution.

Saturation does not simply mean that a solution contains a large amount of solute. It means that the solution has reached the maximum concentration for that solute in that solvent at that temperature. A saturated solution does not have to contain visible undissolved solute: it may contain exactly the maximum amount that can dissolve.

Residue and filtrate in separation

A separation process may leave some material behind while another part passes through or leaves. The material that remains is called the residue. What counts as residue depends on the process being used.

Residue and filtrate in filtration

In filtration, a mixture is poured through a filter. Liquid passes through the filter, while insoluble solid particles are held back.

• The residue is the material retained by the filter.
• The filtrate is the liquid or solution that passes through the filter.

For example, when a mixture of sand and water is filtered, the sand remains on the filter as the residue, and the water that passes through is the filtrate. If a substance is dissolved in the water, it passes through with the water as part of the filtrate.

Residue after evaporation or distillation

The term residue is not limited to filtration. When a solution is evaporated, the dissolved solid left behind is the residue. In distillation, material left in the vessel after some of the mixture has vaporised is also called residue.

Filtrate, however, specifically means material that has passed through a filter; it is not a general name for material separated by evaporation or distillation.