Experimental design

Choosing the apparatus

The distinction that costs marks is between the measuring cylinder, the pipette and the burette. All three measure liquid volume and they are not interchangeable.

• A measuring cylinder is quick and cheap, but a 50 cm³ cylinder can be read only to about the nearest 0.5 cm³. Use it whenever "about 20 cm³" will do.
• A volumetric pipette delivers one fixed volume, usually 25.0 cm³, very accurately. It cannot deliver anything else.
• A burette delivers any volume you choose and is read to two decimal places, so 22.40 cm³ is a proper burette reading and 22.4 cm³ is not.

A stopwatch reads to 0.01 s, but human reaction time is roughly 0.2 s, so quoting a rate experiment result to hundredths of a second claims a precision the method does not have.

Advantages and disadvantages

Questions ask you to compare two methods, so learn one real drawback for each.

Collecting a gas over water is cheap and lets you read the volume directly, but any soluble gas dissolves, so carbon dioxide, sulfur dioxide and ammonia all give readings that are too low. A gas syringe avoids that entirely and measures the volume continuously, but its plunger can stick, it leaks if the bung is loose, and it holds only about 100 cm³.

Measuring mass loss on a balance is a good way to follow a reaction that produces carbon dioxide, since the flask gets lighter as the gas escapes. It is useless for hydrogen, because hydrogen is so light that the mass change is too small for the balance to detect.

A thermometer with a 1 °C scale is fine for a temperature rise of 20 °C but hopeless for one of 0.5 °C.

Solutions

• A solute is the substance that dissolves.
• A solvent is the liquid that dissolves it.
• A solution is the mixture formed when a solute dissolves in a solvent.
• A saturated solution contains as much dissolved solute as it can hold at that temperature, with undissolved solid remaining in contact with it.

The temperature clause matters. Copper(II) sulfate dissolves to about 20 g per 100 g of water at 20 °C but to over 60 g per 100 g at 80 °C, so a solution that is saturated on the bench is no longer saturated once it is warmed. That is the whole basis of crystallisation in 12.4.

Water is the commonest solvent, but it is not the only one. Ethanol and propanone dissolve substances water will not, such as oils and many dyes, which is why they are used in chromatography and in nail varnish remover.

Purity from fixed points

A pure substance melts and boils at a sharp, fixed temperature. Pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure.

An impurity does three things:

• It lowers the melting point.
• It raises the boiling point.
• It makes the change happen over a range of temperature rather than at one value.

Worked example. A white solid is thought to be benzoic acid, whose melting point is listed as 122 °C. The sample begins to melt at 112 °C and is fully molten at 118 °C.

The range is 118 - 112 = 6 °C, and the whole of it lies below the book value.

Both observations point the same way: a melting range rather than a sharp point means impurity, and a depressed melting point confirms it. The conclusion is impure benzoic acid, not a different compound.

Use the same two ideas in the other order to identify a substance: measure the melting point of a purified sample, then compare it with data-book values. A sharp melting point at 122 °C identifies benzoic acid; a sharp melting point at 80 °C does not.

Why purity matters

• Medicines. An impurity may be toxic, or may simply mean the tablet contains less of the active drug than the label claims, so the dose is wrong. A drug given by injection goes straight into the bloodstream, with nothing to filter it.
• Foodstuffs. Impurities can cause illness, and food additives are used in very small quantities, so a contaminated batch of additive affects an enormous amount of food.
• Industrial chemicals. An impurity can poison a catalyst. Sulfur compounds in a feedstock destroy the catalyst in the Haber process, so the gases are purified before they reach it.

Purity is also the reason practical chemistry uses distilled water: tap water carries chloride, calcium and magnesium ions that take part in tests you did not intend, as set out in 10.1.

Common mistakes

• Naming a measuring cylinder where a burette or a pipette is required for an accurate volume.
• Recording a burette reading to one decimal place.
• Saying a gas syringe is more accurate because it is bigger; the real advantage is that no gas dissolves and none escapes.
• Following a reaction by mass loss when the gas produced is hydrogen.
• Saying an impurity raises the melting point. It lowers it.
• Defining a saturated solution without saying "at that temperature".
• Saying a substance is impure because it melts at 122 °C when the book says 122 °C. A sharp value at the book figure is evidence of purity.

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