Learners' AlleyDesigned by Khow Willard

Experimental
Chemistry

1

Measuring Physical Quantities

1.1Quantities, units and apparatus

Quantity	SI unit	Apparatus	Other units
time	second, s	stopwatch	minute (min), hour (h)
temperature	kelvin, K	thermometer	degree Celsius (°C)
length	metre, m	metre rule, measuring tape	cm, dm, mm
mass	kilogram, kg	electronic balance	gram (g), tonne (t)
volume	cubic metre, m³	burette, pipette, measuring cylinder, gas syringe	cm³, dm³

A digital stopwatch reads to ±0.01 s and an analogue stopwatch to ±0.1 s.
Laboratory thermometers are alcohol, mercury or digital. A temperature probe joined to a data logger collects readings automatically.
An electronic balance reads to about ±0.01 g.

1.2Converting units

Time1 min = 60 s 1 h = 60 min = 3600 s

Temperaturetemperature in K = temperature in °C + 273

Length1 m = 10 dm = 100 cm = 1000 mm

Mass1 kg = 1000 g 1 g = 1000 mg

Volume1 dm³ = 1000 cm³ = 1 litre

Volume1 m³ = 1000 dm³

Note1 m = 10 dm, but 1 m³ is not 10 dm³. A cube of side 10 dm has a volume of 10 × 10 × 10 = 1000 dm³.

Worked exampleConvert 250 cm³ to dm³.

1 dm³ = 1000 cm³250 ÷ 1000 = 0.250 dm³

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Measuring Physical Quantities (continued)

1.3Measuring volume

Apparatus	What it measures
pipette	an accurate fixed volume, e.g. 10.0 cm³ or 25.0 cm³
volumetric flask	an accurate fixed larger volume, e.g. 100 cm³ or 250 cm³
measuring cylinder	a range of volumes, to the nearest 0.5 cm³, e.g. 31.5 cm³
burette	a range of volumes, to the nearest 0.05 cm³, e.g. 23.00 cm³
gas syringe	the volume of a gas, up to 100 cm³

Place your eye level with the meniscus to avoid parallax error.
For water, read the bottom of the meniscus.

2

Collecting and Drying Gases

2.1Methods of collecting gases

The method depends on how soluble the gas is in water and how dense it is compared with air.

Water displacementinsoluble or slightly soluble gases

Upward deliverygases less dense than air

Downward deliverygases denser than air

Method	Solubility in water	Density	Examples
water displacement	insoluble or slightly soluble	any	hydrogen, oxygen, carbon dioxide
upward delivery	soluble or insoluble	less dense than air	ammonia
downward delivery	soluble or insoluble	denser than air	chlorine, hydrogen chloride, sulfur dioxide

NoteTo decide if a gas is denser than air, compare its M_r with the average M_r of air, about 29. A gas with M_r above 29 is denser than air (e.g. Cl₂, 71). A gas with M_r below 29 is less dense (e.g. NH₃, 17).

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Learners' AlleyDesigned by Khow Willard

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Collecting and Drying Gases (continued)

2.2Drying agents for gases

Drying agent	Use it for	Do not use it for
concentrated sulfuric acid	most gases, e.g. chlorine and hydrogen chloride	gases that react with it, e.g. ammonia
calcium oxide (quicklime)	ammonia	gases that react with it, e.g. carbon dioxide
fused calcium chloride	most gases	ammonia

NoteCalcium oxide and calcium chloride absorb water from the air, so they must be freshly heated before use.

3

Separation and Purification

3.1Choosing a method

A mixture has two or more substances that are not chemically combined. Each is separated using a difference in a physical property: particle size, solubility, melting or boiling point, or magnetism.

NoteKey terms. A solute dissolves in a solvent to make a solution. Miscible liquids mix completely, while immiscible liquids form separate layers. The filtrate, residue, distillate and sublimate are what each method collects.

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Separation and Purification (continued)

3.2Solid and solid mixtures

Magnetic attraction

A magnet removes magnetic solids (iron, cobalt, nickel), e.g. iron filings from sulfur.

Sieving

Separates solids with different particle sizes.

A suitable solvent

If only one solid dissolves, add the solvent and filter. For sand and salt, the salt solution is the filtrate and the sand is the residue.

Sublimation

A solid that changes straight to a gas, e.g. iodine or naphthalene, is warmed and collected as the sublimate on a cold surface.

3.3Solid and liquid mixtures

Filtration

Separates an insoluble solid from a liquid. The liquid that passes through is the filtrate. The solid left on the paper is the residue.

Evaporation to dryness

Separates a dissolved solid from its solvent by heating until all the solvent has boiled off.

Crystallisation

Used for a solid that decomposes on strong heating, or to obtain pure crystals of a salt.

Heat the solution until it is saturated. Stop before all the solvent evaporates.
Cool the solution so that crystals form.
Filter, wash the crystals with a little cold distilled water, and dry them between filter papers.

Why crystals form: hot water holds more solute than cold. 100 g of water holds 55 g of copper(II) sulfate at 80 °C but only 21 g at 20 °C, so on cooling the other 34 g comes out as crystals.

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Learners' AlleyDesigned by Khow Willard

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Separation and Purification (continued)

3.3Solid and liquid mixtures (continued)

Simple distillation

Separates a pure solvent from a solution, e.g. pure water from salt water. The solvent is collected as the distillate.
The thermometer bulb is level with the side arm, to measure the temperature of the vapour entering the condenser.
Cold water enters the condenser at the bottom, so the condenser stays full of water and cools the vapour efficiently.
Boiling chips give smooth boiling.

3.4Liquid and liquid mixtures

Separating funnel

Separates immiscible liquids. The less dense liquid forms the upper layer.

Run off the lower layer into a beaker.
Close the tap just as the boundary reaches it.
Run the upper layer into a second beaker.

Worked exampleA mixture contains W and X, two immiscible liquids with X denser, Y, a solid that dissolves in W but not in X, and Z, a solid that is insoluble in both liquids. Outline how to separate all four.

Remove the insoluble solid Z by filtration. Use a separating funnel to run off liquid X, the lower layer. Obtain solid Y from its solution in W by evaporation or crystallisation.

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Separation and Purification (continued)

3.4Liquid and liquid mixtures (continued)

Fractional distillation

Separates miscible liquids with different boiling points, e.g. ethanol (78 °C) and water (100 °C).
The glass beads give a large surface area for the vapour to condense and boil again.
The liquid with the lower boiling point distils first. When the temperature rises, change the receiver.

NoteFractional distillation separates crude oil into fractions and liquid air into nitrogen, oxygen and argon.

3.5Chromatography

Chromatography separates a mixture of substances that have different solubilities in a given solvent.

Draw the start line in pencil, as graphite does not dissolve in the solvent.
The solvent level must be below the start line, so the spots do not dissolve into the solvent.
The more soluble a substance, the further it travels. A spot that matches a known pure substance is probably that substance.
Colourless substances are shown with a locating agent or ultraviolet light.

R_f = distance moved by the substancedistance moved by the solvent front both from the start line

4

Purity

4.1Pure substances and mixtures

Pure substance

Melts and boils at one fixed temperature. Gives one spot on a chromatogram.

Mixture

Melts and boils over a range of temperatures. Impurities lower the melting point and raise the boiling point.

Purity matters in medicines and food: impurities in a medicine may cause harmful side effects.

Exam tipTo show a sample is pure, state that it has a sharp, fixed melting point that matches the known value.

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