Experimental
Chemistry

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

Which apparatus should you use to measure exactly 25.0 cm³ of a solution?

2

How do you collect a gas that dissolves in water and is lighter than air?

3

How can you get pure water out of sea water, and salt out of it as well?

4

How do you separate oil from water, and ethanol from water?

5

How does a chromatogram show what is in a food colouring?

6

How do you know if a sample is pure?

Starter

PredictSea water is a mixture of water and salts. You are stranded on an island with a pot, a lid and a fire.

How could you get drinkable water from the sea water?

NoteChemists separate mixtures by using differences in physical properties, such as boiling point. This topic shows how.
1

Measuring Physical Quantities

1.1Quantities, units and apparatus

We use the International System of Units (SI units) as a common standard for recording measurements.

QuantitySI unitApparatusOther units
timesecond, sstopwatchminute (min), hour (h)
temperaturekelvin, Kthermometerdegree Celsius (°C)
lengthmetre, mmetre rule, measuring tapecm, dm, mm
masskilogram, kgelectronic balancegram (g), tonne (t)
volumecubic metre, m3burette, pipette, measuring cylinder, gas syringecm3, dm3
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Measuring Physical Quantities

1.1How accurate is it?

  • A digital stopwatch is accurate 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 is accurate to about ±0.01 g.
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Measuring Physical Quantities

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.
1

Measuring Physical Quantities

Worked exampleConvert 250 cm3 to dm3.

Step 11 dm3 = 1000 cm3

Step 2From the smaller unit (cm3) to the larger unit (dm3), divide: 2501000

Step 3250 cm3 = 0.250 dm3

1

Measuring Physical Quantities

1.3Measuring volume

Each piece of apparatus suits a different volume and accuracy.

1

Measuring Physical Quantities

1.3Choosing the apparatus

ApparatusWhat it measures
pipettean accurate fixed volume, e.g. 10.0 cm3 or 25.0 cm3
volumetric flaskan accurate fixed larger volume, e.g. 100 cm3 or 250 cm3
measuring cylindera range of volumes, to the nearest 0.5 cm3, e.g. 31.5 cm3
burettea range of volumes, to the nearest 0.05 cm3, e.g. 23.00 cm3
gas syringethe volume of a gas, up to 100 cm3
1

Measuring Physical Quantities

1.3Reading the scale

  • Place your eye level with the meniscus, the curve at the surface of the liquid, to avoid parallax error.
  • For water, read the bottom of the curve.
  • Choose apparatus by the volume to be measured and the accuracy needed.
AnalogyReading a scale from above or below is like reading a car's speedometer from the passenger seat: the needle seems to point at the wrong number.
1

Measuring Physical Quantities

Quick checkWhat is an advantage of using a burette instead of a measuring cylinder to measure a volume of liquid?

AIt measures to a smaller division, 0.05 cm³ instead of 0.5 cm³.
BIt measures a fixed volume.
CIt can measure the volume of a gas.
DIt measures larger volumes than a measuring cylinder.
NoteA: a burette measures a range of volumes more accurately. A pipette measures a fixed volume and a gas syringe measures gases.
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
2

Collecting and Drying Gases

2.1Choosing a method

MethodSolubility in waterDensityExamples
water displacementinsoluble or slightly solubleanyhydrogen, oxygen, carbon dioxide
upward deliverysoluble or insolubleless dense than airammonia
downward deliverysoluble or insolubledenser than airchlorine, hydrogen chloride, sulfur dioxide
NoteCompare the gas's Mr with air's average Mr of about 29. Above 29 it is denser than air (Cl2, 71). Below 29 it is less dense (NH3, 17).
2

Collecting and Drying Gases

Your turnGas X is less dense than air and is insoluble in water. Suggest a method that can be used to collect gas X.

Water displacement or upward delivery.[1]
NoteWater displacement works for any gas that is insoluble in water. Upward delivery works because the gas is lighter than air.
2

Collecting and Drying Gases

2.2Drying agents for gases

A wet gas is dried by passing it through a substance that absorbs water.

2

Collecting and Drying Gases

2.2Choosing a drying agent

Drying agentUse it forDo not use it for
concentrated sulfuric acidmost gases, e.g. chlorine and hydrogen chloridegases that react with it, e.g. ammonia
calcium oxide (quicklime)ammoniagases that react with it, e.g. carbon dioxide
fused calcium chloridemost gasesammonia
2

Collecting and Drying Gases

Your turnGas Y reacts with acids. Name one drying agent that can be used to dry gas Y.

Calcium oxide (quicklime).[1]
NoteA gas that reacts with acids cannot be dried with concentrated sulfuric acid, because it would react with the acid. Calcium oxide and calcium chloride absorb water from the air, so they must be freshly heated before use.
3

Separation and Purification

3.1Mixtures

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.

Solute and solventA solute dissolves in a solvent to make a solution. Its solubility is how much can dissolve.
MiscibleLiquids that mix completely. Immiscible liquids do not mix and form separate layers.
What is collectedThe filtrate, residue, distillate and sublimate are the substances each method collects.
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Separation and Purification

3.1Choosing a method

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Separation and Purification

3.2Solid and solid mixtures

Magnetic attractionA magnet removes magnetic solids (iron, cobalt, nickel), e.g. iron filings from sulfur powder.
SievingSolids with different particle sizes are separated by a sieve.
A suitable solventIf only one solid dissolves, add the solvent and filter. Sand and salt: the salt solution is the filtrate.
SublimationA solid that changes straight to a gas, e.g. iodine or naphthalene, forms the sublimate on a cold surface.
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Separation and Purification

3.3Filtration

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

3.3Evaporation to dryness

Separates a dissolved solid from its solvent. Heat until all the solvent has boiled off and only the solid is left.

3

Separation and Purification

3.3Crystallisation

Used when the solid decomposes on strong heating (e.g. sugar), or to obtain pure crystals of a salt such as copper(II) sulfate.

  1. Heat the solution gently until it is saturated, so no more solute can dissolve. Stop before all the solvent evaporates.
  2. Cool the solution slowly so that crystals form.
  3. Filter to collect the crystals, then wash them with a little cold distilled water.
  4. Dry the crystals between filter papers.
3

Separation and Purification

3.3Simple distillation

  • Separates a pure solvent from a solution, e.g. water from salt water. It 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 and leaves at the top, so the condenser stays full of water.
3

Separation and Purification

3.4Separating funnel

Separates immiscible liquids, which settle into layers. The less dense liquid floats on top of the denser one.

  1. Open the tap to run off the lower layer into a beaker.
  2. Close the tap just as the boundary reaches it.
  3. Run the upper layer into a second beaker.
3

Separation and Purification

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.

Step 1Z is the only insoluble solid, so remove it by filtration.

Step 2W and X form layers, so run off X, the lower layer, using a separating funnel.

Step 3Y is dissolved in W, so obtain it by evaporation or crystallisation.

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Separation and Purification

3.4Fractional 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 vapour to condense on and boil again.
  • The liquid with the lower boiling point distils first.
  • When the temperature rises above that boiling point, change the receiver.
NoteFractional distillation separates crude oil into fractions and liquid air into nitrogen, oxygen and argon.
3

Separation and Purification

Quick checkA mixture of two liquids that dissolve in each other, with boiling points of 78 °C and 100 °C, must be separated. Which method is used?

Afiltration
Bseparating funnel
Cfractional distillation
Dcrystallisation
NoteC: the liquids are miscible, so a separating funnel does not work, and their boiling points differ, so fractional distillation separates them.
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Separation and Purification

3.5Chromatography

Chromatography separates a mixture of substances that have different solubilities in a given solvent.
  • Draw the start line in pencil, which does not dissolve. Keep the solvent below the start line.
  • More soluble substances travel further up the paper.
Rf = distance moved by the substancedistance moved by the solvent front = xy
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Separation and Purification

3.5Using Rf values

  • Rf is constant for the same solvent and temperature.
  • A spot with the same Rf as a known pure substance is probably that substance.
  • Colourless substances are shown with a locating agent or ultraviolet light.
4

Purity

4.1Pure substances and mixtures

Pure substanceMelts and boils at one specific temperature under fixed conditions.
MixtureMelts or boils over a range of temperatures. Salt lowers the freezing point of water and raises its boiling point too.
Exam tipTo show a sample is pure, state that it has a sharp, fixed melting point that matches the known value.
4

Purity

Worked exampleThe melting points of water, tridecane and tetradecane are 0 °C, −5 °C and 5 °C. Solid X melts at 0 °C. Solid Y melts over −5 °C to 5 °C. Identify X and Y.

Step 1X melts at one temperature, so X is a pure substance, ice (solid water).

Step 2Y melts over a range, so Y is a mixture, likely tridecane and tetradecane.

Exit ticket

1Name the apparatus that measures 25.0 cm³ of liquid accurately.

A pipette.[1]

2Ammonia is soluble in water and less dense than air. How is it collected?

Upward delivery.[1]

3Which method separates pure water from salt water?

Simple distillation.[1]

4A solid melts from 120 °C to 128 °C. Is it pure?

No, it is a mixture because it melts over a range of temperatures.[1]