Purity, formulations and chromatography

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Pure Substances in Chemistry

In chemistry, a pure substance is a single element or compound that contains no other substances.
For example, pure water contains only H₂O molecules and nothing else.
In everyday language, 'pure' means a substance that is natural or clean and has had nothing added to it, such as pure milk.
A mixture consists of two or more elements or compounds that are physically mixed together, not chemically combined.
The chemical properties of substances in a mixture remain unchanged, and they can be separated by physical means.
Air is a mixture of nitrogen, oxygen, and small amounts of other gases such as carbon dioxide and argon.

Distinguishing Purity Using Melting and Boiling Points

Pure substances melt and boil at specific and sharp temperatures; for example, pure water boils at 100 °C and melts at 0 °C.
Impure substances have a range of melting and boiling points because they contain different substances.
Compared to pure substances, impurities lower the melting point and raise the boiling point.
Melting and boiling point data can be used to distinguish pure substances from mixtures.
Melting point analysis is routinely used to assess the purity of drugs using a melting point apparatus, and the result is compared to data tables.
The closer the measured melting or boiling point is to the actual value, the purer the sample.

Melting point apparatus

Cooling Curves and Purity

A cooling curve shows how the temperature of a liquid changes as it cools and freezes.
A pure substance produces a cooling curve with a horizontal section at the melting point, indicating a sharp melting point.
An impure substance produces a gradual decrease in temperature as it freezes, with no flat plateau.
The shape of the cooling curve can therefore be used to determine whether a substance is pure or impure.

Formulations

A formulation is a mixture that has been designed as a useful product.
Formulations are made by mixing components in carefully measured quantities to ensure the product has the required properties.
Each component in a formulation has a particular purpose.
Examples of formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers, and foods.
Paint is a formulation containing pigment (gives colour), binder (holds pigment), and solvent (dissolves components and alters viscosity).
Formulations are important in the pharmaceutical industry, where exact ratios ensure the drug is delivered safely and effectively.
Formulations are not compounds because the components are not chemically combined.

Chromatography: Separating Mixtures

Chromatography is used to separate substances and provide information to help identify them.
It involves a stationary phase (the chromatography paper) and a mobile phase (the solvent).
Separation depends on the distribution of substances between the two phases.
Substances with higher solubility in the solvent travel further up the paper because they spend more time in the mobile phase.
A pencil line is drawn on the chromatography paper because ink would run into the chromatogram.
The pencil line must be above the level of the solvent so the samples do not wash into the solvent container.
The solvent travels up the paper by capillary action, carrying the substances with it.

Setting up and running paper chromatography

Interpreting Chromatograms and Rf Values

A pure substance produces only one spot on a chromatogram.
An impure substance (mixture) produces more than one spot as the components separate.
If two or more substances are the same, they produce identical chromatograms.
The Rf value (retention factor) is calculated using: Rf = distance moved by substance ÷ distance moved by solvent.
Both distances are measured from the baseline (the pencil start line).
The Rf value is a ratio with no units and always lies between 0 and 1; the closer to 1, the more soluble the substance.
The Rf value of a compound is always the same under the same conditions but changes if the solvent is changed.
Rf values can be compared with reference values to identify unknown substances.

Example chromatogram of a mixture

Required Practical: Investigating Chromatography

The aim is to investigate how paper chromatography can separate and identify a mixture of food colourings.
A horizontal pencil line is drawn 2 cm from the end of the chromatography paper.
Small spots of known colourings (A–D) and an unknown mixture (U) are placed on the line using capillary tubes.
The paper is lowered into water so the pencil line is above the solvent level.
The solvent is allowed to travel at least three quarters of the way up the paper, then the paper is removed and the solvent front is marked.
The distance travelled by the solvent and by each spot is measured, and Rf values are calculated.
The Rf values of the unknown mixture are compared with those of known substances to identify its components.

Key Practical Points and Safety

The pencil line must never be below the level of the solvent, or the samples will be washed away.
The initial line must be drawn in pencil because ink would smudge or run in the solvent.
Solvents used in chromatography (e.g., ethanol, propanone) are often flammable and harmful to health.
Keep solvents away from naked flames and avoid contact with skin or breathing in vapours.
A fume cupboard can be used for harmful solvents.

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연습 문제

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1.In chemistry, which of the following is the definition of a pure substance?
Easy
- AA single element or compound that contains no other substances
- BA substance that has had nothing added to it
- CA useful product made by mixing substances
- DA substance made from only one type of atom
2.Which of the following are examples of pure substances in chemistry terms? (Select all that apply)
Medium
- APure water
- BPure orange juice
- CPure carbon dioxide
- DPure air
- EPure milk
3.Which statement describes the melting and boiling points of a pure substance?
Medium
- AIt has a sharp melting point and a sharp boiling point
- BIt has a range of melting and boiling points
- CIt has a lower melting point and a higher boiling point than an impure sample
- DIt has no fixed melting or boiling point
4.Which of the following are examples of formulations? (Select all that apply)
Medium
- AFertilisers
- BAspirin tablets
- CPure water
- DFuels
- EPure carbon dioxide
5.In paper chromatography, what is the stationary phase?
Medium
- AThe chromatography paper
- BThe solvent
- CThe dye mixture
- DThe solvent front
6.A student sets up paper chromatography but makes two mistakes. Which two of the following are mistakes? (Select all that apply)
Medium
- AThe solvent used is water but the dyes are soluble in propanone, not water
- BThe start line is drawn in ink
- CA lid is placed on the beaker
- DThe dyes are all on the same piece of chromatography paper
- EThe water is not level with the dyes
7.What would the Rf value of the blue dye be if the blue dye is not soluble in the solvent?
Medium
- A0
- B1.0
- CGreater than the Rf value of the yellow dye
- DThere is not enough information to determine the Rf value
8.Green ink is made up of 25% blue ink and 75% green ink. Which word correctly describes the green ink?
Medium
- AFormulation
- BElement
- CCompound
- DMobile phase

더 많은 문제

9.The student repeated the investigation using ethanol as the solvent. The blue dye is not soluble in ethanol. The yellow dye is more soluble in ethanol than in water. Which two statements about the resulting chromatogram are correct? (Select two)
Medium
- AIt would have one spot above the start line and one spot on the start line
- BThe Rf value of the yellow dye would increase
- CIt would have two spots above the start line
- DThe Rf value of the blue dye would be 1.0
- EThe yellow spot wouldn't travel as far up the paper
10.A group of students investigated the purity of water samples. Their boiling points are: A 103°C, B 105°C, C 101°C, D 102–106°C. Which sample is the purest?
Medium
- ASample C
- BSample A
- CSample B
- DSample D
11.A student carried out chromatography on green ink. The solvent front moved 5.8 cm and the yellow dye moved 4.5 cm. Calculate the Rf value of the yellow dye to 2 significant figures.
Medium
- A0.78
- B0.80
- C1.3
- D26
12.A chromatogram was produced from samples A–E and black food colouring. Which of the following conclusions is not true for this chromatogram?
Medium
- ASample B is insoluble in the solvent used
- BSamples C and D are pure substances
- CSample D is the least soluble of the samples
- DThe black ink is a mixture of A, E and an unknown
13.What is a formulation?
Easy
- AA mixture that has been designed as a useful product
- BA pure substance that contains no other substances
- CA compound formed by chemical reaction
- DA single element
- EA mixture that is impure by accident
14.Which of the following statements about paper chromatography are correct? (Select all that apply)
Medium
- AThe stationary phase is the chromatography paper
- BThe mobile phase is the solvent
- CSubstances with higher solubility travel further
- DThe Rf value has units of cm
- EThe start line should be drawn in ink
15.A pure substance will produce only one spot on a chromatogram.
Easy
True or false?
16.Impurities lower the melting point and raise the boiling point of a substance.
Easy
True or false?
17.The Rf value of a substance is always the same regardless of the solvent used.
Easy
True or false?
18.In paper chromatography, the start line should be drawn in ink.
Easy
True or false?
19.A formulation is a mixture that has been designed as a useful product.
Easy
True or false?
20.What is the mobile phase in paper chromatography?
Medium
- AThe solvent
- BThe chromatography paper
- CThe dye mixture
- DThe solvent front
21.Which of the following best describes a mixture?
Medium
- ATwo or more elements or compounds physically mixed together, not chemically combined
- BA single element or compound
- CA compound formed by chemical reaction
- DA pure substance with a sharp melting point
22.Which of the following is not a formulation?
Medium
- APure water
- BPaint
- CFertiliser
- DMedicine
23.Which of the following are correct about formulations? (Select all that apply)
Medium
- AThey are mixtures designed as useful products
- BThey are made by mixing components in carefully measured quantities
- CThey are compounds because the components are chemically combined
- DThey include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods
- EThey are always pure substances
24.Match each term with its correct description.
Medium
- Pure substance
- Mixture
- Formulation
- A single element or compound containing no other substances
- Two or more substances physically mixed, not chemically combined
- A mixture designed as a useful product
25.Match each chromatography term with its correct definition.
Medium
- Mobile phase
- Stationary phase
- Rf value
- The solvent that moves through the paper
- The chromatography paper
- Distance moved by substance divided by distance moved by solvent
26.Place the steps of the paper chromatography practical in the correct order.
Medium
- Draw a pencil line 2 cm from the bottom of the chromatography paper
- Spot the colourings onto the pencil line
- Lower the paper into the solvent so the pencil line is above the solvent level
- Allow the solvent to travel up the paper
- Mark the solvent front and measure distances
27.Which of the following statements about a mixture are correct? (Select all that apply)
Medium
- AThe components are physically mixed together, not chemically combined
- BThe chemical properties of the components stay the same
- CThe components can be separated by physical means
- DThe components always melt at one sharp temperature
- EThe components are joined by chemical bonds
28.Match each separation technique to the mixture it is used to separate.
Medium
- Simple distillation
- Fractional distillation
- Filtration
- Crystallisation
- A liquid and a dissolved solid from a solution
- Two or more miscible liquids
- An undissolved solid from a liquid
- A dissolved solid from a solution
29.Place the main stages used to make fresh water potable into the correct order.
Medium
- Screening and sedimentation
- Filtration through sand and gravel
- Chlorination
30.A student uses paper chromatography to separate a mixture of dyes. Which of the following statements about the process are correct? (Select all that apply)
Medium
- AThe solvent moves up the paper by capillary action
- BThe more soluble a dye is, the further it travels up the paper
- CThe starting line should be drawn in pencil
- DThe starting line should be below the level of the solvent
- EThe dye that is most soluble in the solvent stays closest to the start line
31.A student runs a chromatogram and measures the distance from the baseline to the centre of a spot as 3.0 cm and the distance from the baseline to the solvent front as 6.0 cm. Which of the following is the Rf value of the spot?
Medium
- A0.50
- B2.00
- C0.30
- D3.00
32.Which of the following are formulations? (Select all that apply)
Medium
- APaint
- BFertiliser
- CWashing-up liquid
- DPure carbon dioxide
- EPure water
33.In a chromatogram, sample P produces a single spot with the same Rf value as a spot from an unknown mixture U. Which conclusion is correct?
Medium
- AP is a pure substance that is one component of the mixture U
- BP is a mixture of two or more substances
- CP has the same Rf value as U because it is insoluble in the solvent
- DP must be the solvent used in the chromatogram
34.Deionised water is used in chemical analysis because dissolved ions could react with the substances being tested and give a false result.
Easy
True or false?
35.A student heats a sample of an unknown solid. It begins to melt at 78°C and finishes melting at 85°C. What does this tell you about the sample?
Medium
- AIt is pure because it melts over a range of temperatures
- BIt is impure because it melts over a range of temperatures
- CIt is pure because it has a sharp melting point
- DIt is impure because it melts at a lower temperature than expected
36.A sample of impure water is found to have a boiling point of 102°C. Which statement explains this?
Medium
- AImpurities lower the boiling point
- BImpurities raise the boiling point
- CImpurities have no effect on boiling point
- DImpurities make the boiling point sharper
37.A chromatogram of an unknown mixture U shows spots with Rf values 0.25 and 0.60. Reference substances have Rf values: A = 0.25, B = 0.45, C = 0.60. Which substances are present in U?
Hard
- AA and B
- BB and C
- CA and C
- DA, B and C
38.Match each separation technique with the mixture it is used to separate.
Medium
- Simple distillation
- Fractional distillation
- Filtration
- Crystallisation
- A liquid and a soluble solid from a solution
- Two or more miscible liquids
- An undissolved solid from a liquid
- A dissolved solid from a solution
39.In paper chromatography, why is the pencil line drawn on the chromatography paper rather than using ink?
Medium
- AInk would run into the chromatogram along with the samples
- BInk is insoluble in the solvent
- CInk would fix the samples to the paper
- DPencil is easier to see than ink
40.A student sets up paper chromatography but forgets to add a lid to the beaker. What is the most likely effect on the chromatogram?
Medium
- AThe solvent will evaporate, so the solvent front may not travel far enough
- BThe dyes will not dissolve in the solvent
- CThe dyes will move faster up the paper
- DThe pencil line will dissolve
41.Complete the sentence about the everyday meaning of a pure substance.
Easy
In everyday language, a pure substance means a substance that has had ____ added to it.
42.What name is given to a product such as methylated spirit, which is a mixture designed as a useful product?
Medium
43.Complete the sentence about the phases in paper chromatography.
Easy
In paper chromatography, the ____ phase is the solvent in which the sample molecules can move.
44.In a chromatogram, a colour from dye A moves 3.6 cm and the solvent moves 8.0 cm. Calculate the Rf value for the colour from dye A. Give your answer to 2 significant figures.
Medium
45.A yellow dye moves 4.8 cm and the solvent moves 6.0 cm. Calculate the Rf value of the yellow dye. Give your answer to 2 significant figures.
Medium
46.A sample of methylated spirit contains 260.5 g ethanol, 25.3 g methanol, 2.5 g pyridine and 1.0 g methyl violet. Calculate the percentage by mass of ethanol in the sample. Give your answer to 2 significant figures.
Medium
47.Complete the sentence about Rf values.
Medium
The Rf value is calculated by dividing the distance moved by the ____ by the distance moved by the ____.
48.What is the name given to the visual output of a chromatography run, which is the piece of chromatography paper with visibly separated components?
Medium
49.What term describes a mixture that has been designed as a useful product, such as paint or medicine?
Medium
50.Name the phase in paper chromatography that is the solvent in which the sample molecules move.
Easy
51.A dye in a chromatogram travels 4.5 cm from the baseline while the solvent front travels 7.5 cm from the baseline. Calculate the Rf value of the dye. Give your answer to 2 significant figures.
Medium
52.Complete the sentences about using melting points to judge purity. Use the words: sharp, lower, higher.
Medium
A pure substance melts at a ____ temperature, while an impure substance melts over a range and at a ____ temperature than the pure substance.
53.Aspirin tablets contain aspirin as the active ingredient and other inactive ingredients. Suggest the reason for adding a flavouring to the tablet.
Medium
54.Why could the green ink not be described as a pure substance?
Medium
55.The sample of methylated spirit was analysed and found to contain small amounts of other substances not stated in the formulation. Suggest how this may affect the performance of the formulation.
Hard
56.Give two examples of formulations used in different industries.
Medium
57.Explain what the term ‘pure’ means in chemistry and compare it to the everyday meaning.
Hard
58.In a chromatogram, a colour from dye A moves 3.6 cm and the solvent moves 8.0 cm. Calculate the Rf value for the colour from dye A. Give your answer to 2 significant figures.
Medium
- A2.45
- B0.45
- C1.45
- D-0.55
59.A yellow dye moves 4.8 cm and the solvent moves 6.0 cm. Calculate the Rf value of the yellow dye. Give your answer to 2 significant figures.
Medium
- A-0.2
- B1.8
- C2.8
- D0.8
60.A sample of methylated spirit contains 260.5 g ethanol, 25.3 g methanol, 2.5 g pyridine and 1.0 g methyl violet. Calculate the percentage by mass of ethanol in the sample. Give your answer to 2 significant figures.
Medium
- A99 %
- B81 %
- C90 %
- D108 %
61.A dye in a chromatogram travels 4.5 cm from the baseline while the solvent front travels 7.5 cm from the baseline. Calculate the Rf value of the dye. Give your answer to 2 significant figures.
Medium
- A1.6
- B0.6
- C-0.4
- D2.6

플래시카드

What is the definition of a pure substance in chemistry?

A pure substance is a single element or compound that contains no other substances.

How does the everyday meaning of 'pure' differ from the chemical meaning?

In everyday language, pure means a substance that has had nothing added to it and is in its natural state, such as pure milk. In chemistry, it means a single element or compound with no other substances mixed in.

What are the melting and boiling points of a pure substance like?

A pure substance melts and boils at specific, sharp temperatures.

How do impurities affect the melting and boiling points of a substance?

Impurities lower the melting point and raise the boiling point, and cause the substance to melt or boil over a range of temperatures.

How can melting point data be used to distinguish a pure substance from an impure one?

A pure substance melts sharply at a specific temperature. An impure substance melts over a range of temperatures, and its melting point is lower than the expected value for the pure substance.

What is a formulation?

A formulation is a mixture that has been designed as a useful product, made by mixing components in carefully measured quantities so that it has the required properties.

Give three examples of formulations.

Examples include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Why is each component added to a formulation?

Each component has a particular purpose, and the exact quantities are measured to ensure the product has the required properties.

In paper chromatography, what is the mobile phase?

The mobile phase is the solvent in which the sample molecules can move, such as water or ethanol.

In paper chromatography, what is the stationary phase?

The stationary phase is the chromatography paper itself.

Why is the baseline drawn in pencil rather than ink?

Pencil is used because ink would run or smudge into the chromatogram along with the samples.

Why must the baseline be above the level of the solvent?

If the baseline is below the solvent level, the samples will wash into the solvent container and the chromatogram will be ruined.

How does paper chromatography separate the components of a mixture?

The solvent travels up the paper by capillary action. Components with different solubilities travel at different rates, so they spread apart.

What does a chromatogram with more than one spot tell you about the substance?

It tells you that the substance is a mixture (impure), because the components have separated into different spots.

What does a chromatogram with only one spot tell you about the substance?

It suggests the substance is pure, as it contains only one component.

What is the equation for Rf value?

Rf = distance moved by substance ÷ distance moved by solvent. Both distances are measured from the baseline.

What is the range of possible Rf values?

Rf values always lie between 0 and 1, and have no units because it is a ratio.

Why can Rf values be used to identify unknown substances?

The Rf value of a compound is always the same under the same conditions, so it can be compared with reference values of known substances.

How does changing the solvent affect the Rf value of a substance?

The Rf value depends on the solvent used, so if the solvent is changed, the Rf value changes.

In the required practical, what is the purpose of drawing a solvent front line?

The solvent front line marks how far the solvent has travelled, so you can measure the distance moved by the solvent to calculate Rf values.

What safety precautions should be taken when using solvents like ethanol or propanone in chromatography?

Keep them away from naked flames, avoid contact with skin and breathing in vapour, and use a fume cupboard for harmful solvents.

What is the difference between 'paper chromatography' and a 'chromatogram'?

Paper chromatography is the separation technique; a chromatogram is the visual output—the paper with the separated spots after the run.

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