Purity, Formulations & Chromatography (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards
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- FrontPure Substances
Define pure substance in chemistry.
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Define pure substance in chemistry.
A pure substance is a single element or compound with nothing else added.
Stating "only one type of substance" does not gain credit.
How does the melting point of an impure substance compare with the pure substance?
An impure substance melts over a range of temperatures and its melting point is lower than the pure substance.
A pure substance melts at one specific temperature.
A student tests a sample of sulfur (pure mp = 114 °C) and finds it melts from 100–113 °C.
It is ________ because it melts over a ________ and its melting point is ________ than the known value.
A student tests a sample of sulfur (pure mp = 114 °C) and finds it melts from 100–113 °C.
The sample is impure because it melts over a range and its melting point is lower than the known value.
True or False?
A substance is pure if it has the highest melting point in a data table.
False.
A pure substance is identified by having a specific melting point (a single value, not a range), not by having the highest value.
Choosing the highest or lowest value is a common exam error.
Define formulation.
A formulation is a mixture designed to produce a useful product with specific required properties.
Stating "a mixture" or "chemicals mixed together" without giving the purpose does not gain any marks.
A student is asked to name the type of mixture used in medicines, fuels, and alloys that is carefully designed for a specific purpose.
What is the correct term?
The correct term is formulation.
"Formula" is the most common wrong answer.
A formulation is a mixture designed as a useful product; the word must be recalled, not just described.
Define formulation.
A formulation is a mixture designed as a useful product. Each component is present in carefully measured quantities to give the product specific required properties.
Stating "a mixture" or "chemicals mixed together" alone does not gain credit.
Why is a formulation NOT a compound?
In a formulation, the components are not chemically combined; they are physically mixed.
A compound involves chemical bonding between atoms; a formulation does not.
Paint is a formulation. The ________ gives the paint its colour; the ________ forms a film to hold the pigment in place; the ________ dissolves the other components and alters viscosity.
Paint is a formulation. The pigment gives the paint its colour; the binder forms a film to hold the pigment in place; the solvent dissolves the other components and alters viscosity.
True or False?
A formulation can be defined as "a mixture of chemicals with useful properties."
False.
A formulation must be described as a mixture designed as a useful product or for a specific purpose.
Simply calling it "a mixture" or adding "useful" without stating it is designed for a purpose does not gain credit.
Give four examples of formulations.
Examples of formulations include:
Medicines (pharmaceuticals)
Fuels
Cleaning agents
Fertilisers
Alloys
Foods
Paints.
Each is a carefully designed mixture serving a specific purpose.
In the pharmaceutical industry, formulations are designed to deliver a drug to the correct part of the ________, at the right ________, to be safe to ________, and to have an adequate shelf life.
In the pharmaceutical industry, formulations are designed to deliver a drug to the correct part of the body, at the right concentration, to be safe to consume, and to have an adequate shelf life.
What is paper chromatography used for?
Paper chromatography is used to separate mixtures and identify their components. The mobile phase is the solvent; the stationary phase is the chromatography paper. Substances with greater solubility travel further up the paper.
Why must the start line be drawn in pencil, not pen?
Pencil is used so that the start line does not dissolve in the solvent.
Ink would mix with the solvent and travel up the paper along with the sample, interfering with the results.
The Rf value formula is:
Rf =
The Rf value always has a value ________ than 1.
The Rf value formula is:
Rf =
The Rf value always has a value less than 1.
True or False?
Substances separate in paper chromatography because they have different densities.
False.
Substances separate because they have different solubilities in the solvent and different attractions for the stationary phase (paper).
Density, melting point, and formula mass are all incorrect explanations.
How many spots does a pure substance produce on a chromatogram? What about an impure substance?
A pure substance produces exactly one spot.
An impure substance (mixture) produces more than one spot because its different components have different solubilities and travel different distances.
A student calculates Rf values from a chromatogram of an unknown ink.
What final step is needed to actually identify the colours?
The student must compare the calculated Rf values with known reference Rf values of identified substances under the same conditions.
Calculating Rf values alone is not enough; the comparison step is required for identification.
In a chromatography experiment, the pencil line is drawn below the solvent level. What problem does this cause?
The samples will dissolve directly into the solvent and be washed away rather than travelling up the paper by capillary action.
The pencil line must always be above the solvent level.
In an experiment, a blue dye moves 4.8 cm and the solvent moves 6.0 cm.
Rf = ____ ÷ ____ = ____
In an experiment, a blue dye moves 4.8 cm and the solvent moves 6.0 cm.
Rf = 4.8 ÷ 6.0 = 0.80
True or False?
If a colour moves 3 cm and the solvent moves 6 cm, then Rf = 6 ÷ 3 = 2.0
False.
Rf = distance moved by substance ÷ distance moved by solvent = 3 ÷ 6 = 0.5.
The Rf value must always be less than 1.
Dividing upside-down is the most common error.
How do you locate the solvent front on a chromatogram?
The solvent front is the maximum height the solvent reached on the paper.
It is marked as a pencil line on the dry part of the paper as close to the wet edge as possible, immediately after removing the paper.
It is not the position of the highest colour spot.
Why must a separate capillary tube be used for each food colouring in the chromatography practical?
Using a separate capillary tube for each substance prevents cross-contamination.
If one tube is used for multiple samples, the colours would mix, making it impossible to identify the components of each sample.
A student calculates Rf values for an unknown mixture. What must they do to identify the components?
They must compare the Rf values with known reference Rf values of identified substances, measured under the same conditions (same solvent). Without this comparison step, the unknown components cannot be identified.
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