Elements, Compounds & Mixtures (Edexcel IGCSE Science (Double Award): Chemistry): Flashcards
Exam code: 4SD0
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What is an element?
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What is an element?
An element is a substance made of atoms that all have the same number of protons and cannot be broken down into anything simpler. There are 118 elements in the Periodic Table.
True or False?
A compound can be separated into its elements by physical methods.
False.
A compound cannot be separated by physical means. Its elements are chemically combined and can only be separated by chemical reactions.
What is a compound?
A compound is a pure substance made up of two or more different elements that are chemically combined. Compounds cannot be separated into their elements by physical means.
A mixture contains two or more substances that are .......... combined, and can be separated by .......... methods.
A mixture contains two or more substances that are not chemically combined, and can be separated by physical methods.
How does a compound differ from a mixture?
A compound has its elements chemically combined in fixed proportions and cannot be separated by physical methods. A mixture has substances that are not chemically combined and can be separated physically.
True or False?
CuSO4 is an example of a compound.
True.
Copper(II) sulfate (CuSO4) is a compound because it contains two or more different elements, copper, sulfur and oxygen, that are chemically combined.
Why is a mixture of sand and water classified as a mixture rather than a compound?
Sand and water form a mixture because the substances are not chemically combined. They retain their individual properties and can be separated by physical methods such as filtration.
All substances can be classified as an .........., a .......... or a mixture.
All substances can be classified as an element, a compound or a mixture.
What is a pure substance in chemistry?
A pure substance is a single element or compound that contains no other substances. It has a fixed, sharp melting point and boiling point.
True or False?
Drinking water is classified as a pure substance because it contains H2O molecules.
False.
Drinking water is a mixture because it contains H2O molecules along with dissolved ions and other substances such as chlorine. A pure substance contains only one element or compound.
How can melting point data be used to distinguish a pure substance from a mixture?
A pure substance melts at a single, sharp temperature. A mixture melts over a range of temperatures because it contains different substances with different melting points.
Impure substances generally have .......... melting points and .......... boiling points compared to the pure substance.
Impure substances generally have lower melting points and higher boiling points compared to the pure substance.
True or False?
A pure substance always melts at a single, fixed temperature.
True.
Pure substances have a specific, sharp melting point. Melting over a range of temperatures indicates the presence of impurities.
Why is melting point analysis used to assess the purity of drugs?
Melting point analysis is used because pure substances melt at a precise, known temperature. The closer the measured melting point is to the published value, the purer the sample. A range of melting temperatures indicates impurities are present.
In chemistry, a pure substance consists of a single .......... or compound that contains .......... other substances.
In chemistry, a pure substance consists of a single element or compound that contains no other substances.
A sample of sulfur begins melting at 100 °C and finishes melting at 113 °C. Pure sulfur melts at 115 °C. Explain whether the sample is pure or impure.
The sample is impure because it melts over a range of temperatures (100-113 °C) rather than at a single sharp point. An impure substance has a lower melting point than the pure substance and melts across a range.
What is simple distillation?
Simple distillation is a technique used to separate a liquid from a dissolved solid, or a pure liquid from a mixture of liquids with sufficiently different boiling points. The solution is heated so the liquid evaporates, then the vapour condenses in a condenser and is collected.
How does fractional distillation separate a mixture of ethanol and water?
Fractional distillation separates ethanol and water by exploiting their different boiling points. The mixture is heated to 78 °C, the boiling point of ethanol, which evaporates first, condenses in the condenser and is collected. Heating is stopped before the temperature reaches the boiling point of water (100 °C).
True or False?
Filtration is used to separate a dissolved solid from a solution.
False.
Filtration separates an undissolved (insoluble) solid from a liquid. To separate a dissolved solid from a solution, crystallisation or simple distillation would be used.
In filtration, the liquid that passes through the filter paper is called the .........., while the solid that remains behind is called the ...........
In filtration, the liquid that passes through the filter paper is called the filtrate, while the solid that remains behind is called the residue.
What is crystallisation?
Crystallisation is a separation technique used to obtain a dissolved solid from a solution. The solution is heated to evaporate the solvent, leaving a saturated solution which is then cooled slowly so crystals form.
Why is a pencil line used instead of an ink line in paper chromatography?
A pencil line is used because pencil marks are insoluble in the solvent. Ink is a mixture of dyes that would dissolve and travel up the paper with the solvent, interfering with the chromatogram.
True or False?
In paper chromatography, the pencil baseline must be positioned above the solvent level.
True.
The pencil baseline must sit above the solvent level. If the baseline is submerged, the samples will dissolve directly into the solvent and will not travel up the paper to separate.
In paper chromatography, the solvent travels up the paper by .........., carrying substances with different .......... at different rates.
In paper chromatography, the solvent travels up the paper by capillary action, carrying substances with different solubilities at different rates.
How does crystallisation produce dry crystals of a soluble solid?
The solution is heated to evaporate the solvent until a saturated solution forms. The saturated solution is then cooled slowly so crystals grow. The crystals are collected by filtration, washed with cold distilled water and allowed to dry.
Why is fractional distillation used rather than simple distillation to separate two miscible liquids?
Fractional distillation is used because simple distillation cannot cleanly separate two miscible liquids with close boiling points. Fractional distillation exploits the difference in boiling points, collecting each liquid as it evaporates at its own boiling point.
What is a chromatogram?
A chromatogram is the visual output of a chromatography run: the piece of chromatography paper showing the separated components as distinct spots after the solvent has travelled up the paper.
How does a chromatogram show whether a substance is pure or a mixture?
A pure substance produces only one spot on the chromatogram. A mixture produces more than one spot because its different components travel different distances, separating on the paper.
True or False?
An Rf value can be greater than 1.
False.
The Rf value is a ratio of the distance moved by the substance to the distance moved by the solvent. Because the substance can never travel further than the solvent front, the Rf value is always less than 1.
The Rf value is calculated by dividing the distance moved by the .......... by the distance moved by the ...........
The Rf value is calculated by dividing the distance moved by the substance by the distance moved by the solvent.
Why is a reference spot included in a chromatography experiment?
A reference spot of a known substance is included so that unknown spots can be compared to it. If an unknown spot travels the same distance as the reference spot, the two substances are likely to be the same compound.
True or False?
The Rf value of a compound is the same regardless of which solvent is used.
False.
The Rf value depends on the solvent used. If the solvent is changed, the Rf value changes. Rf values are only comparable when measured under the same conditions.
A substance travels 3 cm and the solvent travels 6 cm. What is the Rf value of the substance?
The Rf value = distance moved by substance ÷ distance moved by solvent = 3 ÷ 6 = 0.5. Rf values have no units and are always less than 1.
In chromatography, both the distance moved by the substance and the solvent are measured from the .......... to the .......... of the spot.
In chromatography, both the distance moved by the substance and the solvent are measured from the baseline to the centre of the spot.
How can a chromatogram be used to identify which components are present in an unknown mixture?
The unknown mixture is run alongside known reference substances on the same chromatogram. Any spot in the unknown mixture that is at the same height as a reference spot (same Rf value) indicates that component is present in the mixture.
Why must the baseline be drawn in pencil and not ink in a paper chromatography practical?
The baseline must be drawn in pencil because pencil marks do not dissolve in the solvent. Ink is a mixture of dyes that would travel up the paper with the solvent, interfering with the chromatogram and making results unreliable.
True or False?
In a paper chromatography practical, the solvent level in the beaker must be kept below the pencil baseline.
True.
The solvent must be kept below the pencil baseline. If the solvent covers the baseline, the samples dissolve directly into the solvent and are washed away rather than travelling up the paper to separate.
In paper chromatography, the distance moved by the solvent is measured from the .......... line to the ...........
In paper chromatography, the distance moved by the solvent is measured from the baseline line to the solvent front.
What is the solvent front in paper chromatography?
A solvent front is the furthest point reached by the solvent on the chromatography paper. It is marked with a pencil line as soon as the paper is removed from the beaker, and is used to calculate Rf values.
Why should sample spots be kept as small as possible in paper chromatography?
Large sample spots cause separated spots to merge or spread, making it difficult to measure the distance moved accurately. Keeping spots small and concentrated produces a clearer chromatogram and more accurate Rf values.
True or False?
Covering the beaker during chromatography helps prevent the solvent from evaporating.
True.
Covering the beaker creates a saturated atmosphere of solvent vapour. This prevents the solvent from evaporating off the paper surface, which would slow separation and affect the final position of the spots.
Why must the chromatogram be removed from the beaker before the solvent front reaches the top of the paper?
If the solvent runs off the top of the paper, the distance moved by the solvent front cannot be measured. Without this value, the Rf values for the separated components cannot be calculated.
In a paper chromatography practical, a different .......... tube is used for each food colouring to avoid .......... the samples.
In a paper chromatography practical, a different capillary tube is used for each food colouring to avoid cross-contaminating the samples.
How are the Rf values of known food colourings used to identify the components of an unknown mixture in this practical?
The Rf values of the known colourings A, B, C and D are calculated and compared to the Rf values of the spots produced by the unknown mixture U. A matching Rf value indicates that the unknown mixture contains that colouring.
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