An element is a substance made of atoms that all contain the same number of protons , and cannot be broken down into anything simpler. There are 118 elements in the Periodic Table — for example, copper, iron and magnesium.

How does a compound differ from a mixture?

A compound differs from a mixture in that its elements are chemically combined , not just mixed together. This means a compound cannot be separated into its elements by physical methods , whereas a mixture can.

A compound is a pure substance made up of two or more elements that are chemically combined . There are an unlimited number of compounds. Examples include CO 2 and CaCO 3 .

True or False? A compound can be separated into its elements by filtration.

False. A compound cannot be separated into its elements by physical methods such as filtration. Only a mixture can be separated this way.

A mixture is a combination of two or more substances (elements and/or compounds) that are not chemically combined . Mixtures can be separated by physical methods such as filtration or evaporation. Examples include sand and water, and oil and water.

A compound is formed when two or more elements are .......... combined, whereas a mixture contains substances that are .......... chemically combined.

A compound is formed when two or more elements are chemically combined, whereas a mixture contains substances that are not chemically combined.

Why can mixtures be separated by physical methods?

Mixtures can be separated by physical methods because their components are not chemically combined — they are simply mixed together. The individual substances keep their own properties, so techniques like filtration or evaporation can pull them apart.

Complete the table by classifying each substance. Substance Classification Copper Carbon dioxide (CO 2 ) Sand and water Magnesium Copper(II) sulfate (CuSO 4 )

Substance Classification Copper Element Carbon dioxide (CO 2 ) Compound Sand and water Mixture Magnesium Element Copper(II) sulfate (CuSO 4 ) Compound

What is a pure substance (in chemistry)?

A pure substance is a single element or compound that contains no other substances. For example, pure water contains only H 2 O molecules and nothing else.

How does a pure substance's melting point differ from that of an impure substance?

A pure substance's melting point differs in that it melts at a specific, sharp temperature . An impure substance melts over a range of temperatures because it contains different substances with different melting points.

True or False? A pure substance melts at a specific, sharp temperature.

True. A pure substance melts at one fixed temperature. An impure substance melts over a range of temperatures, which is how you can tell the two apart.

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.

Why do impure substances have a lower melting point than the pure substance?

Impure substances have a lower melting point because the added impurities disrupt the regular structure of the pure substance. This means less energy is needed to break the particles apart, so melting begins at a lower temperature .

What is meant by purity in a chemical context?

Purity refers to the extent to which a substance contains only one element or compound, with no other substances mixed in. The closer a substance's measured melting or boiling point is to the known value, the purer the sample.

True or False? Drinking water is a pure substance because it is clean and safe to drink.

False. In chemistry, pure means a single element or compound with nothing else present. Drinking water contains dissolved ions , chlorine and other substances, so it is a mixture , not a pure substance.

How is melting point used to assess the purity of a substance?

Melting point is used to assess purity by heating a small sample slowly and recording the temperature at which it melts. A pure substance melts sharply at one temperature. An impure substance melts over a range. The measured value is compared to known data tables to judge purity.

Filtration is a separation technique used to separate an undissolved solid from a liquid or solution. The mixture is poured through filter paper in a funnel. Small liquid particles pass through as filtrate , while large solid particles are left behind as residue .

When would you use crystallisation rather than simple distillation?

You would use crystallisation rather than simple distillation when you want to obtain the dissolved solid from a solution, not the liquid. Simple distillation is used when you want to collect the pure liquid (for example, water from salt water).

True or False? In filtration, the solid particles pass through the filter paper and are collected as the filtrate.

False. In filtration , it is the liquid that passes through the filter paper as filtrate . The solid particles are too large to pass through and remain as residue .

What is simple distillation ?

Simple distillation is a technique used to separate a pure liquid from a solution (for example, water from salt water). The solution is heated so the liquid evaporates , passes through a condenser where it cools and condenses , and is collected as a pure liquid.

Paper chromatography separates substances based on their different .......... in a solvent. Substances with higher .......... travel further up the paper.

Paper chromatography separates substances based on their different solubilities in a solvent. Substances with higher solubility travel further up the paper.

How does fractional distillation separate ethanol from water?

Fractional distillation separates ethanol from water by heating the mixture to 78 °C — the boiling point of ethanol. Ethanol evaporates first, passes through the condenser and is collected. When the temperature begins to rise to 100 °C , heating is stopped so the two liquids are now separated.

What is crystallisation ?

Crystallisation is a technique used to separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold. The solution is heated to evaporate the solvent, leaving a saturated solution that is then cooled slowly so crystals form.

True or False? Paper chromatography separates substances based on their different solubilities in a solvent.

True. Substances with higher solubility in the solvent travel further up the chromatography paper . This difference in solubility causes the substances to separate.

Complete the table by matching each separation technique to the type of mixture it is used to separate. Technique Mixture type separated Filtration Simple distillation Crystallisation Fractional distillation Paper chromatography

Technique Mixture type separated Filtration Undissolved solid from a liquid Simple distillation Soluble solid from a solution (or pure liquid from a mixture) Crystallisation Dissolved solid from a solution Fractional distillation Two or more miscible liquids Paper chromatography Substances with different solubilities in a solvent

Why must the pencil line be above the level of the solvent in paper chromatography?

The pencil line must be above the solvent level so that the samples are not washed away into the solvent. If the spots were submerged, the substances would dissolve directly into the solvent rather than travelling up the chromatography paper by capillary action.

What is an R f value?

R f values are used to identify the components of mixtures in chromatography. The R f value is a ratio with no units, calculated by dividing the distance moved by the substance by the distance moved by the solvent . It is always less than 1.

How can you tell from a chromatogram that a substance is pure?

You can tell a substance is pure from a chromatogram because it produces only one spot . If more than one spot appears, the substance is a mixture containing more than one component.

True or False? The R f value of a substance can be greater than 1.

False. The R f value is always less than 1 because a substance can never travel further than the solvent front . The solvent always moves the greatest distance.

The R f value is calculated by dividing the distance moved by the .......... by the distance moved by the .......... .

The R f value is calculated by dividing the distance moved by the substance by the distance moved by the solvent .

How do you use a chromatogram to identify the components of an unknown mixture?

You identify the components of an unknown mixture by comparing the position of its spots to those of known reference substances on the same chromatogram. If a spot from the unknown is at the same height as a known substance, or has the same R f value, they are the same substance.

True or False? A pure substance produces only one spot on a chromatogram.

True. A pure substance contains only one component, so it produces only one spot. A mixture produces multiple spots as its different components separate and travel different distances.

What does it mean if two spots on a chromatogram are at the same height?

If two spots are at the same height on a chromatogram, it means they have travelled the same distance and have the same R f value. This indicates the two substances are identical — they are the same compound.

Complete the table with conclusions about each chromatogram observation. Observation Conclusion One spot produced Multiple spots produced Two spots at the same height

Observation Conclusion One spot produced Substance is pure Multiple spots produced Substance is a mixture Two spots at the same height The two substances are identical

Why does changing the solvent change the R f value of a substance?

Changing the solvent changes the R f value because different solvents interact differently with each substance. A substance's solubility in a solvent determines how far it travels up the paper, so a different solvent gives a different distance — and therefore a different R f value.

Why is a pencil line used to mark the baseline in paper chromatography, rather than a pen?

A pencil line is used because pencil (graphite) does not dissolve in the solvent. If a pen were used, the ink would run up the paper along with the samples and interfere with the chromatogram.

True or False? The chromatography paper should be placed so that the baseline dips into the solvent.

False. The baseline must sit above the level of the solvent. If the baseline is submerged, the sample spots will dissolve into the solvent and be washed away rather than travelling up the paper.

In paper chromatography, the solvent is poured to a depth of no more than .......... cm, and the solvent is allowed to travel at least .......... quarters of the way up the paper before the paper is removed.

In paper chromatography, the solvent is poured to a depth of no more than 1 cm, and the solvent is allowed to travel at least three quarters of the way up the paper before the paper is removed.

What is the solvent front ?

The 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 solvent, and is used to measure the distance the solvent has travelled when calculating R f values.

True or False? A separate capillary tube should be used for each sample in paper chromatography.

True. Using a different capillary tube for each sample prevents cross-contamination between food colourings. Mixing tubes would introduce impurities and affect the results.

How do you calculate the R f value from measurements taken from a chromatogram?

You calculate the R f value by dividing the distance moved by the substance by the distance moved by the solvent . Both distances are measured in mm from the baseline to the centre of the spot (or solvent front). The R f value has no units.

IGCSEChemistryEdexcelChemistry (Modular)Unit 1Flashcards1. Principles of Chemistry: Part 1Elements, compounds & mixtures

Elements, compounds & mixtures (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards

Exam code: 4XCH1

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FrontElement, Compound or Mixture
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Cards in this collection (41)

Define element.
An element is a substance made of atoms that all contain the same number of protons, and cannot be broken down into anything simpler.
There are 118 elements in the Periodic Table — for example, copper, iron and magnesium.
How does a compound differ from a mixture?
A compound differs from a mixture in that its elements are chemically combined, not just mixed together.
This means a compound cannot be separated into its elements by physical methods, whereas a mixture can.
Define compound.
A compound is a pure substance made up of two or more elements that are chemically combined.
There are an unlimited number of compounds. Examples include CO₂ and CaCO₃.
True or False?
A compound can be separated into its elements by filtration.
False.
A compound cannot be separated into its elements by physical methods such as filtration. Only a mixture can be separated this way.
Define mixture.
A mixture is a combination of two or more substances (elements and/or compounds) that are not chemically combined.
Mixtures can be separated by physical methods such as filtration or evaporation. Examples include sand and water, and oil and water.
A compound is formed when two or more elements are .......... combined, whereas a mixture contains substances that are .......... chemically combined.
A compound is formed when two or more elements are chemically combined, whereas a mixture contains substances that are not chemically combined.
Why can mixtures be separated by physical methods?
Mixtures can be separated by physical methods because their components are not chemically combined — they are simply mixed together.
The individual substances keep their own properties, so techniques like filtration or evaporation can pull them apart.
Complete the table by classifying each substance.
Substance
Classification
Copper
Carbon dioxide (CO₂)
Sand and water
Magnesium
Copper(II) sulfate (CuSO₄)
Substance
Classification
Copper
Element
Carbon dioxide (CO₂)
Compound
Sand and water
Mixture
Magnesium
Element
Copper(II) sulfate (CuSO₄)
Compound
What is a pure substance (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.
How does a pure substance's melting point differ from that of an impure substance?
A pure substance's melting point differs in that it melts at a specific, sharp temperature.
An impure substance melts over a range of temperatures because it contains different substances with different melting points.
True or False?
A pure substance melts at a specific, sharp temperature.
True.
A pure substance melts at one fixed temperature. An impure substance melts over a range of temperatures, which is how you can tell the two apart.
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.
Why do impure substances have a lower melting point than the pure substance?
Impure substances have a lower melting point because the added impurities disrupt the regular structure of the pure substance.
This means less energy is needed to break the particles apart, so melting begins at a lower temperature.
What is meant by purity in a chemical context?
Purity refers to the extent to which a substance contains only one element or compound, with no other substances mixed in.
The closer a substance's measured melting or boiling point is to the known value, the purer the sample.
True or False?
Drinking water is a pure substance because it is clean and safe to drink.
False.
In chemistry, pure means a single element or compound with nothing else present. Drinking water contains dissolved ions, chlorine and other substances, so it is a mixture, not a pure substance.
How is melting point used to assess the purity of a substance?
Melting point is used to assess purity by heating a small sample slowly and recording the temperature at which it melts.
A pure substance melts sharply at one temperature. An impure substance melts over a range. The measured value is compared to known data tables to judge purity.
What is filtration?
Filtration is a separation technique used to separate an undissolved solid from a liquid or solution.
The mixture is poured through filter paper in a funnel. Small liquid particles pass through as filtrate, while large solid particles are left behind as residue.
When would you use crystallisation rather than simple distillation?
You would use crystallisation rather than simple distillation when you want to obtain the dissolved solid from a solution, not the liquid.
Simple distillation is used when you want to collect the pure liquid (for example, water from salt water).
True or False?
In filtration, the solid particles pass through the filter paper and are collected as the filtrate.
False.
In filtration, it is the liquid that passes through the filter paper as filtrate. The solid particles are too large to pass through and remain as residue.
What is simple distillation?
Simple distillation is a technique used to separate a pure liquid from a solution (for example, water from salt water).
The solution is heated so the liquid evaporates, passes through a condenser where it cools and condenses, and is collected as a pure liquid.
Paper chromatography separates substances based on their different .......... in a solvent. Substances with higher .......... travel further up the paper.
Paper chromatography separates substances based on their different solubilities in a solvent. Substances with higher solubility travel further up the paper.
How does fractional distillation separate ethanol from water?
Fractional distillation separates ethanol from water by heating the mixture to 78 °C — the boiling point of ethanol.
Ethanol evaporates first, passes through the condenser and is collected. When the temperature begins to rise to 100 °C, heating is stopped so the two liquids are now separated.
What is crystallisation?
Crystallisation is a technique used to separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold.
The solution is heated to evaporate the solvent, leaving a saturated solution that is then cooled slowly so crystals form.
True or False?
Paper chromatography separates substances based on their different solubilities in a solvent.
True.
Substances with higher solubility in the solvent travel further up the chromatography paper. This difference in solubility causes the substances to separate.

Substance	Classification
Copper
Carbon dioxide (CO₂)
Sand and water
Magnesium
Copper(II) sulfate (CuSO₄)

Substance	Classification
Copper	Element
Carbon dioxide (CO₂)	Compound
Sand and water	Mixture
Magnesium	Element
Copper(II) sulfate (CuSO₄)	Compound

Complete the table by matching each separation technique to the type of mixture it is used to separate.

Technique	Mixture type separated
Filtration
Simple distillation
Crystallisation
Fractional distillation
Paper chromatography

Technique	Mixture type separated
Filtration	Undissolved solid from a liquid
Simple distillation	Soluble solid from a solution (or pure liquid from a mixture)
Crystallisation	Dissolved solid from a solution
Fractional distillation	Two or more miscible liquids
Paper chromatography	Substances with different solubilities in a solvent

Why must the pencil line be above the level of the solvent in paper chromatography?
The pencil line must be above the solvent level so that the samples are not washed away into the solvent.
If the spots were submerged, the substances would dissolve directly into the solvent rather than travelling up the chromatography paper by capillary action.
What is an R_f value?
R_f values are used to identify the components of mixtures in chromatography.
The R_f value is a ratio with no units, calculated by dividing the distance moved by the substance by the distance moved by the solvent. It is always less than 1.
How can you tell from a chromatogram that a substance is pure?
You can tell a substance is pure from a chromatogram because it produces only one spot.
If more than one spot appears, the substance is a mixture containing more than one component.
True or False?
The R_f value of a substance can be greater than 1.
False.
The R_f value is always less than 1 because a substance can never travel further than the solvent front. The solvent always moves the greatest distance.
The R_f value is calculated by dividing the distance moved by the .......... by the distance moved by the ...........
The R_f value is calculated by dividing the distance moved by the substance by the distance moved by the solvent.
How do you use a chromatogram to identify the components of an unknown mixture?
You identify the components of an unknown mixture by comparing the position of its spots to those of known reference substances on the same chromatogram.
If a spot from the unknown is at the same height as a known substance, or has the same R_f value, they are the same substance.
True or False?
A pure substance produces only one spot on a chromatogram.
True.
A pure substance contains only one component, so it produces only one spot. A mixture produces multiple spots as its different components separate and travel different distances.
What does it mean if two spots on a chromatogram are at the same height?
If two spots are at the same height on a chromatogram, it means they have travelled the same distance and have the same R_f value.
This indicates the two substances are identical — they are the same compound.
Complete the table with conclusions about each chromatogram observation.
Observation
Conclusion
One spot produced
Multiple spots produced
Two spots at the same height
Observation
Conclusion
One spot produced
Substance is pure
Multiple spots produced
Substance is a mixture
Two spots at the same height
The two substances are identical
Why does changing the solvent change the R_f value of a substance?
Changing the solvent changes the R_f value because different solvents interact differently with each substance.
A substance's solubility in a solvent determines how far it travels up the paper, so a different solvent gives a different distance — and therefore a different R_f value.
Why is a pencil line used to mark the baseline in paper chromatography, rather than a pen?
A pencil line is used because pencil (graphite) does not dissolve in the solvent.
If a pen were used, the ink would run up the paper along with the samples and interfere with the chromatogram.
True or False?
The chromatography paper should be placed so that the baseline dips into the solvent.
False.
The baseline must sit above the level of the solvent. If the baseline is submerged, the sample spots will dissolve into the solvent and be washed away rather than travelling up the paper.
In paper chromatography, the solvent is poured to a depth of no more than .......... cm, and the solvent is allowed to travel at least .......... quarters of the way up the paper before the paper is removed.
In paper chromatography, the solvent is poured to a depth of no more than 1 cm, and the solvent is allowed to travel at least three quarters of the way up the paper before the paper is removed.
What is the solvent front?
The 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 solvent, and is used to measure the distance the solvent has travelled when calculating R_f values.
True or False?
A separate capillary tube should be used for each sample in paper chromatography.
True.
Using a different capillary tube for each sample prevents cross-contamination between food colourings. Mixing tubes would introduce impurities and affect the results.
How do you calculate the R_f value from measurements taken from a chromatogram?
You calculate the R_f value by dividing the distance moved by the substance by the distance moved by the solvent.
Both distances are measured in mm from the baseline to the centre of the spot (or solvent front). The R_f value has no units.

Observation	Conclusion
One spot produced
Multiple spots produced
Two spots at the same height

Observation	Conclusion
One spot produced	Substance is pure
Multiple spots produced	Substance is a mixture
Two spots at the same height	The two substances are identical

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