Pure Substances and Mixtures Introductory Chemistry Tutorial

Key Concepts

Matter can be classified as either a:
(a) pure substance

(b) mixture

A pure substance:
⚛ cannot be separated into 2 or more substances by physical or mechanical means

⚛ is homogeneous, that is, has uniform composition throughout the whole sample

⚛ has properties that are constant throughout the whole sample⁽¹⁾

A pure substance has a sharp melting point (melts at one temperature) and a sharp boiling point (boils at one temperature).

⚛ has properties that do not depend on how it is prepared or purified

⚛ has constant chemical composition

A pure substance is either:
(a) element

(b) compound

A mixture:
⚛ can be separated into 2 or more substances by physical or mechanical means

⚛ displays the properties of the pure substances making it up

A mixture melts over a range of temperatures and boils over a range of temperatures.

⚛ has a composition that can be varied by changing the proportion of pure substances making it up

A mixture can be either:

(a) homogeneous (uniform composition throughout the sample)⁽²⁾
Homogeneous mixtures are called solutions.

(b) heterogeneous (non-uniform composition throughout the sample).⁽³⁾
Heterogeneous substances are always mixtures.

Understanding the difference between pure substances and mixtures, and, knowing how to separate mixtures into their component pure substances is important, not only to Chemistry, but to maintaining your own health!

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Pure Substances

A pure substance cannot be separated into 2 or more substances by physical means such as crystallization or distillation nor by mechanical means such as sifting, filtering, or using a magnet.

If you grind some pure iron (Fe) up until it is in small pieces and then bring a magnet close to the pieces, all the pieces will be attracted to the magnet. You cannot separate the iron into simpler substances by grinding it into smaller pieces and using a magnet because it is a pure substance and all the smaller pieces have the same magnetic property as the original sample.

Water (H₂O)⁽⁴⁾ is a pure substance because it can NOT be separated into simpler substances by physical or mechanical means.
Distilling pure water, for example, does NOT separate water into simpler substances which would be the elements hydrogen (H) and oxygen (O), it only produces water vapour (the same substance but in a different phase or state).

The compound iron sulfide⁽⁵⁾ (FeS) is a pure substance because it can NOT be separated into simpler substances by physical or mechanical means.
Putting a magnet next to the pure substance iron sulfide will NOT pull out the magnetic iron (Fe) leaving the non-magnetic sulfur⁽⁶⁾ (S) behind.

A pure substance is homogeneous, that is, has uniform composition throughout the whole sample.

A sample of the element iron (Fe) is composed only of iron atoms so its composition is said to be uniform throughout the sample.
Any sample of iron is 100% iron.

A sample of the element sulfur⁽⁷⁾ is composed only of sulfur atoms (S) so its composition is said to be uniform throughout the sample.
Any sample of sulfur is 100% sulfur.

A sample of the compound iron sulfide (FeS) is always made up of 63.5% iron and 36.5% sulfur so its composition is said to be uniform throughout the sample.

A pure substance has properties that are constant throughout the whole sample.

This means that a pure substance will have a constant appearance, colour, density, melting point and boiling point throughout the sample.
It also means that if you take a small sample of the original pure substance, the sample will undergo the same chemical reactions as the original substance.

If you broke a piece of pure elemental iron (Fe) up in to smaller pieces, then each piece will
⚛ have the same metallic lustre as each other and the original sample

⚛ have the same dark-grey colour as each other and the original sample

⚛ have the same density as each other and the original sample

⚛ display the same magnetic behaviour, attraction to a magnet, as each other and the original sample

⚛ have the same melting point and boiling point as each other and the original sample

⚛ produce the same odourless gas, hydrogen gas, when acid is added to the samples

If you took samples of pure elemental sulfur (S), then each sample will
⚛ have the same dull lustre

⚛ have the same yellow colour

⚛ have the same density

⚛ display the same non-magnetic behaviour, no attraction to a magnet

⚛ have the same melting point and boiling point

⚛ not react when acid is added

If you hit a sample of the solid compound iron sulfide (FeS) with a hammer, smashing it up into smaller pieces, then each of these smaller pieces would:
⚛ have the same colour as each other and the original sample

⚛ have the same density as each other and the original sample

⚛ have the same magnetic behaviour as each other and the original sample

⚛ have the same melting point and boiling point as each other and the original sample

⚛ produce the same gas, hydrogen sulfide (rotten egg gas), when acid is added to the samples

Because pure substances have constant properties throughout the whole sample it means that pure substances display a sharp melting point and a sharp boiling point.
On a graph of temperature vs time, this is shown as a horizontal flat line where the temperature does not change over time until all the pure substance has melted or boiled.
The graph below shows the changes in temperature over time for a pure substance:

Notice that the temperature remains constant at 160°C between 4 minutes and 11 minutes. This horizontal flat line part of the graph is typical of a pure substance.

Melting point of a solid substance is often used to determine whether the substance is pure.
Similarly, the boiling point of a "liquid"⁽⁸⁾ substance can be used to determine whether the substance is pure.

A pure substance has properties that do not depend on how it is prepared or purified.

There are a number of different ways that you could prepare iron sulfide (FeS). For example, you could

grind up solid metallic iron and solid yellow sulfur together then heat it to make solid iron sufide in a synthesis reaction

add a solution of iron ions (Fe²⁺) to a solution of sulfide ions (S^2-) to make solid iron sulfide in a precpitation reaction (a type of double displacement reaction)

If you tested the properties of the iron sulfide prepared in these two different ways, you would find that the two samples would have the same colour, density, magnetic behaviour, melting point, boiling point, and would produce the same rotten gas (hydrogen sulfide, H₂S) when treated with acid.

A pure substance has constant chemical composition.

This means that a pure substance can be either an element or a compound.

The element sulfur (S) is a pure substance, it is made up ONLY of sulfur atoms.

The element iron (Fe) is a pure substance, it is made up ONLY of iron atoms.

The compound iron sulfide (FeS) is a pure substance, it is always made up of 1 iron atom chemically bonded to 1 sulfur atom.

The element hydrogen exists as H₂ molecules and is a pure substance, it is made up of 2 hydrogen atoms chemically bonded to each other.

The element oxygen exists as O₂ molecules and is a pure substance, it is made up of 2 oxygen atoms chemically bonded to each other⁽⁹⁾.

The compound water exists as H₂O molecules and is a pure substance, it is always composed of 2 hydrogen atoms and 1 oxygen atom chemically bonded to each other.

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Mixtures

A mixture displays the properties of the pure substances making it up.

If you grind up some metallic iron (Fe) and yellow sulfur (S) up in a mortar and pestle this is a mixture because

the iron is still a dark-grey colour with metallic lustre that is attracted to a magnet and will react with acid to produce hydrogen gas

the sulfur is still a dull yellow colour that is not attracted to a magnet and will not react with acid.

A mixture can be separated into 2 or more substances by physical or mechanical means because each pure substance retains its own properties.

If you grind up some metallic iron (Fe) and yellow sulfur (S) up in a mortar and pestle this is a mixture because you can separate the magnetic iron from the non-magnetic sulfur using a magnet.

Some methods for separating the components of a mixture are given in the table below:

Methods for Separating the Components in a Mixture⁽¹⁰⁾
Separation technique	Property used for separation	Example
Sifting (sieving)	particle size	Alluvial gold is separated from smaller soil particles using a sieve.

Visual Sorting	colour, shape or size	Gold nuggets can be separated from crushed rock on the basis of colour.⁽¹¹⁾

Magnetic Attraction	magnetism⁽¹²⁾	Magnetic iron can be separated from non-magnetic sulfur using a magnet.

Decanting	density and solubility	Liquid water can be poured (decanted) off insoluble solid sand sediment. Less dense "liquid" oil can be poured (decanted) off more dense liquid water.

Separating Funnel	density of liquids	In a separating funnel, less dense "liquid" oil floats on top of more dense liquid water, when the stopcock at the bottom of the separating funnel is open the water flows out from under the oil.

Filtration	solubility	Insoluble solid calcium carbonate can be separated from liquid water by filtration.

Evaporation	solubility and boiling point	Soluble salts like sodium chloride in sea water can be separated from water by evaporation.

Crystallization (crystallisation)	solubility	Slightly soluble copper sulfate can be separated from water by crystallization.

Distillation	boiling point	Liquid ethanol (ethyl alcohol) can be separated from liquid water by distillation because ethanol has a lower boiling point than water.
Distillation	boiling point	Kerosene, diesel and petrol can be separated from crude oil by distillation because each of these substances has a different boiling point.

A mixture has a composition that can be varied by changing the proportion of pure substances making it up.

If you grind up some metallic iron (Fe) and yellow sulfur (S) up in a mortar and pestle this is a mixture because you can change the proportions (relative amounts) of iron and sulfur that you use.
You could use equal amounts of iron and sulfur to make the mixture, or you could use more iron and less sulfur, or you could use more sulfur and less iron.

Water out of the tap is a mixture because it contains varying amounts of a range of dissolved salts as well as micro-organisms.
The variation in the dissolved salts in tap water is responsible for the taste, and for its "hardness".
The variation in the number and type of micro-organisms can make the water unhealthy to drink.

If you add a drop of red food dye to a glass of water the resulting homogeneous mixture (solution) will be a pale red colour.
If you change the proportion of red food dye to water by adding more drops of red food dye, the solution will become a deeper red colour.
Red food dye dissolved in water is a mixture because you can vary the proportions of red food dye and water.

A mixture can be a heterogeneous substance, one with a non-uniform composition throughout the sample. Heterogeneous substances are always mixtures.

Rocks from which metals are extracted (ores) are heterogeneous substances because the composition of the rock can vary.
One sample of the ore may contain a larger percentage of the metal than another sample within the same rock formation.

Muddy water such as you find in a lake is a mixture composed of fine clay particles suspended in water as well as dissolved salts and micro-organisms. The composition of this mixture near the surface of the lake will be different to the composition of the mixture near the bottom of the lake. Because the mixture in the lake does not have a uniform composition throughout it is said to be a heterogeneous mixture.

A mixture can also be a homogeneous substance, one with a uniform composition throughout the sample. Not all homogeneous substances will be mixtures however!

If you add a drop of red food dye to a glass of water and stir it, then the red food dye disperses uniformly throughout the water resulting in a solution that is the same colour in all places within the solution. This solution is a mixture which has a uniform composition throughout so it is an example of a homogeneous mixture.

If you add a small amount of solid blue copper sulfate to some warm water and stir it, the blue colour is dispersed uniformly throughtout the water indicating that the mixture has a uniform composition throughout.

During your chemistry course you will frequently use homogeneous mixtures known as "aqueous solutions", that is, substances dissolved in water.

Mixtures made up of solids do not display a sharp melting point, they melt over a range of temperatures.
Consider the graph below which shows how the temperature of a homogeneous mixture changes with time:

Notice that on a temperature vs time graph there is no horizontal flat line during which the temperature remains constant over time. Instead, there will be a slope indicating that the components of the mixture are melting.

Sharpness of the melting point is often used to determine whether a substance is pure or impure.
An impure substance is a mixture composed mostly of one pure substance but with trace amounts of other substances which are then called impurities.

Sharp melting point (horizontal flat line on the graph) indicates that the substance is pure.

Substance melts over a range of temperatures (no one horizontal flat line on the graph) indicates that the substance is not pure (it is "impure", a mixture of substances).

Some examples of different types of mixtures are given in the table below:

Examples of Different Types of Mixtures
Type of Mixture	Example
gas in gas	The atmosphere is a mixture of gases, mostly nitrogen gas and oxygen gas.

liquid in liquid	Wine is a mixture of liquid water with other substances like liquid ethanol (ethyl alcohol) dissolved in it.

solid in solid	Alloys, such as steel, brass and bronze, are made up of a mixture of solid metals.

gas in liquid	Soft drinks, such as cola, are mixtures of liquid water with other substances such as carbon dioxide gas dissolved in it.

solid in liquid	Sea water is a mixture of liquid water with other substances like soluble salts dissolved in it.

solid in gas	Smoke is a mixture of tiny solid particles suspended in atmospheric gases.

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Relevance of Pure Substances and Mixtures to You and to Chemistry

As discussed above, if your drinking water is a mixture that contains micro-organisms that can make you sick, this is a problem! Similarly, there are some salts which can be dissolved in the water that can make you very sick.
A lot of time, effort, and expense goes into removing these potentially dangerous substances from drinking water in a process often referred to as "water purification".

Similarly the food you eat contains a mixture of different substances. Most of these substances are beneficial like carbohydrates, proteins, fats and oils and vitamins, but some can be detrimental to your health.
Cycads are a plant generally found in the tropics and can be used as food by traditional peoples like Australian Aborigines because they contain large amounts of the carbohydrate known as starch. Unfortunately, cycads also contain other substances toxic to humans. Thankfully, the toxic substances are soluble in water, so people that use cycad seeds for food first remove the husk then dry them or cook them before leaching out the toxic substances in running water overnight.⁽¹³⁾
Sometimes humans add toxic substances to food which must be removed before we eat it. For example, some plants that are used for food are sprayed with insecticides to kill insects, some of these insecticides can harm humans. So it is important to separate and remove the harmful parts of the mixture from the beneficial parts.

The composition of our atmosphere changes with time, that is, the proportion of different gases making up the mixture of gases in our atmosphere is changing.
In recent years we have noted an increase in the amount of carbon dioxide gas in the atmosphere.
Carbon dioxide is a "greenhouse gas", so-called because it is one of the gases in the atmosphere that keeps Earth warm enough for living things to survive.
If the amount of carbon dioxide gas increases too much however, this could change Earth's climate sufficiently to be detrimental to the health and on-going survival of living things.
Research continues into how to address this on-going problem.

Crude oil is a mixture of different substances. This mixture is separated into different "fractions" such as kerosene, diesel and petrol which can all be used as fuels.

The whole word uses about 200 million tonnes of sodium chloride (salt⁽¹⁴⁾) every year. The source of most of this salt is the world's oceans because ocean water is a mixture of water and dissolved salts and gases. Salt is separated from the ocean water mixture by evaporating off the water in a process known as "solar evaporative salt production".⁽¹⁵⁾

Chemistry is the study of how atoms are arranged in matter, and changing the arrangement of those atoms in chemical reactions.
Therefore it is important to know just what atoms you started with before the chemical reaction.
If the substance you start with is a pure substance, then you know exactly what atoms are present before your chemical reaction takes place, and therefore, you know what atoms will be re-arranged during the chemical reaction.
If substances are pure, you can predict how the atoms will be re-arranged in a chemical reaction.
If the substance is not pure, if it is impure, then the original atoms can be re-arranged in more than one way, and you can no longer easily predict how the atoms will be re-arranged in a chemical reaction.

Chemists are notoriously finicky when it comes to using clean glassware!
If you are making up your sodium chloride in water solution you need to use clean glassware because, if the glassware is dirty it will add other substances to your solution, but you won't know what they are!
You will no longer know the amount of sodium chloride in your solution, that is, if sodium chloride was part of the "dirt" in the glassware then the amount of sodium chloride in your solution has increased by an unknown amount.
Other unknown substances may be present in the "dirt", so you can no longer predict how the atoms will be rearranged in a chemical reaction because there are unknown atoms present as well as the sodium chloride.

For the same reason, Chemists are diligent in keeping their "pure substances" pure.
If you look at the jar of sodium chloride in the laboratory it will have a label on it which includes information about how pure the sodium chloride is.
If you dip a dirty spatula into the jar to extract some of the sodium chloride, then some of this "dirt" will stay in the jar.
This jar of sodium chloride is now a mixture NOT a pure substance, we say the sodium chloride has been contaminated.
We no longer know what atoms are present in the jar, so next time we scoop some of it out with a clean spatula we won't know how much sodium chloride is actually present because some it will be made up of these unknown substances.

Chemists use the properties of homogeneous mixtures all the time.
If you make up 1 L (1000 mL) of sodium chloride solution by dissolving 100 g of sodium chloride in the water and mix it thoroughly by stirring or shaking, then all of the sodium chloride will be dispersed evenly throughout the water. This means that every 1 mL volume of this solution will contain 100 g/1000 = 0.1 g of sodium chloride (0.1 grams of sodium chloride per litre of solution, 0.1 g mL^-1).⁽¹⁶⁾
If you pour off 100 mL of this solution into a beaker, then the beaker will contain 0.1 g mL^-1 × 100 mL = 10 g
If you pour off 10 mL of this solution into a beaker, then the beaker will contain 0.1 g mL^-1 × 10 mL = 1 g
This is very useful! It means you can make up 1 large volume of solution and use smaller volumes of it as required instead of having to make up new solutions every time you want to use it.

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Footnotes

1. A region in which all the physical and chemical properties are uniformly the same is referred to as a phase.
A pure substance is composed of only one phase.
A given phase may be solid, liquid or gaseous.

2. An homogeneous mixture is composed of one phase.
Because gases mix completely, any mixture of gases will be composed of 1 phase and will be homogeneous.
A soluble salt dissolved in liquid water to make an aqueous solution of salt is also composed of only phase.
A soluble liquid like ethanol dissolved in liquid water is also composed of only one phase.

3. A heterogeneous mixture is composed of more than one phase.

4. H₂O is the molecular formula for water, it tells us that 2 hydrogen atoms and 1 oxygen atom are present in each molecule of water.
You can find out more in the molecular formula tutorial.

5. Iron sulfide is the name given to the compound containing 1 "atom" of iron and 1 "atom" of sulfur based on its composition.
You could name this as a binary inorganic ionic compound (a salt) in which case the preferred IUPAC name is iron(2+) sulfide.
An older IUPAC name for the same compound is iron(II) sulfide (or iron(II) sulphide).
An even older name for the same compound is ferrous sulfide (or ferrous sulphide).

6. The IUPAC preferred spelling for the element with the chemical symbol S is sulfur not sulphur.
You may still find the old spelling of "sulphur" used to refer to the element sulfur.

7. Atoms of sulfur can be arranged in different ways. You can read more about this in the allotropes tutorial.

8. The term liquid in chemistry refers to a state (or phase) of matter in which all the "particles" (molecules for example) are identical. Therefore, a liquid is, by definition, a pure substance. In common useage, people refer to many types of mixtures as "liquids", for example the cordial in a glass might be referred to as a liquid by a non-chemist but a chemist thinks of this as a mixture known as an aqueous solution because there are a lot of substances dissolved in the water (aqua) to give it flavour and colour.
The most general term for a substance that flows is "fluid". A liquid is a fluid, an aqueous solution is a fluid, and gases are fluids.

9. Oxygen can also exist in a different form known as ozone in which 3 oxygen atoms are chemically bonded to each other (O₃). O₂ and O₃ are known as allotropes of oxygen.

10. This is NOT a comprehensive list of separation methods. You might also like to read about some techniques chemists use to separate substances from mixtures in the following AUS-e-TUTE tutorials:
Chromatography, Electrophoresis and Mass Spectroscopy
And, there are numerous methods used commercially to separate metals from the ores. You might like to read the following AUS-e-TUTE tutorials on metal extraction:
Metal Extraction Concepts, Carbon Reduction Method of Metal Extraction, Copper Extraction by Smelting, Electrolytic Extraction of Aluminium, Electrolytic Extraction of Sodium, Electrowinning Copper

11. Gold nuggets can be large enough to see lying on the surrounding ground. A gold nugget found like this led to the establishment of the enormous Kalgoorlie gold fields in Western Australia.
You can read more about this in the December 2012 issue of AUS-e-NEWS.

12. Strictly speaking we should refer to this property as ferromagnetism.
Ferromagnetism refers to a strong attraction to a magnetic field and is displayed by the elements iron (Fe), cobalt (Co) and nickel (Ni) in the solid state.
In common useage "magnetism" refers to "ferromagnetism".
There are other types of magnestism:
⚛ paramagnetism, substances with unpaired electrons experience a weak attraction to a magnetic field
⚛ diamagnetism, a very feeble repulsion out of a magnetic field which is experienced by all matter

13. At one time it was thought that the disease Guam Dementia was caused by flour made from toxic cycad seeds.
You can read more about this in the September 2017 issue of AUS-e-NEWS.

14. In common useage, sodium choride (NaCl) is referred to as salt or table salt. To a chemist, sodium chloride is just one example of a salt. In chemistry, salts are compounds made up of cations (positively charged ions) and anions (negatively charged ions) which are held together in a three dimensional lattice using electrostatic forces of attraction (ionic bonds). A salt is therefore an ionic compound made up of two ions, a binary ionic compound.

15. Solar Evaporative Salt Production was discussed in detail in the September 2016 issue of AUS-e-NEWS.

16. When a substance dissolves in the water, the resulting mixture is known as an aqueous solution. The amount of substance dissolved in a given volume of water is known as the concentration of the solution.
You can read more about solutions and concentration in the Solutions Concepts Tutorial.