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Pure Substances and Mixtures Introductory Chemistry Tutorial

Key Concepts

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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.

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

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

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.

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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

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

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

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

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

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.

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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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.