Elements, Compounds and Mixtures
In this lesson we will learn how matter can be categorised based on the subunits making it up, and how this affects the separability of substances.
By the end of this lesson you will be able to:
Describe atoms and molecules.
Compare pure substances and mixtures.
Compare elements and compounds.
Discuss the separation of elements, compounds and mixtures.
Particles in Matter: Atoms and Molecules
- All matter is made up of subunits.
- There are two main types of subunits: atoms and molecules.
- Atoms are the basic unit of matter.
- There are about 90 different types of atoms that exist naturally and more than 20 that have been synthesised artificially.
- Examples of atoms include helium atoms, carbon atoms and copper atoms.
Atoms are too small to see individually, but they can be represented as simple spheres.
- Molecules are chemical structures made up of two or more atoms held in a fixed arrangement.
- These atoms are held together by forces of attraction known as chemical bonds.
- Molecules can consist of just one type of atom, but more often they consist of more than one type of atom.
- Molecules can contain as littles as two atoms in total, or they can contain many thousands of atoms.
- Due to the infinite number of ways that different atoms can combine, there is an almost unlimited number of different molecules that can exist.
Hydrogen molecules consist of two hydrogen atoms.
Ethanol molecules consist of two carbon atoms, six hydrogen atoms and one oxygen atom.
Types of Matter: Pure Substances and Mixtures
- Matter can be divided into pure substances and mixtures, based on the number of different subunits it contains.
- Pure substances are substances that are made up of one type of subunit.
- This subunit may be an atom or a molecule.
Pure substances contain only one type of subunit.
- Mixtures are substances that are made up of more than one type of subunit.
- Mixtures are physical combinations of two or more pure substances.
Mixtures contain more than one type of subunit.
Types of Pure Substances: Elements and Compounds
- Pure substances can be divided into elements and compounds, based the number of different atoms they contain.
- Elements are substances that are made up of subunits containing one type of atom.
- Examples of elements include helium, carbon and copper. They are made up only of helium atoms, carbon atoms and copper atoms respectively.
Helium, carbon and copper are each made up of only one type of atom, therefore they are elements.
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- In some elements the atoms exist as molecules, containing two or more atoms of the same type.
- The element chlorine is made up of chlorine molecules, which contain two chlorine atoms bonded together.
- The element phosphorus is made up of phosphorus molecules, which contain four phosphorus atoms bonded together.
- The element sulfur is made up of sulfur molecules, which contain eight sulfur atoms bonded together.
Although chlorine, phosphorus and sulfur are made up of molecules, they are still elements because their molecules are made up of only one type of atom.
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- Compounds are substances that are made up of subunits containing more than one type of atom.
- Examples of compounds include: water, which is made up of molecules containing hydrogen and oxygen atoms; carbon dioxide, which is made up of molecules containing carbon and oxygen atoms; and sodium chloride (salt), which is made up of subunits containing sodium and chlorine atoms.
Water, carbon dioxide and salt are each made up of subunits containing more than one type of atom, therefore they are compounds.
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Properties of Elements, Compounds and Mixtures
- Pure substances (elements and compounds) have fixed properties, such as melting and boiling point and chemical reactivity.
- For example, water is a pure substance. It freezes at 0 °C and boils at 100 °C. Crude oil is a mixture. It has multiple boiling points, where different components boil and vaporise at different temperatures.
Water has fixed melting and boiling points.
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- Compounds have properties that are different to the elements they are formed from.
- For example, water molecules are made up of hydrogen and oxygen atoms, but water has very different properties to either hydrogen or oxygen. Water is a liquid, whereas hydrogen and oxygen are both gases. Water also reacts with other substances very differently to how hydrogen or oxygen do.
- Mixtures have variable properties, which are a combination of the properties of their components.
- For example, crude oil is a mixture. It has multiple boiling points, where different components, such as natural gas and kerosene, boil and vaporise at different temperatures.
Crude oil has multiple boiling points as it is a mixture of several substances.
(Image: Psarianos, Wikimedia Commons)
Separation of Pure Substances and Mixtures
- Since elements consist of only one type of atom, they cannot be physically or chemically separated into simpler substances.
- Compounds cannot be physically separated into their component elements, but they can be chemically separated.
- For example, haematite is a compound containing iron and oxygen. The iron can only be extracted from haematite by a chemical reaction.
A blast furnace chemically extracts the element iron from compounds containing iron.
(Image: Psarianos, Wikimedia Commons)
- Mixtures can be physically separated into their component substances, by processes such as filtration and distillation.
- For example, pure alcohol (ethanol) can be physically separated from mixtures containing alcohol by distilling it.
Distillation can separate pure alcohol from alcohol mixtures.
(Image: GOKLuLe, Wikimedia Commons)
- The chemical separation of compounds involves the breaking of chemical bonds, whereas the physical separation of mixtures does not.
- The breaking of chemical bonds requires energy. Therefore, the separation of mixtures into their component substances is easier (requires less energy) than the separation of compounds into their component elements.
- For example, the distillation of alcohol requires much less energy than the extraction of iron.
- There are two main types of subunits that make up matter: atoms and molecules.
- Atoms are the most basic type of subunit.
- Molecules consist of two or more atoms held in a fixed arrangement by chemical bonds.
- Matter can be classified into pure substances or mixtures, based on the number of different subunits it contains.
- Pure substances are made up of one type of subunit.
- Mixtures are made up of more than one type of subunit.
- Pure substances can be classified into elements or compounds, based the number of different atoms they contain.
- Elements are made up of one type of atom.
- Compounds are made up of more than one type of atom.
- Pure substances (elements and compounds) have fixed properties.
- Mixtures have variable properties.
- Elements cannot be separated into other substances.
- Compounds can be chemically separated into their component elements.
- Mixtures can be physically separated into their component substances.
The classification of matter.
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