GCSE Chemistry revision

Mixtures

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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A molecule is a group of two or more atoms held together by chemical bonds.
Molecules can consist of atoms of the same element, such as oxygen (O₂), or different elements, such as water (H₂O).
A single atom, like helium, does not count as a molecule because it requires at least two atoms.

A compound is a substance that contains two or more different elements chemically bonded together.
Water (H₂O) and carbon dioxide (CO₂) are compounds because they each contain more than one type of element.
Molecules such as oxygen (O₂), chlorine (Cl₂), and nitrogen (N₂) are not compounds because they only contain one type of element.
In a compound, the elements are always present in fixed proportions, for example water always has two hydrogen atoms for every one oxygen atom.

2. What is a Compound? AQA

A compound is a substance that contains two or more different elements chemically bonded together.
Water (H₂O) and carbon dioxide (CO₂) are compounds because they each contain more than one type of element.
Molecules such as oxygen (O₂), chlorine (Cl₂), and nitrogen (N₂) are not compounds because they only contain one type of element.
In a compound, the elements are always present in fixed proportions, for example water always has two hydrogen atoms for every one oxygen atom.

Chemical formulae use element symbols and subscript numbers to show how many atoms of each element are in a molecule, e.g. H₂O means two hydrogen atoms and one oxygen atom.
If only one atom of an element is present, no subscript number is written, as seen with the single carbon in CO₂.
A more complex example is H₂SO₄ (sulfuric acid), which contains two hydrogen atoms, one sulfur atom, and four oxygen atoms.
Brackets in formulae group atoms together, and the number outside the bracket shows how many of those groups are present, e.g. Ca(OH)₂ means one calcium and two groups of OH.

3. Chemical Formulae AQA

Chemical formulae use element symbols and subscript numbers to show how many atoms of each element are in a molecule, e.g. H₂O means two hydrogen atoms and one oxygen atom.
If only one atom of an element is present, no subscript number is written, as seen with the single carbon in CO₂.
A more complex example is H₂SO₄ (sulfuric acid), which contains two hydrogen atoms, one sulfur atom, and four oxygen atoms.
Brackets in formulae group atoms together, and the number outside the bracket shows how many of those groups are present, e.g. Ca(OH)₂ means one calcium and two groups of OH.

Some compounds, such as sodium chloride (NaCl), exist as giant structures containing millions or billions of atoms rather than small individual molecules.
Compounds that form large structures rather than molecules typically contain ionic bonds between their atoms.
The formula of a non-molecular compound acts as a ratio; for example, NaCl shows that sodium and chloride ions are present in a 1:1 ratio.

4. Molecular vs Non-Molecular Compounds AQA

Some compounds, such as sodium chloride (NaCl), exist as giant structures containing millions or billions of atoms rather than small individual molecules.
Compounds that form large structures rather than molecules typically contain ionic bonds between their atoms.
The formula of a non-molecular compound acts as a ratio; for example, NaCl shows that sodium and chloride ions are present in a 1:1 ratio.

A mixture consists of two or more substances that are not chemically combined together.
In a mixture, the individual substances are physically combined but no chemical bonds form between them.
Because the substances in a mixture are not chemically bonded, they retain their individual properties and can be separated by physical methods.

5. What is a Mixture? AQA

A mixture consists of two or more substances that are not chemically combined together.
In a mixture, the individual substances are physically combined but no chemical bonds form between them.
Because the substances in a mixture are not chemically bonded, they retain their individual properties and can be separated by physical methods.

Mixtures can be separated using physical methods such as filtration, crystallisation, or distillation.
These separation techniques work because the components of a mixture are not chemically bonded and have different physical properties.

A molecule contains two or more atoms bonded together, whereas a compound specifically requires two or more different elements.
All compounds are chemically bonded substances with fixed ratios of elements, while mixtures have no chemical bonds and can vary in composition.
Compounds cannot be separated by physical methods, whereas the components of a mixture can be separated physically.

7. Key Differences: Molecules, Compounds & Mixtures AQA

A molecule contains two or more atoms bonded together, whereas a compound specifically requires two or more different elements.
All compounds are chemically bonded substances with fixed ratios of elements, while mixtures have no chemical bonds and can vary in composition.
Compounds cannot be separated by physical methods, whereas the components of a mixture can be separated physically.