Elements, Compounds and Mixtures

• Elements are the basic building blocks of all living and non-living matter.
• Elements discovered on Earth can be found in the Periodic Table.
• Each element has a unique chemical symbol (atomic symbol), which can be found in the Periodic Table. For example, the element "magnesium" has the chemical symbol Mg.

Elements can be classified as metals or non-metals:

• Metals are found to the left of the zig-zag line
• Non-metals are found to the right of the zig-zag line

• Mercury has a silvery appearance and is the only metal that exists as a liquid at room temperature.
• The volume of a fixed mass of mercury increases uniformly with temperature.
• Hence, mercury is commonly used as the liquid in liquid-in-glass thermometers.

Elements are made up of only one type of atom and exist as either single atoms or molecules:

Elements that are made up of single atoms are known as monoatomic elements.

• Noble gases, found in the last column (Group 18) of the Periodic Table, are monoatomic elements.
• Examples of noble gases include helium (He) and argon (Ar).

• A molecule is a group of two or more atoms that are chemically combined together.
• A molecule of an element is a group of two or more of the same type of atom chemically combined together.
• Most non-metals are made up of molecules.

These elements exist as diatomic molecules (two atoms chemically combined):

• An element has fixed physical properties, such as melting point, boiling point and density.
• An element has fixed chemical properties, i.e. how they react with other substances.

• A molecule of a compound is a group of two or more different types of atoms that are chemically combined together.
• Compounds containing only non-metals are made up of molecules.

• A compound has different properties from its constituent elements.
• A compound has a fixed proportion by mass of its constituent elements — it has a fixed composition, hence its own unique chemical formula.
• A pure compound has fixed melting and boiling points.

A compound can only be broken down (decomposed) to simpler substances using chemical methods, but not by physical methods. This is because chemical methods are required to break the chemical bonds in compounds.

Mixtures may exist in any of the three physical states — i.e. in solid, liquid or gaseous state at room temperature and pressure.

• A mixture has the same properties as its constituent substances.
• A mixture has variable proportions by mass of its constituent substances — it has a variable composition.
• A mixture has variable melting and boiling points — it melts and boils over a range of temperatures.
• A mixture can be separated into its constituent substances using physical methods, e.g. filtration and distillation, because no chemical changes are involved in the formation of mixtures.

• Solutions have a homogeneous composition with uniform physical and chemical properties throughout. For example, the colour, density and appearance of a solution are uniform throughout. The solute particles cannot be seen.
• When a solution is filtered, no residue is left on the filter paper.
• When a solution is left to stand, the solute does not separate from the solvent to form solid deposits. Hence, there is no sedimentation.
• Liquid solutions usually allow light to pass through fully (they are transparent).

Metals can be mixed with other elements to form solid solutions known as alloys:

• Liquid solvents containing dissolved solutes — e.g. sugar solution, salt solution and soda water (solid + liquid).
• Mixtures of miscible liquids — e.g. ethanol (alcohol) and water (liquid + liquid).

• Air is a solution of gases. It mainly contains elements such as nitrogen and oxygen. Noble gases are also present in smaller percentages, and it also contains compounds such as carbon dioxide and water vapour.
• Natural gas is a solution of gases such as methane and ethane.

Example: the solubility of Substance X may be 10 g per 100 cm³ of water at 25 °C.

• Suspensions have a heterogeneous composition with different physical and chemical properties throughout the mixture. The insoluble particles can be seen.
• The components of a suspension can be easily separated using physical methods such as filtration. The insoluble particles in the medium are collected as residue while the medium is collected as the filtrate.
• When a suspension is left to stand, it separates into distinct layers. The insoluble particles usually settle to the bottom of the container to form solid deposits. This is known as sedimentation.
• Suspensions usually appear cloudy and do not allow light to pass through fully — some of the dispersed particles cause light to scatter when light passes through.

In particle diagrams, each circle represents one atom. Different colours/sizes represent different types of atoms; touching circles are chemically combined into one molecule.

1. Count the types of atom (different colours) — 1 type = element; 2+ types bonded together = compound; 2+ types not bonded = mixture.
2. Check if atoms are bonded — separate circles = single atoms; touching circles = molecules.

"Which of the following shows an element that exists as molecules?"

"Which of the following represents a pure compound?"