Sec 1 Science Elements, Compounds & Mixtures notes

An element is a pure substance made of only one kind of atom. It cannot be split into simpler substances by chemical reactions. Oxygen, iron, carbon and gold are elements. Each has a chemical symbol, and the symbols are arranged on the Periodic Table.

Metals

Often shiny, and many conduct electricity and heat. Examples: iron, copper, aluminium.

Non-metals

Often dull, and many are poor conductors. Examples: oxygen, sulfur, carbon. Some non-metals are gases at room temperature.

A compound is made of two or more elements that are chemically combined in a fixed way. Water is a compound of hydrogen and oxygen. You can put out some fires with water, even though hydrogen burns and oxygen supports burning. The compound’s properties are different from the elements in it.

• The elements are chemically joined, not just sitting together.
• A compound has a fixed composition. You cannot change the recipe and still have the same compound.
• Splitting a compound back into elements needs a chemical change, not a simple physical separation such as filtration.

Knowing what a material is made of matters in everyday products. A plastic used for a food container can be safe in that form, while the same name used in the wrong place, such as a contaminant in food, can be harmful. The chemical composition is the difference.

A mixture contains two or more elements and/or compounds that are not chemically combined. Air is a mixture of gases, including nitrogen and oxygen. Sea water is a mixture of water and dissolved salts. Iron filings mixed with sulfur still look like grey metal and yellow powder, and a magnet can still pick out the iron.

• Dissolves

  Solute

  The substance that dissolves, such as salt.

• Does the dissolving

  Solvent

  The liquid it dissolves in, such as water.

• The result

  Solution

  A mixture that looks uniform. The solute does not settle and cannot be filtered out.

• Not dissolved

  Suspension

  Solid bits spread in a liquid. They can settle and can often be removed by filtration.

Both a solution and a suspension are mixtures. The difference is whether the solid has dissolved. Muddy water is a suspension. Salt water, once the salt has dissolved, is a solution.

• Higher temperature — particles move faster, so the solid dissolves faster.
• Stirring — brings fresh solvent into contact with the solid.
• Smaller pieces — more surface touches the solvent, so dissolving is faster.

Solubility is how much solute dissolves in a given amount of solvent at a given temperature. The rate is how quickly that happens. Crushing a tablet makes it dissolve faster. It does not mean more of the tablet can dissolve in the end.