Elements , Compounds and Mixtures y9

What is an element?

A substance made of only one type of atom, which cannot be broken down into anything simpler by chemical means.

What is a compound?

A substance that contains two or more different elements chemically bonded together in fixed proportions.

What is a mixture?

A combination of different elements and/or compounds not chemically bonded and can exist in varying proportions.

What is a molecule?

Two or more atoms chemically bonded together. (Can be an element like H₂ or a compound like H₂O.)

What is a pure substance?

A material with a fixed melting and boiling point, made up of a single element or compound.

What are the key features of elements?

Made of one type of atom only; Found on the Periodic Table.

What are the key features of compounds?

Two or more different atoms chemically bonded; Properties differ from original elements; Difficult to separate original elements.

What are the key features of mixtures?

Elements/compounds not bonded together; Easily separated by physical methods; Properties are a mix of the substances’ properties; Have a range of melting/boiling points.

What is filtration used for?

Separating an insoluble solid from a liquid or solution.

What are the steps in filtration?

1. Pour mixture onto filter paper; 2. Insoluble solid stays as residue; 3. Liquid/solution passes through as filtrate. Example: Sand and water.

What is crystallisation used for?

To separate a soluble solid (solute) from a solution.

What is the principle behind crystallisation?

Solubility decreases as temperature decreases — causing crystals to form.

How do you obtain pure salt from rock salt?

1. Grind rock salt; 2. Add water, stir and heat gently; 3. Filter to remove insoluble sand; 4. Evaporate filtrate to form saturated solution; 5. Cool to crystallise pure salt; 6. Filter and dry crystals.

What is simple distillation used for?

To obtain a pure liquid from a solution when there’s a large difference in boiling points.

Describe how simple distillation works.

1. Heat solution; 2. Lowest boiling liquid evaporates first; 3. Vapour passes through condenser; 4. Vapour cools and condenses into pure liquid (distillate).

Give an example of simple distillation.

Separating water from salt water; Distilling blue ink to collect pure water.

What is fractional distillation used for?

To separate a mixture of miscible (soluble) liquids with similar boiling points.

How does fractional distillation work?

1. Uses a fractionating column; 2. Lowest boiling liquid evaporates first and rises; 3. Vapour condenses and is collected; 4. Temperature increased to collect next liquid. Examples: Separating hydrocarbons in crude oil; Separating gases in air.

What is chromatography used for?

To separate substances based on different solubilities in a solvent (e.g., coloured dyes).

What are the steps in paper chromatography?

1. Draw pencil baseline; 2. Add ink spots; 3. Place in solvent below baseline; 4. Solvent carries dyes up paper; 5. Remove when solvent nearly reaches top; 6. Mark solvent front.

What does each spot on a chromatogram represent?

A different substance in the mixture.

What does it mean if a spot stays on the baseline?

The substance is insoluble in the solvent or has high affinity for the paper.

How do you calculate the Rf value?

Rf = Distance travelled by solute ÷ Distance travelled by solvent (ratio between 0 and 1, no units).

What does matching Rf values indicate?

Substances are likely the same; check in different solvents to confirm.