Elements, Compounds & Mixtures and Atomic Structure

What is an element?

One type of atom only (e.g. oxygen O₂).

What is a compound?

Two or more elements chemically bonded in fixed ratio (e.g. water H₂O).

What is a mixture?

Substances not chemically bonded, can be separated physically (e.g. air or saltwater).

Why does a pure substance have a sharp melting point?

All particles are identical, melting at one fixed temperature.

Why does a mixture melt over a range of temperatures?

Different particles have different melting points.

Name five experimental techniques used to separate mixtures.

Simple distillation, fractional distillation, filtration, crystallisation, paper chromatography.

How does simple distillation work?

Heat mixture, lowest boiling point liquid evaporates first, vapour condenses and is collected separately.

Give an example of when simple distillation is used.

Separating water from saltwater.

What is the main difference between simple distillation and fractional distillation?

Fractional distillation uses a fractionating column for liquids with close boiling points.

Give an example use for fractional distillation.

Separating crude oil fractions or ethanol + water.

How can filtration and crystallisation be used together to obtain pure salt from rock salt?

Dissolve rock salt in water, filter to remove insoluble sand, evaporate water slowly to form salt crystals.

What information does a chromatogram give about a mixture?

Number of spots = number of different substances; position/Rf value of spots = identity of substances.

Define Rf value.

Rf = distance travelled by substance / distance travelled by solvent front.

Calculate the Rf value of spot A if it travelled 4.2 cm and the solvent front travelled 7.0 cm.

Rf = 4.2 / 7.0 = 0.60.

Describe the practical steps for investigating paper chromatography.

Draw pencil baseline, spot different inks/food colours, place paper in beaker with solvent, cover beaker, let solvent rise, remove paper, mark solvent front, calculate Rf values.

What is an atom?

Smallest particle of an element (e.g. He).

What is a molecule?

Two or more atoms chemically bonded (e.g. O₂, H₂O).

What is atomic number?

Number of protons, defines the element.

What is mass number?

Protons + neutrons.

What are isotopes?

Atoms of the same element with different number of neutrons.

What is relative atomic mass (Aᵣ)?

Weighted average mass of all isotopes relative to ¹²C.

Why is relative atomic mass not usually a whole number?

It's an average of different isotope masses and abundances.

Calculate the relative atomic mass of chlorine with isotopes ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance).

(35 × 75 + 37 × 25) / 100 = 35.5.

Calculate Aᵣ for magnesium with isotopes ²⁴Mg (79%), ²⁵Mg (10%), ²⁶Mg (11%).

(24 × 79 + 25 × 10 + 26 × 11) / 100 = 24.32.