Exercises: Pure Substances, Mixtures and Separation

Classify each of the following as an element, a compound, a homogeneous mixture or a heterogeneous mixture:

1. a) sodium chloride
2. b) sea water from Dublin Bay
3. c) copper
4. d) muesli

A student has a mixture of sand and water. Name the separation technique that would be used to separate the sand from the water, and state the property of the sand that makes this technique work.

State whether each of the following is a physical change or a chemical change:

1. a) ice melting on a lake in Connemara
2. b) iron rusting on a gate
3. c) magnesium ribbon burning in air
4. d) sugar dissolving in tea

A sample of crude oil from an Irish refinery is separated by fractional distillation into petrol, kerosene and diesel.

1. a) Explain why crude oil is a mixture and not a compound.
2. b) Explain, in terms of boiling point, how fractional distillation separates the components.
3. c) Is this separation a physical or a chemical process? Justify your answer.

A mixture contains salt (sodium chloride), iron filings and sand. Describe a sequence of separation steps, with the technique used at each step, that would give all three components in separate and dry form. Justify the technique chosen at each step by referring to a property of the component being separated.

Iron filings $(7.0\ \text{g})$ are heated with sulfur $(4.0\ \text{g})$. The product is iron(II) sulfide, $\mathrm{FeS}$, and the iron and sulfur react in the mass ratio $7:4$.

1. a) What mass of iron(II) sulfide forms?
2. b) A magnet is passed over the mixture before heating and again after heating. Describe and explain what is observed each time.
3. c) State two pieces of evidence that a chemical change has taken place.

A water sample from a river contains dissolved salt, suspended clay particles and a small amount of dissolved ethanol from an industrial discharge.

1. a) Design a sequence of separation techniques to obtain pure water, clay and ethanol. Justify each step.
2. b) Explain why simple distillation would be unsuitable for separating the ethanol from the water if a high-purity ethanol sample were required.
3. c) Suggest how a student could show that the final water sample is pure.

Paper chromatography is used to analyse a green food dye. Three spots appear on the chromatogram, with $R_f$ values of $0.25$, $0.55$ and $0.80$. The solvent front has moved $12.0\ \text{cm}$ from the baseline.

1. a) Calculate the distance moved by each spot.
2. b) Explain why the dye is a mixture.
3. c) Explain, in terms of the stationary and mobile phases, why different components travel different distances.
4. d) Explain why the baseline is drawn in pencil and not in ink.

When $2.43\ \text{g}$ of magnesium is burned completely in air, $4.03\ \text{g}$ of magnesium oxide forms.

1. a) Calculate the mass of oxygen that combined with the magnesium.
2. b) Explain how this result illustrates the law of conservation of mass.
3. c) A student finds that the product weighs $3.90\ \text{g}$ when the experiment is performed in an open crucible. Suggest two reasons for the discrepancy.
4. d) Justify why the burning of magnesium is classified as a chemical change.