Eutectic mixtures

These are mixtures which at constant pressure freeze at a constant temperature to give a solid of the same composition.

Similarities between a eutectic mixture and pure compound

1. both have a sharp freezing point at constant pressure
2. Have the same composition in liquid and liquid

Reasons why eutectic mixtures are not compound

1. their composition varies with pressure
2. can be separated by other physical methods as distillation and  evaporation to dryness
3. X-ray analysis shows that eutectic mixtures are not pure compound
4. Microscopic analysis shows that eutectic mixtures are heterogeneous, made of a mixture crystals of separate substances
5. Chemical properties of eutectic mixtures are those of individual components making up the mixture for example when solder is reacted with dilute hydrochloric acid, tin reacts whereas lead does not.

Conditions necessary for formation of eutectic mixtures

(a) pure crystals of the components should separate from mixture on cooling.

(b) The two substance involved must be completely miscible when melted

Consider a solution of NaCl in water; when a solution containing less than 23.3% NaCl is cooled, pure crystals of Ice will separate out at a temperature below 0⁰C. As this occurs the solution become richer in NaCl leading to further depression of the freezing point until about -21 ⁰C. At this temperature the composition of the solution is 23.3% NaCl and further cooling temperature remains constant until both sodium chloride and water have frozen together to form a solid of the same composition.

Alternatively; when a solution containing more than 23.3% of sodium chloride is cooled, NaCl separate out as temperature falls until the concentration of the solution is about 23% and this occurs at about -21⁰C. Here water and NaCl freeze to form a solid of the same composition.

Graphically, the above information can be represented on eutectic diagram below

The mixture of salt and ice which crystallizes at -21⁰C from either a dilute or concentrated solution is called eutectic mixture and the temperature at which the mixture freezes is called Eutectic temperature.

Other substances that can form eutectic mixtures are

• Tin (mpt. 232) and lead (mpt.227); Eutectic composition is 66% Pb and eutectic temperature 183⁰C.
• Zinc (mpt. 491) and cadmium (mpt. 321⁰C); eutectic composition 75% Zn and temperature 270⁰C.

Example 1

(a) Explain the following term

(i) solubility              (3 marks)

(ii) Eutectic mixture (3 marks)

(b)The equilibrium diagram for the sodium nitrate-water system is shown below

(i) State what lines DE and EF represent (2 mark)

DE- freezing/melting point of solutions containing less than 38.62% sodium nitrate

EF – solubility curve for solutions containing more than 38.62% sodium nitrate at various temperatures

(ii) Determine the % of sodium nitrate at points A, B and C       (3marks)

A: 26% (draw a horizontal line to touch the curve and a vertical line to read the concentration)

B: 52%

C: 38.62%

(iii) Name the substances at A, B, and C (4½mk)

A: ice and sodium nitrate

B: solid NaNO₃ and solution

C: Eutectic + solid NaNO₃

(c) (i) What would be observed if a solution   having the composition K was cooled slowly

The solution remains in liquid form until -17.5⁰C; here the temperature remains constant until all the liquid has turned into a solid, and then the temperature of the solid falls.

(ii) State two reasons why eutectic mixtures are not compounds.

they can be separated by other physical means such as evaporation to dryness

their composition varies with pressure

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Eutectic mixtures

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Compiled by Dr. Bbosa Science