Purity, Formulations and Chromatography

1 In chemistry, what does "pure" mean?

Answer: A pure substance is a single element or compound, not mixed with any other substance.

2 How can the melting and boiling points of a substance be used to show it is pure?

Answer: Pure elements or compounds melt and boil at specific temperatures, they do not melt or boil over a range of temperatures.

3 What is a "formulation" ?

Answer: A formulation is a mixture that has been designed as a useful product.

4 How are they made?

Answer: Mixing the components in carefully measured quantities to ensure that the product has the required properties.

5 Give an example of a formulation.

Answer: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods

6 What is "chromatography" ?

Answer: Chromatography can be used to separate mixtures and can give information to help identify substances.

7 What is an "Rf value" ?

Answer: Chromatography can be used to separate mixtures and can give information to help identify substances.

8 Give the expression to calculate Rf values.

Answer: Rf = distance moved by substance ÷ distance moved by solvent

9 What are the two phases used in chromatography?

Answer: mobile phase and stationary phase

10 How does chromatography separate substances?

Answer: It depends on the distribution of substances between the phases (mobile and stationary). The more attracted the substance is to the stationary phase, the less it moves and the smaller the Rf value. The more attracted the substance is to the mobile phase, the more it moves and the larger the Rf value.

11 Most people use the term "pure milk" in everyday speech.

a) A scientist uses the word "pure" in a different way and would not consider milk to be a pure substance. Explain why.

Answer: In everyday language, a pure substance can mean a substance that has had nothing added to it. It is unadulterated and in its natural state. But chemically pure means that there is only one element or compound in the sample and not mixed with any other substance. So, pure milk is not chemically pure.

b) The composition of whole milk is 4.9% lactose (carbohydrate), 3.4% fat, 3.3% protein, 0.7% minerals, and water. Calculate the percentage amount of water in whole milk.

Answer: 100 - (4.9 + 3.4 + 3.3 + 0.7) = 87.7%

c) The boiling point of water is exactly 100°C. Predict the boiling point of whole milk and justify your prediction.

Answer: Between 100.1°C and 110°C. Milk is mainly water so will have similar boiling point to water, but as it is a mixture it will be slightly higher than water. Milk is a mixture so it will boil over a range.

12 Chlorophyll was obtained by stirring a shredded green leaf in a suitable solvent. A fine line of the solution was drawn 2.0 cm from the bottom of a piece of chromatography paper and allowed to dry. The paper was placed in another solvent (mobile phase), making sure that the dried line of pigment was above the surface of the solvent. The figure below shows the results of the chromatography.

a) State the number of pigments in the sample of chlorophyll.

Answer: four

b) State the pigment that was insoluble in the mobile phase.

Answer: Pigment A

c) State the pigment was most attracted to the mobile phase.

Answer: Pigment D

d) Calculate the Rf value of pigment D.

Answer: (11.5 - 2.0) ÷ (12 - 2.0) = 0.95

e) State the pigment that had a Rf value of 0.33

Answer: If pigment moved x cm, then x ÷ 10 = 0.33  →  x = 3.3 cm. Since the sample started at the 2.0 cm line on the paper, the pigment would be at 5.3 cm. This is pigment C.

13 Steel is an alloy made from iron with carbon added to it. The figure below shows how the strength of steel is affected by the amount of carbon added.

a) Explain why steel can be considered to be a formulation.

Answer: Steel is an alloy, which is a mixture. Each chemical has a particular purpose, and so is a special type of mixture called a formulation.

b) Describe what the graph in the figure above shows.

Answer: As the percentage carbon increases, so does the strength. From about 0.8% there is very little change in strength as more carbon is added.

c) State the tensile strength of steel with 0.6% carbon. Give your answer in Pa using standard form.

Answer: 800 MPa  or  8 × 1010 Pa

14 Aspirin (C9H8O4 ) can be made into a tablet and is a medicine that can help to relieve pain. A pure sample of aspirin has a melting point of 136°C.

a) Explain why the melting point of an aspirin tablet from a blister pack will not be exactly 136°C.

Answer: The tablet is not made just from aspirin and instead is a formulation (mixture). It will not have a definite melting point or will melt over a range.

b) A tablet of a painkilling medication contains aspirin (to relieve pain), chalk (to bulk out the tablet) and artificial sweeteners (to make the tablet taste better). Classify this substance.

Answer: formulation