1(b) - Elements, Mixtures and Compounds

Question 1

What is an element?

Answer 1

The simplest type of substances made up of only one type of atom.

Question 2

What is a compound?

Answer 2

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

Question 3

What is a mixture?

Answer 3

A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined.

Question 4

What is oxygen classified as?

Answer 4

oxygen (O2) is diatomic because they contain two atoms.

Question 5

What is the list of elements that are diatomic?

Answer 5

• Hydrogen (H2)
• Nitrogen (N2)
• Fluorine (F2)
• Oxygen (O2)
• Iodine (I2)
• Chlorine (Cl2)
• Bromine (Br2)

Question 6

What is the melting/ boiling point like of pure substances?

Answer 6

Pure substances, such as an element or a compound, melt and boil at fixed temperatures.

Question 7

What is the melting/boiling point like for mixtures, give examples using water?

Answer 7

mixtures melt and boil over a range of temperatures.

Example: although pure water boils at 100⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will raise the boiling point to 100.2⁰C.

Example: although pure water melts at 0⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will lower the melting point to -0.6⁰C.

Question 7

What is the melting/boiling point like for mixtures, give examples using water?

Answer 7

mixtures melt and boil over a range of temperatures.

Example: although pure water boils at 100⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will raise the boiling point to 100.2⁰C.

Example: although pure water melts at 0⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will lower the melting point to -0.6⁰C.

Question 8

What is simple distillation used for?

Answer 8

This method is used to separate a liquid from a solution. For example: separating water from salt water.

Question 9

How does simple distillation work?

Answer 9

The saltwater is boiled. The water vapour condenses back into a liquid when passed through the condenser. The salt is left behind in the flask.

Note: cold water is passed into the bottom of the condenser and out through the top so that the condenser completely fills up with water.

Question 10

What is fractional distillation?

Answer 10

This method is used to separate a mixture of different liquids that have different boiling points. For example, separating alcohol from a mixture of alcohol and water.

Question 11

How does fractional distillation work?

Answer 11

Water boils at 100oC and alcohol boils at 78oC. By using the thermometer to carefully control of temperature of the column, keeping it at 78oC, only the alcohol remains as vapour all the way up to the top of the column and passes into the condenser.

The alcohol vapours then condense back into a liquid.

Question 12

What is filtration?

Answer 12

This method is used to separate an insoluble solid from a liquid. For example: separating sand from a mixture of sand and water.

Question 13

How does filtration work (sand from water)?

Answer 13

The mixture is poured into the filter paper. The sand does not pass through and is left behind (residue) but the water passes through the filter paper and is collected in the conical flask (filtrate).

Question 14

What is crystallisation?

Answer 14

This method is used to obtain a salt which contains water of crystallisation from a salt solution. For example: hydrated copper sulfate crystals (CuSO4.5H2O(s)) from copper sulfate solution (CuSO4(aq)).

Question 15

How does crystallisation work?

Answer 15

• Gently heat the solution in an evaporating basin
• until crystals form on a glass rod (which shows that a hot saturated solution has formed).
• Leave to cool and crystallise.
• Filter to remove the crystals.
• Dry by leaving in a warm place.
  If instead the solution is heated until all the water evaporates, you would produce a powder of anhydrous copper sulfate (CuSO4(s)).

Question 16

What is paper chromatography?

Answer 16

This method can be used to separate the parts of a mixture into their components. For example, the different dyes in ink can all be separated and identified.

Question 17

How does paper chromatography work?

Answer 17

The coloured mixture to be separated (e.g. a food dye) is dissolved in a solvent like water or ethanol and carefully spotted onto the chromatography paper on the baseline, which is drawn in pencil so it doesn’t ‘run or smudge’.

The paper is carefully dipped into the solvent and suspended so the baseline is above the liquid solvent, otherwise all the spots would dissolve in the solvent. The solvent is absorbed into the paper and rises up it as it soaks into the paper. The choice of solvent depends on the solubility of the dye. If the dye does not dissolve in water then normally an organic solvent (e.g. ethanol) is used.

As the solvent rises up the paper it will carry the dyes with it. Each different dye will move up the paper at different rates depending on how strongly they stick to the paper and how soluble they are in the solvent.

Question 18

What is used paper chromatography used for?

Answer 18

Paper chromatography can be used to investigate the composition of a mixture.

Question 19

How does the chromatogram provide information about the composition of a mixture?

Answer 19

A baseline is drawn on the paper. The mixture is spotted onto the baseline alongside known or standard reference materials. The end of the paper is then put into a solvent which runs up the paper and through the spots, taking some or all of the dyes with it.

Different dyes will travel different heights up the paper.

The resulting pattern of dyes is called a chromatogram.

Question 20

Rf Value

Answer 20

Calculated as the distance moved by a spot divided by the distance moved by the solvent front on a chromatogram.

Question 21

How do you calculate the Rf value?

Answer 21

Rf = distance of dye from baseline/distance of solvent from baseline

For each dye, the Rf value is calculated. To do this, 2 distances are measured:

• The distance between the baseline and the dye
• The distance between the baseline and the solvent front, which is how far the solvent has travelled from the baseline

Question 22

What does the Rf Value tell you?

Answer 22

If the Rf value of one of the components of the mixture equals the Rf value of one of the standard reference materials then that component is know to be that reference material.

Note that because the solvent always travels at least as far as the highest dye, the Rf value is always between 0 and 1.

Question 23

What does the Rf value tell you about how soluble dyes are?

Answer 23

Dyes that are more soluble will have higher Rf values than less soluble dyes. In other words, more soluble dyes move further up the paper. The extreme case of this is for insoluble dyes which don’t move at all (Rf value = 0). The other aspect affecting how far a dye travels is the affinity that dye has for the paper (how well it ‘sticks’ to the paper).

Question 24

Explain the practices using paper chromatography using inks/food colourings?

Answer 24

1. A pencil line (baseline) is drawn 1cm from the bottom of the paper. Pencil will not dissolve in the solvent, but if ink were used instead it might dissolve and interfere with the results of the chromatography.
2. A spot of each sample of dye is dropped at different points along the baseline.
3. The paper is suspended in a beaker which contains a small amount of solvent. The bottom of the paper should be touching the solvent, but the baseline with the dyes should be above the level of the solvent. This is important so the dyes don’t simply dissolve into the solvent in the beaker.
4. A lid should cover the beaker so the atomosphere becomes saturated with the solvent. This is so the solvent does not evaporate from the surface of the paper.
5. When the solvent has travelled to near the top of the paper, the paper is removed from the solvent and a pencil line drawn (and labelled) to show the level the solvent reached up the paper. This is called the solvent front.
6. The chromatogram is then left to dry so that all the solvent evaporates.

Common solvents are water or ethanol. The choice of solvent depends on whether most of the dyes are soluble in that solvent.