Chemistry – Key Topic 8
4.8 Chemical analysis
4.8.1 Purity, formulations and chromatography
4.8.1.1 Pure substances
In chemistry, a pure substance is a single element or compounds not mixed with any
other substance
Pure elements and compounds melt and boil at specific temperatures. Melting point
and boiling point data can be used to distinguish pure substances from mixtures
- The closer your measured value is to the actual melting or boiling point, the purer
the sample is
- Impurities will lower the melting point and increase the melting range of a
substance
- They will also increase the boiling point and may result in a sample boiling over a
range of temperatures
In everyday language, a pure substance can mean a substance that has had nothing
added to it, so it is unadulterated and in its natural state
4.8.1.2 Formulations
A formulation is a mixture that has been designed as a useful product. Many products
are complex mixtures in which each chemical has a particular purpose. Formulations
are made by mixing the components in carefully measured quantities to ensure that
the product has the required properties. Formulations include fuels, cleaning agents,
paints, medicines, alloys, fertilisers and foods.
- they are really important in the pharmaceutical industry., e.g. by altering the
formulation of a pill, chemists can make sure it delivers the drug to the correct part of
the body at the right concentration, that its consumable and has a long enough shelf
life
Paints are a common formulation. They will contain:
- pigment to provide colour
- a binder to help the paint attach itself to an object
- a solvent to help it spread out well
, 4.8.1.3 Chromatography
Chromatography can be used to separate mixtures and can give information to help
identify substances. Chromatography involves a stationary phase and a mobile phase.
Separation depends on the distribution of substances between the phases.
- The mobile phase moves through the stationary phase carrying components of the
mixture with it
- A substance with stronger forces of attraction between itself and the stationary
phase will be carried at a greater distance in a given time
The ratio of the distance moved by a compound (centre of spot from origin) to the
distance moved by the solvent can be expressed as its Rf value:
- Rf = distance moved by substance/distance moved by solvent
Different compounds have different Rf values in different solvents, which can be used
to help identify the compounds. The compounds in a mixture may separate into
different spots depending on the solvent but a pure compound will produce a single
spot in all solvents.
4.8.2 Identification of common gases
4.8.2.1 Test for hydrogen
The test for hydrogen uses a burning splint held at the open end of a test tube of the
gas. Hydrogen burns rapidly with a pop sound
4.8.2.2 Test for oxygen
The test for oxygen uses a glowing splint inserted into a test tube of gas. The splint
relights in oxygen
4.8.2.3 Test for carbon dioxide
The test for carbon dioxide uses an aqueous solution of calcium hydroxide. When
carbon dioxide is shaken with or bubbled through limewater, the limewater turns
milky
4.8.2.4 Test for chlorine
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine
gas the litmus paper is bleached and turns white.
4.8.3 Identification of ions by chemical and spectroscopic means
4.8 Chemical analysis
4.8.1 Purity, formulations and chromatography
4.8.1.1 Pure substances
In chemistry, a pure substance is a single element or compounds not mixed with any
other substance
Pure elements and compounds melt and boil at specific temperatures. Melting point
and boiling point data can be used to distinguish pure substances from mixtures
- The closer your measured value is to the actual melting or boiling point, the purer
the sample is
- Impurities will lower the melting point and increase the melting range of a
substance
- They will also increase the boiling point and may result in a sample boiling over a
range of temperatures
In everyday language, a pure substance can mean a substance that has had nothing
added to it, so it is unadulterated and in its natural state
4.8.1.2 Formulations
A formulation is a mixture that has been designed as a useful product. Many products
are complex mixtures in which each chemical has a particular purpose. Formulations
are made by mixing the components in carefully measured quantities to ensure that
the product has the required properties. Formulations include fuels, cleaning agents,
paints, medicines, alloys, fertilisers and foods.
- they are really important in the pharmaceutical industry., e.g. by altering the
formulation of a pill, chemists can make sure it delivers the drug to the correct part of
the body at the right concentration, that its consumable and has a long enough shelf
life
Paints are a common formulation. They will contain:
- pigment to provide colour
- a binder to help the paint attach itself to an object
- a solvent to help it spread out well
, 4.8.1.3 Chromatography
Chromatography can be used to separate mixtures and can give information to help
identify substances. Chromatography involves a stationary phase and a mobile phase.
Separation depends on the distribution of substances between the phases.
- The mobile phase moves through the stationary phase carrying components of the
mixture with it
- A substance with stronger forces of attraction between itself and the stationary
phase will be carried at a greater distance in a given time
The ratio of the distance moved by a compound (centre of spot from origin) to the
distance moved by the solvent can be expressed as its Rf value:
- Rf = distance moved by substance/distance moved by solvent
Different compounds have different Rf values in different solvents, which can be used
to help identify the compounds. The compounds in a mixture may separate into
different spots depending on the solvent but a pure compound will produce a single
spot in all solvents.
4.8.2 Identification of common gases
4.8.2.1 Test for hydrogen
The test for hydrogen uses a burning splint held at the open end of a test tube of the
gas. Hydrogen burns rapidly with a pop sound
4.8.2.2 Test for oxygen
The test for oxygen uses a glowing splint inserted into a test tube of gas. The splint
relights in oxygen
4.8.2.3 Test for carbon dioxide
The test for carbon dioxide uses an aqueous solution of calcium hydroxide. When
carbon dioxide is shaken with or bubbled through limewater, the limewater turns
milky
4.8.2.4 Test for chlorine
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine
gas the litmus paper is bleached and turns white.
4.8.3 Identification of ions by chemical and spectroscopic means
