Topic 8 - Chemical Analysis Flashcards
What is a pure substance in chemistry?
In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
What is a pure substance in everyday language?
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, eg pure milk.
How can you measure the purity of an element?
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.
What will impurities in a sample do?
- Lower the melting point and increase the melting range of the substance
- Increase the boiling point and many result in the substance boiling over a range of temperatures
What is a formulation?
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.
What are some examples of formulations?(8)
Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers, cosmetics and foods.
What is Chromatography used for?
Chromatography can be used to separate mixtures and can give information to help identify substances.
What are the phases of chromatography?
Chromatography involves a stationary phase (where the molecules can’t move), and a mobile phase (where the molecules can move).
What does separation (the amount of time spent by the molecules in each phase) depend on?
Separation depends on the distribution of substances between the phases.
The amount of time the molecules spend in each phase depends on;
* how soluble they are in the solvent
* how attracted they are to the paper
How does Chromatography work?
- The solvent moves up the paper carrying the substances in the mixture - the more soluble a chemical, the more time it spends in the solvent, meaning it will move further up the paper.
- 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.
What is the Rf value?
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.
How do you calculate the Rf value?
Rf = distance travelled by substance / distance travelled by solvent
How are Rf values helpful?
Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. Rf values can be used to identify the components of an unknown mixture by comparison with Rf values of known substances.
What is the aim of required practical 6?
To investigate how paper chromatography can be used to separate and tell the difference between coloured substances. Students should calculate Rf values.
What is the method of Required Practical 6?(10)
- Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper.
- Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line.
- Use the fifth tube to put a small spot of the unknown mixture U on the line.
- Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil.
- Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots.
- Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper should dip into the solvent.
- Allow the solvent to travel undisturbed at least three quarters of the way up the paper.
- Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line.
- Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent.
- For each food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot.
What is the conclusion of required practical 6?
The use of chromatography and Rf values is a viable method of identifying unknown mixtures given reference material.
What is the 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 ‘squeaky pop’ sound.
What is the test for oxygen?
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
