Topic 8 Chemical analysis

What is an element?

• Made up of only one atom

• Pure

What is a compound?

• Made up of two or more different types of atoms chemically bonded together

• Pure

What is a mixture?

• Made up of two or more different types of elements or compounds NOT chemically bonded together

• Not pure

What is a pure substance?

A single element or compound that is not mixed with any other substance

How can pure elements and compounds be identified using physical properties?

• Pure elements and compounds melt and boil at specific temperatures

• Whereas impure substances melt and boil over a range of temperatures

Why is melting point and boiling point data useful?

It can be used to distinguish pure substances from mixtures

What does “pure” mean in everyday language?

A substance that has had nothing added to it (unadulterated) and in its natural state - e.g pure milk

Why is this water pure?

Because both the melting and boiling points are at specific temperatures

Why is this water impure?

Because the water melts and boils over a range of temperatures

What is a formulation?

• A formulation is a mixture that has been designed as a useful product

• In a formulation, the quantity of each component is carefully measured so that the product has the properties needed

Examples of formulations

• Fuels

• Cleaning prodcuts

• Paints

• Medicines

• Alloys

• Fertilisers

• Foods

What is chromatography used for?

To separate mixtures based on their solubilities and provide information to help identify substances

What are the two phases involved in chromatography?

• Stationary phase (does not move)

• Mobile phase (moves through the stationary phase, carrying substances)

How does a pure compound appear on a chromatogram?

It produces a single spot in all solvents

How can chromatography distinguish pure from impure substances?

Impure substances produce multiple spots

Why do we draw the starting line in pencil?

If we drew the line in pen, the pen ink would move up the paper with the solvent

What determines how substances separate in chromatography?

The distribution of substances between the stationary and mobile phases

How is the Rf value calculated?

• Rf​ = distance moved by substance / distance moved by solvent

• Rf values do not have a unit

How can Rf values help identify substances?

Different compounds have different Rf values in different solvents, so comparing Rf values with known data can help identify them

Other factors to consider in paper chromatography

• Several different substances could have the same Rf value. We need to repeat the experiment using a different solvent

• If this substance has never been analysed before, then there will not be an Rf value on the database

Chromatography practical

1) Draw a pencil baseline 2 cm from the bottom of chromatography paper and mark spots for samples

2) Use a capillary tube to place small spots of known substances and the unknown on the line

3) Pour solvent (e.g water) into a beaker so the depth is below the baseline

4) Attach the paper to a glass rod using tape and lower the paper in the beaker so the bottom touches the solvent but the spots stay above it - the sides of the paper must not touch the walls of the beaker

5) Cover the beaker with a lid and allow the solvent to travel up the paper (do not move the beaker)

6) When the solvent is about ¾ up the paper, remove it and mark point where the water reached with a pencil (solvent front)

7) Let the paper dry, then compare the spots

8) Measure distances to calculate Rf values to find chemicals in the colours

Why must the solvent be above the baseline?

• Because the samples would dissolve directly into the solvent causing the dyes to wash off the paper instead of separating

Why must the paper not touch the sides of the paper?

Because the solvent would not be able to travel evenly up the paper causing separation to become uneven and results are less reliable

How do you test for hydrogen gas and what is the result?

• A test tube contains a gas which might be hydrogen

• To test for hydrogen, insert a burning splint into test tube

• Hydrogen burns rapidly with a pop sound

How do you test for oxygen and what is the result?

• A test tube contains a gas which might be oxygen

• To test for oxygen, use a glowing splint and insert into a test tube of the gas

• The splint relights in oxygen

How do you test for carbon dioxide and what is the result?

• To test for carbon dioxide, use limewater which is an aqueous solution of calcium hydroxide

• When carbon dioxide is shaken with or bubbled through limewater the limewater turns cloudy (milky)

What is limewater?

• An aqueous solution of calcium hydroxide

How do you test for chlorine and what is the result?

• To test for chlorine, insert damp litmus paper into the test tube

• When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white