Chemical analysis

Define element

A substance made of 1 type of atom

Define compound

A pure substance containing two or more elements that have chemically combined

Define solute

A substance that has been dissolved

Define solvent

A liquid that dissolves a substance

Define solution

A liquid that has a substance dissolved in it

How can melting points help to identify a substance?

If the compound melts over a very narrow range (e.g 1⁰), it can be assumed that the substance is relatively pure

Define mixture

A group of different elements or compounds which are not chemically bonded together

Define molecule

a substance that contains two or more atoms held together by chemical bonds

Define chromatography

A chemical analysis technique, used to separate substances in a mixture

What is paper chromatography used for?

Separate a mixture of substances based on their solubility in a solvent

what is the name of the pencil lines in paper chromatography?

baseline

Why is a pencil used for the baseline?

pen ink would dissolve in the solvent and move up the paper

What are the phases of chromatography?

1. mobile phase - in which molecules can move, usually a liquid or gas

2. Stationary phase - in which molecules can’t move, usually a solid or a really thick liquid

What determines the amount of time molecules spend in the mobile and stationary phases?

1. How soluble they are in the solvent (mobile phase)

2. How attracted they are to the paper (stationary phase)

What happens to the chemicals that have greater solubility in the mobile phase?

A substance which is more soluble in the mobile phase will spend more time in the mobile phase. This means it will move faster, and therefore travel a further distance up the paper than a less soluble chemical

Rf formula

distance moved by substance/ distance travelled by the solvent

Define analytical chemistry

The science of separating, identifying and quantifying different substances

What is a chemically pure substance?

A single element or compound that hasn’t been mixed with any other substance

How can impurities affect the melting and boiling point of the sample?

1. Lower bp

2. increase bp

3. increase the range over which a substance melts or boils

What physical test can you do to check that a substance is pure?

Heat it until it melts or boils. If it melts or boils at a specific temperature (rather than over a range of temperatures) than it is pure

What is a formulation?

Mixtures that have been prepared for a specific purpose using a specific formula

What are formulations made from?

precise amounts of different components, and each component has a particular function

What substances might contain a formulation?

1. cleaning products

2. fuels

3. cosmetics

4. medicines

Oxygen test

oxygen will relight a glowing splint

Hydrogen test

Hydrogen will make a squeaky pop sound when exposed to a lit splint

Carbon dioxide test

When bubbled through limewater, it will turn the limewater cloudy

Chlorine gas test

It turns a damp litmus paper from red to white

What is used as the solvent in the chromatography rp?

Water

Where should the origin line be drawn on the chromatography paper?

2cm from the bottom short edge of the paper

What is the purpose of the solvent front line?

To mark how far the solvent travelled up the paper

What measurements are needed to calculate Rf values?

1. Distance between start line and solvent front line (solvent distance)

2. Distance between start line and the middle of each colour spot

What does a higher Rf value mean?

The higher the Rf value, the further up the chromatography paper the substance travelled

Why dry the paper before calculating Rf value?

To prevent the spots from continuing to move or diffuse, allowing accurate distance measurements to calculate Rf