topic 8- chemical analysis

What is a pure substance

Something that contains only one type of compound or element

What information can you use to identify an unknown substance

The melting point and boiling point

What will a chemically pure substance do

Will melt or boil at a specific temperature

What is a physical test

Testing (observing, measuring) the physical properties of a substance eg melting point

What is a chemical reaction

A reaction with another chemical to identify a substance

Impure substances melt and boil…

…over a range of temperatures (depending on how much of each substance there is in the mixture)

What does having impurities in a substance do to the melting and boiling point

Lowers melting point, increases boiling point

What is a formulation

Useful mixtures with a precise purpose that are made by following a specific formula with precise amounts of different compounds

What are some examples of formulations

Fuels, paint, medicines, alloys, fertilisers, cleaning agents, food and drink

Different components are always…

…present in the same proportions and each of the different components may contribute to a different property

What is chromatography

An analytical technique used to separate the different substances in a mixture, so it can be identified.

What is paper chromatography used for

Used to separate the different dyes in an ink

Different dyes will travel up the paper at…

…different rates

What does it mean if the ink sample stays on baseline

Chemicals aren’t soluble in the solvent; can’t dissolve and move up the paper

What is a chromatogram

The pattern of doors left in paper chromatography

What determines how far each chemical travels

The properties of the substance

What is the mobile phase

(Substance) where molecules can move in, usually liquids or gas

What is the stationary phase

(Substance or material) where molecules can’t move in, usually solid or really thick liquid

What determines how fast the chemicals move up the paper

How much time chemicals in ink sample spend in mobile or stationary phase

Where Does chemicals more soluble in the solvent spend more time in

More time in mobile phase so moves faster

Where does chemicals less soluble in the solvent or more attracted to the paper spend more time in

Stationary phase so moves slower

Write a method for chromatography

1) take a piece of filter paper and use a pencil to draw a line near the bottom of the sheet (baseline)

2) add sample of ink to the pencil line

3) fill a beaker with a shallow amount of solvent

4) place filter paper into solvent making sure not to submerge the pencil line and spot(s) of ink

5) wait for solvent to travel up paper as it does the different dyes that make up the ink will dissolve in solvent and move up with it

How can you stop the solvent from evaporating

Placing a lid on top of beaker

What is the formula for Rf value

Rf value = distance travelled by substance/ distance travelled by solvent

For which type of substance will there be a single spot on the chromatogram

Pure substance

You can calculate a ____ Rf value for pure substances

Specific

Why could the Rf value of the same substance vary

If type of paper and solvent used is different

what is the test and result for chlorine

Damp blue litmus paper into test tube of sample gas - bleaches it

What might briefly happen with chlorine test

Turns red as solution is acidic

What is the test and result for oxygen

Glowing splint inside test tube of sample gas - relights glowing splint

What is the test and result for hydrogen

Lit splint at open end of test tube of sample gas - burns with a squeaky pop

Why do we get a squeaky pop for hydrogen test

The hydrogen is burning quickly with oxygen in the air to form water

What is the test and result for carbon dioxide

Bubble gas through solution of calcium hydroxide in water (lime water) - solution turns cloudy

Why does the solution become cloudy in the carbon dioxide test

caused by the carbon dioxide and calcium hydroxide reacting to form calcium carbonate

What is the flame tests for metal ions

Lithium = crimson, sodium = yellow, potassium = lilac, calcium = orange-red, copper = green

How do you do the flame tests

By dipping a clean niche one wire loop in some hydrochloric acid and then holding it in a blue flame from a Bunsen burner until it burns without any colour. Then dipping the loop into the sample you want to test and put it back in the flame

Why does the flame tests only work for samples that contain a single metal ion

The flame colours of some ions may be hidden by the colours of others

What are the precipitation (colour) reactions (with sodium hydroxide) test for metal ions

Calcium and magnesium = white, copper = blue, iron(II) = green, iron(III) = brown, aluminium = white at first then rediscover in excess sodium hydroxide to form a colourless solution

What is the test and result for sulfate ions

Add dilute hydrochloric acid followed by barium chloride solution - forms a white precipitate of barium sulfate

Why is dilute hydrochloric acid used for sulfate ions test

It reacts with and removes any carbonate ions present since if carbonate ions are present when adding barium chloride it would produce a white precipitate which would confuse the results

What is the test for halide ions

Add dilute nitric acid (reacts with and removes any carbonate and sulphite impurities) followed by silver nitrate solution

What are the results for halide ions

Chloride = white precipitate, bromide = cream precipitate, iodide = yellow precipitate

Why is dilute nitric acid used instead of hydrochloric acid for halide ions test

If hydrochloric acid is used it would introduce chloride ions to solution so a white precipitate would be formed regardless

How does flame emission spectroscopy work

When metal ions are heated they emit light, the light passes through a spectroscope which can detect different wavelengths of light to produce a line spectrum

What does the combination of wavelengths emitted by an ion depend on

Charge and electron arrangement

Why do different ions emit different wavelengths of light

No two ions have the same charge and election arrangement

What does the intensity of lines on spectrum indicate

Concentration of ion in solution

What are the advantages of using instrumental analysis

Very sensitive, fast, accurate

What are the advantages of manual tests

Basic and cheap