Chemistry - 12 Chemical Analysis

Pure substance

sample of matter made up of just one element or compound

Fixed point

melting and boiling point of a substance

How can you test that a sample is pure?

sample changes state all at once, rather than over a range of temperatures

A pure substance changes state...

at a fixed temperature

A mixture changes state...

over a range of temperatures

Impurities tend to ... the melting point and ... the boiling point

lower, raise

Formulation

mixture that has been designed as a useful product

Common formulations [5]

- medicine

- paint

- cleaning agents

- fuels

- food products

Non-active ingredients in medicine [7]

- colorants

- sweeteners

- coatings

- fillers

- binding agents

- ingredients to aid dissolving in effective part of GI tract

- ingredients to prevent upset stomach

Ingredients in paint [3]

- pigment

- binder

- solvent

Function of binder in paint

causes paint to adhere to surfaces and form a protective film

Function of solvent in paint

causes pigment and binder to spread well by thinning

Ingredients in cleaning agents [4]

- surfactant

- water

- colouring and fragrance additives

- rinse agent

Function of surfactant in cleaning agents

detergent which removes grease

Function of rinse agent in cleaning agents

allows water to drain off crockery

Chromatography

laboratory technique used to separate mixtures of molecules based on solubility

Paper chromatography

technique used to separate and observe the different pigments in a material by their solubility in a solvent on paper

Phases of chromatography [2]

- mobile phase

- stationary phase

Mobile phase

phase that moves in chromatography

Stationary phase

phase that does not move in chromatography

The substance which travels the most has the strongest attraction to the (solvent/paper)

solvent

The substance which travels the least has the strongest attraction to the (solvent/paper)

paper

Adsorb

to attract and stick to a surface

How can unknown substances be identified using chromatography?

separated substances on a chromatogram can be compared with a database of other substances

Retention factor

ratio that represents how far a compound travels in a particular solvent

Retention factor (Rբ) =

(distance moved by substance) ÷ (distance moved by solvent)

Why is chromatography data stored as retention factors?

it eliminates the effects of other factors, so variables do not need to be closely controlled each time a chromatogram is created

Positive test for hydrogen gas

lit splint makes a 'squeaky pop' sound

Positive test for oxygen gas

glowing splint relights

Positive test for carbon dioxide gas

limewater turns milky/cloudy white when gas is bubbled through it

Positive test for chlorine gas

damp blue litmus paper is bleached white

Tests for positive ions [2]

- flame tests

- sodium hydroxide

What kind of ions are tested for with flame tests?

metal cations

Flame test

testing chemicals by burning them to look at their flame colours

Performing a flame test [4]

- nichrome wire loop dipped in hydrochloric acid, heated, re-dipped

- loop dipped in metal compound being tested

- loop held in roaring blue flame of Bunsen burner

- colour of flame observed to determine chemical

Why is the nichrome wire loop dipped in hydrochloric acid and heated?

to clean it and avoid contamination

Limitations of flame tests [2]

- mixture of metals may produce indistinguishable flame colours

- colours are subjective (human error)

Lithium ion (Li⁺) flame colour

crimson

Sodium ion (Na⁺) flame colour

yellow

Potassium ion (K⁺) flame colour

lilac

Calcium ion (Ca²⁺) flame colour

orange-red

Copper ion (Cu²⁺) flame colour

green

What should be observed when sodium hydroxide is added to a metal compound?

formation of a precipitate and its colour

Metals ions forming a white precipitate [3]

- Aluminium (Al³⁺)

- Calcium (Ca²⁺)

- Magnesium (Mg²⁺)

What is different about the white precipitate formed with sodium hydroxide and aluminium ions?

it re-dissolves in excess sodium hydroxide

Metal ions forming a coloured precipitate [3]

- Copper(II) (Cu²⁺)

- Iron(II) (Fe²⁺)

- Iron(III) (Fe³⁺)

Colour of copper(II) (Cu²⁺) precipitate

blue

Colour of iron(II) (Fe²⁺) precipitate

green

Colour of iron(III) (Fe³⁺) precipitate

brown

Positive test for carbonate ions

gas produced by adding a dilute acid tests positive as carbon dioxide

Limewater test [3]

- acidic carbon dioxide gas reacts with alkaline calcium hydroxide

- white precipitate of calcium carbonate formed

- this causes cloudiness/milkiness

Positive test for halide ions

precipitate is formed when dilute nitric acid and silver nitrate added

Why is nitric acid used in the test for halide ions rather than any other acid?

any other acid will produce precipitates with the silver ions

What is the function of the nitric acid in the test for halide ions?

dissolves the compound and removes any carbonate ions, as they would form a precipitate with silver ions

Iodide ions (I⁻) precipitate colour with nitric acid and silver nitrate

yellow

Bromide ions (Br⁻) precipitate colour with nitric acid and silver nitrate

cream

Chloride ions (Cl⁻) precipitate colour with nitric acid and silver nitrate

white

What is the precipitate formed in the test for halide ions with nitric acid and silver nitrate?

silver halide

Positive test for sulfate ions

white precipitate formed when hydrochloric acid and barium chloride added

Why is hydrochloric acid used in the test for sulfate ions rather than any other acid?

any other acid will produce a precipitate with barium ions

What is the function of the hydrochloric acid in the test for sulfate ions?

removes carbonate ions that would otherwise form a precipitate with barium ions

What is the precipitate formed in the test for sulfate ions with hydrochloric acid and barium chloride?

barium sulfate

Instrumental analysis

chemical tests using machines to make them faster and more accurate

Advantages of instrumental analysis [3]

- highly accurate and sensitive

- faster than manual tests

- enable analysis of small samples

Disadvantages of instrumental analysis [3]

- usually very expensive

- special training is needed to use machines

- results can often only be interpreted by comparison with data from known substances

Spectroscope

device which measures spectrum of light

Flame emission spectroscopy

method of instrumental analysis that records the colour of light given out by a metal compound when it is burned

What can a spectrometer measure? [2]

- frequency of light waves (colour)

- intensity/absorbance of light

What releases the light when a metal ion is heated? [3]

- heat causes electrons in metal ions to move to higher energy levels/shells

- when they return, the energy is released in the form of light

- different metals release light with distinct wavelengths

Flame emission spectroscopy [3]

- sample is heated and releases light of specific wavelength, unique to particular metal

- spectrometer detects wavelength and compares against known ions

- spectrometer detects intensity/absorbance to determine concentration of ions

What does the wavelength of light detected by the spectrometer inform us about the metal ions?

type of metal ion

What does the intensity/absorbance of light detected by the spectrometer inform us about the metal ions?

concentration of metal ion

Uses of flame emission spectroscopy [2]

- testing drinking water for heavy metals

- controlling trace metals in steel industry