4.8 Chemical analysis

Completed flashcard set

Define pure substance in chemistry and in everyday language

Chemistry: A pure substance is a single element or compound not mixed with any other substance

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

What is true about pure elements and compounds?

Pure elements and compounds melt and boil at specific temperatures

State a way that you can distinguish a pure substance from a mixture

Pure elements and compounds melt at specific temperatures.

• Melting and boiling point data can be used to distinguish pure substances from mixtures

Define formulation

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

• Many products are complex mixtures in which each chemical is present in a measured quantity and has a particular purpose

Describe how formulations are made

Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties

Give examples of formulations

• Fuels

• Cleaning agents

• Paints

• Medicines

• Alloys

• Fertilisers

• Foods

What can chromatography be used for?

• To separate substances out of mixtures based on their solubilities

• It can then be used to identify the substances

Describe the two phases of chromatography

• Mobile phase: where the molecules can move

  • The solvent is the mobile phase because it moves

• Stationary phase: where the molecules cannot move

  • The paper is the stationary phase because it does not move

What is the R_f value and give its formula

R_f value is the ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent

R_f = distance moved by substance / distance moved by solvent

Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances

• Different compounds have different R_f values in different solvents

• 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 determines which substances move further up the chromatography paper

• Molecules with higher solubility in the solvent and which are less attracted to the paper will spend more time in the mobile phase and will therefore move further

Explain what separation of a substance in chromatography depends on

• The mobile phase moves through the stationary phase and anything dissolved in the mobile phase moves with it

• How quickly a substance moves depends on its distribution between the two phases - whether it spends more time in the mobile phase or the stationary phase

• Substances that spend more time in the mobile phase than the stationary phase will move further

Label the chromatogram

• Solvent front: the furthest part reached by the solvent

• Origin: must be in pencil as pencil is insoluble so will not dissolve in solvent and be carried up the paper

State the test for hydrogen

Give the result

• Burning splint held at the open end of a test tube of the gas

• Hydrogen burns rapidly with a squeaky pop sound

State the test for oxygen

Give the result

• Glowing splint is inserted into a test tube of the gas

• The splint relights in oxygen

State the test for carbon dioxide

Give the result

• Carbon dioxide is shaken with/ bubbled through an aqueous solution of calcium hydroxide (limewater)

• The limewater goes cloudy

State the test for chlorine

Give the result

• Damp litmus paper is put into chlorine gas

• The litmus paper is bleached and turns white

What is limewater?

An aqueous solution of calcium hydroxide

• (Think: hydroxide, alkaline, limewater used in cleaning)

What type of tests can be used to identify metal ions?

• Flame tests

  • Lithium, sodium, potassium, calcium and copper compounds produce distinctive colours in flame tests

• Sodium hydroxide solution

  • A precipitate can be formed which have characteristic colours

State the results of the flame tests

Memorise:

• Crimson lipstick

  • Crimson, Li (lithium)

  • Lithium compounds result in a crimson flame

• The curious green frog

  • Cu (copper), green

  • Copper compounds result in a green flame

• A yellow sun

  • Yellow, Sodium Na+

  • Sodium compounds result in a yellow flame

• California is hot! (red/orange)

  • Calcium, red

  • Calcium compounds result in an orange-red flame

• A pretty lilac in a flower Pot

  • Lilac, potassium

  • Potassium compounds result in a lilac flame

If a sample for a flame test contains a mixture of ions, what happens?

Some flame colours can be masked

Describe the sodium hydroxide tests for metal ions

Forms white metal hydroxide precipitates:

• Solutions of magnesium ions

• Solutions of calcium ions

• Solution of aluminium ions

  • Only aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution

* Remember: magnesium + calcium white food supplements, aluminium foil used for food, odd one out so dissolves

Forms coloured precipitates

• Solutions of copper(II) form blue

• Solutions of iron (II) form green

• Solutions of iron (III) form brown

* Remeber: copper = young boy, green brown is earthy (II, III)

Name the negative ions you need to be able to identify

• Carbonate CO₃^2- ions

• Halide ions

• Sulfates SO₄^2-

Describe how to identify carbonates

Carbonates contain CO₃^2- ions

• Add a sample in a test tube and add a few drops of dilute acid - carbon dioxide will be produced. Connect the tube to a tube of limewater. If carbonate ions are present, the carbon dioxide will released will turn limewater cloudy when it bubbles through it

Describe how to identify halide ions

Halide ions in a solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid

• Halide ions with silver nitrate solution

• Presence of dilute nitric acid

A silver halide forms

• Silver chloride: white

• Silver bromide: cream

• Silver iodide: yellow

Memorisation: swedish, hallo, silver jewellery, n, chlorine suncream pool, bro.., idied yellow gon

Describe how to identify sulfates

Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid

White precipitate barium sulfate forms

• Sulfate ions and barium chloride solution

• In the presence of dilute hydrochloric acid

If sulphate ions are present a white precipitate of barium sulfate will form.

What is instrumental method analysis?

Give advantages of this over chemical tests

Using specific scientific instruments instead of conducting tests

Advantages:

• Very sensitive; they can detect even very small amounts of substances

• Very fast

• Very accurate

• For flame emission spectroscopy:

  • If a sample contains multiple different metal ions, the line spectrum produced will show the lines for all of them

Give an example of an instrumental method used to identify substances

Flame emission spectroscopy

• Used to analyse metal ions in solutions

Describe the method of flame emission spectroscopy

• A sample is placed in a flame

• As the ions heat up, their electrons become excited (absorb energy and move up to a higher energy level)

• When they drop back to their original energy level, they transfer energy as light

• The light passes through a spectroscope which can detect different wavelengths of light to produce a line spectrum

• The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement

  • Since no two ions have the same charge and the same electron arrangement, different ions emit different wavelengths of light

• Therefore, each ion produces a different pattern of wavelengths and has a different line spectrum

• The intensity of the spectrum indicates the concentration of ion in the solution

Therefore, line spectrums can be used to identify ions in solution calculate their concentrations

What do spectroscopes do?

• Spectroscopes can detect different wavelengths of light to produce a line spectrum

Every different metal ions produces a unique pattern of wavelengths (as no two metal ions have the same electron arrangement and charge) so they all produce different line spectrums

The green ink contains more than two compounds. Suggest two reasons why only two spots are seen on the chromatogram

• Some of the compounds are colourless (in solution)

• Some compounds have the same Rf values