Chemical Analysis

Write a 6 Mark answer for a chromatography experiment

• Draw pencil start line

• Place spot of ink on start line (using glass capillary tube)

• Name suitable solvent

• Place suitable solvent in beaker.

• Place paper in solvent so solvent is below start line

• Use a lid

• Allow dyes to travel at least ¾ up paper

• Allow it to dry

• Draw solvent line on wet edge

• Count spots

• Measure distance between 2 lines for:
  Rf = Distance moved by solute/distance moved by solvent

Why do you draw the start line in chromatography in pencil

Pencil won’t dissolve in solvent and be carried up paper

Why is the solvent line below the start line

Otherwise ink would dissolve in solvent and not be carried up

In chromatography, what type of spots will pure chemicals produce

one spot in all solvents

What is the stationary phase in chromatography

Paper

What is the mobile phase in chromatography

Solvent

How far would chemicals strongly attracted to the stationary phase move

Very far

What is the test for hydrogen

Lighted splint makes ‘pop’ sound and goes out

What is the test for oxygen

Glowing splint relights

What is the test for CO2

Bubbles through limewater. Turns cloudy white

What is the test for chlorine

Damp blue litmus paper turns white (gets bleached)

What are pure substances

Made up of only one type of element or compound. Can’t physically simplify anymore. Specific fixed boiling and melting points. Is important in like medicine but could have unwanted side effects.

What are impure substances

mixture of 2 or more different elements or compounds. Change way something looks, works or tastes. Decrease melting point and increase boiling point

What is the melting point depression

Lowers melting point

What is a formulation

Mixture with many different chemicals that don’t react with each other, but when mixed together, have desired effect. ALWAYS fixed amounts.

What type of ions are flame tests used for

positive

What is the method for the flame test

• Dip nichrome wire in concentrated HCl or heat it (sterilises/cleans it)

• Dip nichrome wire in solution/powder and hold tip of wire in blue bunsen burner flame.

• Observe colour change of flame

• Clean with HCl for reuse

Flame test Colour: Lithium

Crimson

Flame test Colour: Sodium

Yellow-orange

Flame test Colour: Potassium

Lilac

Flame test Colour: Calcium

Brick red/orange-red

Flame test Colour: Copper

Green

Flame test Colour: Magnesium

No colour

What is the test for metal cations

Sodium hydroxide

Metal cation NaOH: Calcium and magnesium

White precipitate. Use flame test to differentiate

Metal cation NaOH: Copper

Blue precipitate

Metal cation NaOH: Iron (II)

Green

Metal cation NaOH: Iron (III)

Brown

Metal cation NaOH: Aluminium

Soluble and dissolves in excess sodium hydroxide

What is the method to determine Negative carbonate ions

• Pour solution in tube

• Pour limewater in another tube

• Add HCl to solution

• Only if bubbles seen, use teat pipette to transfer gas produced to limewater

• Record results

• Only a carbonate solution would get bubbles as adding dilute acid produces CO2.

• Most carbonates don’t dissolve in water but group 1 carbonates do

• CO2 turns limewater cloudy

What is the method for negative Halide ions

• Pour solution in test tube

• Add few drops of dilute nitric acid then silver nitrate solution

• Nitric acid dissolves compound to remove carbonate ions

Negative Halide ions result: Iodide

Yellow precipitate

Negative Halide ions result: Bromide

Cream precipitate

Negative Halide ions result: Chloride

White precipitate

What is the method for negative sulfate ions

• Pour solution in test tube

• Add dilute HCl then barium chloride solution. HCl removes carbonate ions

• Observe results

Negative Sulfate ions results

White precipitate = is insoluble salt barium sulfate. Shows sulfate ions present. Sodium sulfate and sodium chloride turn cloudy

What is Flame emission Spectroscopy

• Method telling us metal ions are present in something from their characteristic line spectra and concentration of metal ions

How does FES work

• Sample heated in flame

• Energy excited electrons in metal ions. Jump into higher energy levels and releases light

• In spectrometer, wavelength of light analysed by passing through spectroscope.

• Each type of metal ion gives its own characteristic pattern of radiation called ‘line spectrum’

• Measure intensity (absorbance) of light with specific wavelength to show concentration

What are the advantages of FES

• Highly accurate. More sensitive that flame tests.

• Can save lives

• Quicker

• Small samples recognised easily

• Identify ion and concentration

What are the disadvantages of FES

• Delicate = expensive

• Complicated = Needs special training to use

• Gives results only interpreted by comparison with data from known substances