Ch [T6] - Chemical Analysis

Question 1

What is a pure substance?

Answer 1

A substance that contains one __compound__ or one __element__ throughout

Question 2

How can you test if a substance is pure?

Answer 2

• Pure substances have fixed, specific melting and boiling points

Therefore…

• Measure the MP or BP and check it against the known data in a data book

Question 3

What is a formulation?

Answer 3

A carefully designed mixture with a specific purpose containing many ingredients in very specific amounts

Question 4

How can you test for hydrogen gas, and what will you see?

Answer 4

Test: hold a lit splint near the gas

Observe: hear a squeaky pop

Question 5

How can you test for oxygen gas, and what will you see?

Answer 5

Test: hold a glowing splint in the gas

Observe: splint relights

Question 6

How can you test for carbon dioxide gas, and what will you see?

Answer 6

Test: bubble the gas through limewater

Observe: limewater turns milky

Question 7

How can you test for chlorine gas, and what will you see?

Answer 7

Test: place damp blue litmus paper in the gas

Observe: damp blue litmus paper turns __red__ then __white__

Question 8

What is an Rf value, and how do you calculate it?

Answer 8

(Retention factor)

An Rf value is a ratio that’s calculated for each substance which is compared with published reference values to identify a substance

Rf = __distance travelled by sample__
distance travelled by solvent

Question 9

Flame Tests: ions and their flame colours

Answer 9

• Crimson: lithium Li+ ions present
• Yellow: sodium Na+ ions present
• Lilac: potassium K+ ions present
• Orange-red: calcium Ca²+ ions present
• Green: copper Cu²+ ions present

Question 10

Flame Tests: disadvantages

Answer 10

• Only works with samples with single ions
• If sample contains >1 ion, then 1 or more colours may be masked by the other

Question 11

Instrumental Analysis: how does flame emission spectroscopy work?

Answer 11

• Sample is put into a flame and the light given out from the sample is placed through a spectroscope - The output of this is a line spectra
• A line spectra can be used to identify metal ions in the solutions and measure their concentrations
• Line spectra of unknown sample is compared with known “reference” line spectra in data books

Question 12

Instrumental Analysis: how can flame emission spectroscopy be used to measure concentration?

Answer 12

• Absorbance of a particular wavelength of light is measure for solutions of metal ions of a known concentration
• Concentration determined using a graph: absorbance (y axis) against concentration in ppm (x axis)

Question 13

Sodium hydroxide solution Tests: how it works

Answer 13

Place a pipette of the solution containing ions in a test tube.

Add ~0.5cm³ of sodium hydroxide (NaOH) solution in drops.

If a change happens, add a further pipette of NaOH - one substance that has a change occur will change again.

Question 14

Sodium hydroxide solution Tests: ions and their changes

Answer 14

• Copper (II) Cu²+: blue precipitate, no change after second pipette
• Iron (II) Fe²+: green precipitate, no change after second pipette
• Iron (III) Fe³+: brown precipitate, no change after second pipette
• Aluminium Al³+: white precipitate, redissolves into colourless solution after second pipette
• Magnesium Mg²+: white precipitate, no change after second pipette
• Calcium Ca²+: white precipitate, no change after second pipette

Question 15

Sodium hydroxide solution Tests: ions and their equations

Answer 15

Tip: you only need to memorize one equation and all the ion formulas - there’s a pattern!

Cu²+ + 2OH —–> Cu(OH)2
Fe²+ + 2OH —–> Fe(OH)2
Fe³+ + 2OH —–> Fe(OH)3
Al³+ + 2OH —–> Al(OH)3
Mg²+ + 2OH —–> Mg(OH)2
Ca²+ + 2OH —–> Ca(OH)2

Question 16

Halide Ions Tests: how it works

Answer 16

Add a pipette of silver nitrate (AgNO3) to a pipette of each chalice solution in a test tube

Question 17

Halide Ions Tests: ions and their reactions

Answer 17

• Chloride Cl-: white precipitate
• Bromide Br-: cream precipitate
• Iodide I-: yellow precipitate

Question 18

Halide Ions Tests: equations

Answer 18

Tip: only learn one and all the ion formulas - there’s a pattern!

• Ag+ + Cl- —–> AgCl
• Ag+ + Br- —–> AgBr
• Ag+ + I- —–> AgI

Question 19

Carbonate ions Test: how it works and ion formula

Answer 19

• Add 5cm³ HCl acid to a spatula of the solid carbonate in a boiling tube
• Bubble the gas produced through lime water using a delivery tube

Carbonate: Co3²-

Question 20

Carbonate ions Test: what will you see if there’s carbonate ions present?

Answer 20

Lime water goes milky

Question 21

Sulfate ions Test: how it works and ion formula

Answer 21

• Add a few drops of HCl acid to a pipette of a sulfate solution in a test tube
• Then add a pipette of barium chloride (BaCl2)

Sulfate: SO4²-

Question 22

Sulfate ions Test: what will you see if there are sulfate ions present?

Answer 22

Milky white BaSo4 precipitate