Chemistry Qualitative

Qualitative Analysis

11AT/12RT

What exactly is qualitative analysis?

Qualitative Analysis is the branch of chemistry concerned with identifying which elements/ group of elements may be present in a compound.

How does it work?

Some elements have special behaviours/ properties that we can use to identify if they are present

Sometimes we can identify elements based on their general appearance alone

• Bromine - bromine is the only liquid non-metal and it is dark brown in colour

• Mercury- mercury is the only liquid metal and it is silver in colour

Other elements/ions with unique appearances are

• Copper- has a blue/green colour

• Iron (II) - pale green colour

• Iron (III)- orange brown colour

• Chlorine- yellow green gas

Multiple Choice

Identify the metal that has a blue-green appearance

• Iron (III)

• Bromine

• copper

• Mercury

Multiple Choice

An unlabelled bottle contains a pure element. It is in a liquid state and is dark-brown in colour. What is it most likely to be?

• Water

• Bromine

• Iodide

• Iron (III)

Identifying gases

Some gases can be identified by simple chemical tests or general observations

For example:

• Ammonium gas has a strong pungent odour

• Sulphur Dioxide gas has a strong, irritating odour similar to that of a just struck match

• Oxygen relights a glowing splint

• Nitrogen dioxide gas is orange-brown in colour and relights a glowing splint

• Carbon dioxide gas turns limewater a milky white colour

Multiple Select

What's happening in the chemical test pictured?

• A colourless gas

• A green gas

• A brown gas

• splint goes out

• splint relights

Multiple Choice

Identify the gas most likely to produce the result in the chemical test pictured?

• Sulphur dioxide

• Carbon dioxide

• Oxygen

• Nitrogen dioxide

Multiple Choice

A sample of an unknown colourless gas is flowed through lime water and the following reaction occurs. Which gas is most likely to cause this reaction?

• Oxygen

• carbon dioxide

• nitrogen dioxide

• sulphur dioxide

We can conduct flame tests to identify certain metals

Some metal elements, when exposed to an open flame, burn a characteristic colour. When we conduct a flame test, the colour that is produced can tell us which metals are present in the substance being burned

Poll

Think you can remember these colours?

• I think I got it

• What even is this?

Multiple Choice

A group one metal is introduced to a bunsen flame and it burns a brilliant lavender colour. Which element is most likely present?

• Sodium

• Potassium

• Barium

• Magnesium

Similarly to flame tests we can use heat to determine what a substance may be

• When exposed to heat, hydrogen makes a squeaky, popping sound

• Heated hydrogen sulphate leaves deposits of sulphur

• Heating ammonium will produce ammonia gas which sublimates on the cool part of the test tube

Litmus tests

That's right, litmus tests can tell us more than just if something is acidic or basic. It can be used to help identify some compounds

Here are some examples

• Hydrogen chloride will turn turn blue litmus paper red

• Chlorine will bleach blue litmus paper

• Ammonia turns red litmus paper blue

• Nitrogen dioxide turns red litmus paper blue

• Sulphur dioxide turns blue litmus paper red

Other paper tests include:

• Chlorine turns iodide paper a blue-black colour

• Sulphur dioxide turns dichromate paper green

• Hydrogen sulphide turns lead ethanoate paper a brown-black colour

• water vapour turns cobalt chloride paper pink

Multiple Choice

Your lab tech has brought you a sample of a strange yellow green gas. Which paper test would you use to confirm that it may be chlorine?

• Red Litmus Paper

• Cobalt chloride paper

• Blue litmus paper

• Dichromate paper

Precipitate tests

We can also conduct simple reactions and analyze the precipitates that form. Most commonly we conduct precipitate tests with either NaOH (sodium hydroxide) or NH₄OH (aqueous ammonia)

The following precipitates are common when the substance is reacted with NaOH

• sodium, calcium, magnesium, aluminium, lead and zinc all produce a white precipitate

• copper produces a pale blue, gelatinous precipitate

• ammonium produces ammonia gas

• Iron (II) produces a dirty green, gelatinous precipitate

• Iron (III) produces a rusty brown gelatinous precipitate

Multiple Choice

An unknown metallic compound was reacted with a sample of NaOH. A gelatinous precipitate with a brownish colour was achieved. Which metal ion is most likely present?

• Cu

• Fe ²⁺

• Zn

• Fe ³⁺

The following reactions are common with NH₄OH solution

• Sodium, magnesium, zinc, aluminium will produce a white precipitate

• Copper produces a pale blue solid precipitate or a royal blue liquid

• lead produces a green precipitate

• Iron (II) produces a dirty green gelatinous precipitate

• Iron (III) produces a rusty brown gelatinous precipitate

Multiple Choice

An unknown compound reacts with NaOH to produce a white precipitate. When reacted with NH₄OH a green precipitate is formed. Which metal ion is most likely present?

• Fe²⁺

• copper

• Sodium

• lead

Multiple Choice

Your lab tech has just brought you a sample of an unknown compound. It appears to be metallic in nature and you conduct a precipitate test with NaOH a solid white precipitate forms. Which test would you conduct next?

• Paper test

• Flame test

• Smell test

• throw it away

Multiple Choice

You conduct a flame test and the following result is produced, which metal ion is most likely present?

• Magnesium

• Sodium

• Calcium

• Barium

Solubility Tests

Sometimes the solubility of a substance gives it away

Soluble Salts

All salts formed from: Sodium, Potassium and ammonium are soluble

All nitrates are also soluble

Most chlorides are soluble except for:

• Silver chloride

• Mercury Chloride

• Lead Chloride

Most Sulphates are soluble except for:

• Barium Sulphate

• Lead Sulphate

• **Calcium sulphate is partially soluble

Most carbonates are INSOLUBLE except for

• Sodium Carbonate

• Potassium Carbonate

• Ammonium Carbonate

Multiple Choice

A metallic substance is being tested in the lab. It is found to be partially soluble in water and produces a brick red flame test. Which metal ion is definitely present?

• Calcium

• Magnesium

• Sodium

• Barium

Silver Nitrate solution can detect several ions based on their precipitates as well

• Chloride forms a white precipitate

• Iodide forms a yellow precipitate

• Bromide forms a cream-white precipitate

Specific tests

When all else fails there are specific tests that produce characteristic results with some compounds

Sulphate Ions

If we add hydrochloric acid and barium nitrate/chloride to a sulphate a white precipitate of BaSO₄ forms

Poll

Nitrate Ions can be identified by reacting with iron sulphate and an acid. A characteristic brown ring forms inside the reaction vessel

• I see it

• Wow

Carbonates

For a suspected carbonate, react it with a sample of acid and collect the gas that forms. All carbonates produce carbon dioxide gas with acid

And of course

Carbon Dioxide can be identified by passing it through lime water which will turn milky white in contact with the gas

Poll

Do you think you can handle qualitative analysis?

• This is a lot

• I need to practice