Qualitative Analysis

Qualitative Analysis

​What is Qualitative Analysis?

​Qualitative Analysis is a technique used in chemistry to determine the components of an unknown compound by examining its qualities

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​Some chemicals have very unique traits that make them easily identifiable through general observation or confirmatory tests

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**quantitative ana​lysis conversely involves examining numerical data e.g. moles/ concentration etc.

​Appearance

​The first means by which an unknown substance can be identified is by examining its physical appearance.

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​Some metals and gases have very unique appearances or smells

​Metal Ions with Unique Colours

​When copper is present in a compound, its ions cause a blue green appearance

​The presence of some metals in a compound produce unique colours based on the ion present:

​Fe²⁺ ions produce a pale green colour

​Fe³⁺ ions produce a rusty/ yellow brown colour

​Fe²⁺

​Fe³⁺

​Gases with Unique Colours

​Chlorine gas has a unique yellow-green colour

​Nitrogen Dioxide has a unique orange brown colour

​Unique textures caused by Ions

Chloride compounds and nitrate compounds have a unique deliquescent nature

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​this means that they have a tendency to absorb moisture from the air and dissolve themselves eventually becoming liquids

​Smell Tests

​Some chemical compounds have unique smells that can be used to identify them:

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​Ammonia: Unique pungent smell

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​Hydrogen Chloride: pungent smell

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​Sulphur dioxide: pungent, burning smell

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​Hydrogen sulphide: bad eggs smell

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​Flame Tests

​Flame tests can also be used to identify specific metals

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​When some metal ions are exposed to flames, they burn in the air producing unique colours

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​Examining these colours can help identify if a certain metal is present in a compound

​These common metals produce the following flame colours when introduced to a burning flame

​blue

Gas Tests​

​what is a gas test?

​Some gases also have unique qualities that can be tested for

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​The following gases can be tested for with specific tests:

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​Hydrogen

​Oxygen

​Hydrogen Sulphide

​Carbon Dioxide

​Chlorine

​Gas Tests

• ​H₂ burns with a squeaky popping sound

• ​O₂ can relight a glowing (almost out) splint

• ​CO₂ when passed through lime water produces a milky white colour

• ​Cl₂ bleaches litmus paper

• ​HS produces sulphur deposits if introduced to a lit splint

​Litmus Paper

​Moist litmus paper can be used in gas testing to determine if a gaseous compound is acidic or basic. This allows us to narrow down the identity of an unknown gas.

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​Hydrogen chloride: blue litmus paper turns red (acidic)

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​Sulphur dioxide: blue litmus paper turns red (acidic)

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​Ammonia: turns red litmus paper blue (basic)

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​Nitrogen dioxide: blue litmus paper turns red (acidic)

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​Other Paper Tests

​Other types of papers can be used to test for the presence of specific ions and compounds

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​The following compounds have unique reactions with special papers:

• ​Water

• ​Chlorine

• ​Sulphur Dioxide

• ​Hydrogen Sulphide

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​Other Paper Tests

​In the presence of chlorine gas, iodide paper turns blue black in colour

​in the presence of sulphur dioxide potassium dichromate paper turns green

​Other Paper tests

​Hydrogen sulphide causes lead ethanoate paper to turn brown-black in colour

​Water changes cobalt chloride paper from a blue colour to pink

​NaOH and NH₄OH tests

​NaOH and NH₄OH

​Sodium Hydroxide and Ammonium Hydroxide solutions can be used to confirm the presence of certain compounds

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​when reacted with either solutions some compounds produce precipitates with unique properties

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​Precipitates are reaction products that appear as (usually) solid, insoluble deposits that come out of a previously aqueous solution

​NaOH Precipitates

​When reacted with a compound containing the following elements, unique precipitates are formed:​

• ​calcium - white liquid

• ​aluminium - white solid

• ​copper - pale blue gelatinous sub.

• ​magnesium - white liquid

• ​zinc - white solid

• ​Fe ²⁺ - dirty green gelatinous sub.

• ​Fe ³⁺ - rusty brown gelatinous sub.

  ** when reacted with a compound containing ammonium, ammonia gas is produced

​NH4OH Precipitates

• ​Aluminium- white liquid

• ​copper - royal blue solid

• ​Magnesium - white liquid

• ​zinc - white solid

• ​Fe²⁺ - dirty green gelatinous sub

• ​Fe ³⁺ - rusty brown gelatinous sub

Specific Tests​

​Specific Tests

In addition to prior tests, there are specific tests that can be used to confirm the presence of some compounds

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​These tests can produce unique reactions, unique precipitates or products like gases that can in turn confirm exactly what a compound may consist of

​Testing for Sulphates

​Compounds containing sulphates can be tested for by reacting the suspected substance with a mixture of Hydrochloric acid and Barium Chloride/ Barium Nitrate

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​If the compound is a sulphate, a white, liquid precipitate will form

​Using Silver Nitrate

Silver Nitrate solution can test for Halide (group 7) compounds

• ​Chlorides in silver nitrate form greyish white precipitate (may turn purple on standing)

• ​Iodides in silver nitrate form yellow precipitates

• ​Bromides in silver nitrate produce cream coloured precipitates

​Iodide

​Chloride

​Bromide

​Testing for Carbonates

​Carbonates and Hydrogen Carbonates all react with acids to form carbon dioxide gas

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​Carbon dioxide, when bubbled through lime water, turns the solution milky white

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​Adjacent is the lab set-up when testing a carbonate

​Testing for Nitrates

A specific test is used for substances suspected of being nitrates: Reacting with a solution of acid and iron sulphate

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​When a nitrate is exposed to this solution a unique reaction takes place

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​A distinct brown ring forms in the reaction vessel confirming that a nitrate is present

​Solubility Tests

​Solubility is a substances ability to dissolve especially in water

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​The presence of some ions determines if a compound is soluble or not. Solubility tests can be used as an initial method of testing to narrow down a substance's identity

​Soluble and Insoluble Compounds

​All salts containing sodium, potassium, ammonium are soluble

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​All common nitrates are soluble

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​All Chlorides except: silver chloride, mercury chloride and lead chloride are soluble

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​All Sulphates except: Barium sulphate and lead sulphate are soluble **Calcium sulphate is slightly soluble

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​All carbonates are insoluble except those of: sodium, potassium and ammonium