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Analysing Substances - Qualitative
To test for sulfate ions, we add hydrochloric acid and barium chloride solution.

Analysing Substances - Qualitative

In GCSE Chemistry students will look at the various methods and techniques used when analysing substances. In this, the second of three quizzes on the topic, we look at qualitative analysis. This helps us to identify what is present, but does not tell us how much of it there is.

Analysing is the identification of substances by investigating their physical and chemical properties. It is always important to be able to identify unknown substances, particularly in the the food industry and the field of medicine. Many metals are poisonous in large enough amounts and analysis can identify them should they accidentally find their way into the food chain. In cases of actual poisoning, medical staff need to be able to reliably analyse body tissue and blood to find out what is there. Only then can they decide on the correct treatment.

How do we test for sulfate ions?
Add dilute nitric acid
Heat and look for colour changes
Add hydrochloric acid and barium chloride solution
Add sodium hydroxide and warm it, then add aluminium powder
When sulfate ions combine with barium ions, they form barium sulfate which is insoluble and therefore appears as a precipitate
The copper oxide that is produced when copper carbonate is heated is what colour?
The copper is reduced to copper (I) and the carbonate decomposes, releasing carbon dioxide into the air
A positive test for nitrate ions would give which of the following observations?
A lemon-yellow colour when hot and turns to white when cool
A pale yellow precipitate
A dirty green precipitate
Damp red litmus paper turning blue
Red litmus turning blue shows that an alkali has been formed
If iodine solution is added to ethane, what observation would be made?
Remains brown
Changes from brown to black
Changes from brown to purple
Changes from brown to brick red
Ethane is saturated and does not have a double bond that can react with the iodine
If ammonia gas is produced when sodium hydroxide is added to an unknown solution, what ions were present in the unknown solution?
Ammonium + ions
Ammonium - ions
Copper 2+ ions
Iron 3+ ions
The ammonium ions have the formula NH4+. They donate a proton to the hydroxide forming water and NH3 which is a gas at room temperature
Some metal ions form coloured precipitates with sodium hydroxide. Which metal ion forms a light blue precipitate when reacted with sodium hydroxide?
Copper (II) ions
Iron (II) ions
Iron (III) ions
Cobalt ions
Transition metal ions often form coloured compounds, copper is no exception
Which of the following ions dissolves in excess sodium hydroxide solution?
Magnesium 2+
Aluminium 2+
Calcium 2+
Aluminium 3+

 Both magnesium and calcium ions are insoluble in excess sodium hydroxide solution

What is used to identify an unsaturated compound?
Bromine water
Dilute nitric acid
Barium sulfate
Aluminium powder
This is used to test for unsaturated hydrocarbons and oils. Iodine can be used instead of bromine
Flame tests can be used to identify metals in groups 1 and 2 of the Periodic Table. Which metal burns with an intense yellow/orange flame?
Street lights are yellow/orange due to the sodium they contain
If dilute nitric acid and silver nitrate are added to an unknown solution and a cream precipitate appears, we can tell that the ions that are present are...
chloride ions
bromide ions
iodide ions
chlorine ions
You need to know the results of this test off by heart for the exams
You can find more about this topic by visiting BBC Bitesize - Analysing substances

Author:  Kate Gardiner

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