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Using Chemical Tests to Identify Unknown Ions
(TRIPLE ONLY)
Background Information and Keywords
Background Information and Keywords
Keywords
Keywords
- Soluble - When a substance can be dissolved in the solvent
- Insoluble - When a substance cannot be dissolved in the solvent
- Solution - When a solute is dissolved in a solvent
- Precipitate - an insoluble solid formed from the reaction of two liquids/solutions
- Precipitation reaction - a reaction that produces a precipitate
- Anion - a negatively charged ion
- Cation - a positively charged ion
Background Information
Background Information
This practical is all about trying to identify compounds.
In chemistry A LOT of substances are white powders. It's important for chemists to be able to identify what these similar looking compounds are made of.
All ionic substances are made of a positive ion (cation) which is a metal, and a negative ion (anion) which is a non-metal
There are several methods we can use to identify different types of positive and negative ions.
These include flame tests and precipitate tests.
Other things you need to be able to do
Other things you need to be able to do
To do the practical
To do the practical
Be able to follow written instructions carefully
Know what is meant by a solution and a precipitate
To be able to record observations clearly and accurately
To handle the data and make conclusions
To handle the data and make conclusions
To be able to use your knowledge of these tests to identify unknown compounds (usually by using more than one test)
To be able to describe each test and remember the names of the chemicals which need to be added
To be able to write basic ionic equations for the formation of the precipitates (Higher ONLY)
"How To Do It" Video
"How To Do It" Video
Watch this video to remind yourself how the practical in this PAG are carried out.
What are the key steps?
What are the key chemicals called which are used in each test?
What are the positive results for each test?
What are the names of the solid precipitates formed?
Diagrams
Diagrams
Practical 1: Metal Ion Flame Tests
Practical 1: Metal Ion Flame Tests
Key Steps
Key Steps
Flame tests can be used to identify metal ions (Li+, Na+, K+, Ca+ and Cu2+) in solids or solutions, by using this method:
Dip a metal inoculating loop or a wooden spill into a sample of the sample
Turn on a bunsen burner and adjust it to a blue or roaring flame
Put the inoculating loop or spill with the sample into the flame
Observe what colour the flame goes with the metal
Practical 2: Metal Ion Precipitate Tests
Practical 2: Metal Ion Precipitate Tests
Key Steps
Key Steps
We can also test for metal ions (Cu2+, Fe2+, Fe3+, Ca2+, Mg2+ and Al3+) in solutions using precipitate tests using this method:
Place a small sample of a the metal ion solution into a test tube
Add small quantities of sodium hydroxide solution (NaOH) to the metal ion solution
Wait to see if a precipitate (solid) forms
Add excess sodium hydroxide solution to see if the preciptate dissolves
Use the colour of the precipitate to decide what metal ion is present
Practical 3: Non-Metal Ion Tests
Practical 3: Non-Metal Ion Tests
Key Steps - Halide Test
Key Steps - Halide Test
We can test for halide ions (Cl-, Br- and I-) by using this test:
Add a small amount of halide ion solution to a test tube
Add some dilute nitric acid and silver nitrate solution
If it's a halide a precipitate will form
See what colour the precipitate is to identify which halide ion is present.
Key Steps - Sulfate Test
Key Steps - Sulfate Test
We can test for sulfate ions (SO42-) in solution using this method:
Add a small volume of your unknown ion solution into a test tube.
Add a small amount of dilute nitric acid
Add a few drops of barium chloride OR barium nitrate solution to the test tube
If a white precipitate forms, then sulfate ions are present.
Key Steps - Carbonate Test
Key Steps - Carbonate Test
To test for carbonate ions (CO32-) in solution we use this method:
Add a small volume of your ion solution to a test tube.
Add a small amount of dilute acid to the test tube
If the test tube solution fizzes, then a carbonate ion is present (as carbon dioxide gas has been made).
OPTIONAL - we can also collect and test the gas as shown in the diagram.
Pass the gas through a sample of lime water. If the lime water goes cloudy, the gas made is carbon dioxide.
Key Questions from this Practical
Key Questions from this Practical
How do you carry out a flame test?
What are some of the advantages of flame tests over flame spectra when identifying metal ions?
What are some of the advantages of flame spectra over flame tests when identifying metal ions?
How do you carry out a precipitation test to test for metal ions?
What chemical do you use in a precipitation test to test for metal ions?
What are the positive results of these tests?
How do you carry out a precipitation test to test for halide, sulfate and carbonate ions?
What chemical do you use in a precipitation test to test for halide, sulfate and carbonate ions?
What are the positive results of these tests?
Question Practice and Exam Questions
Question Practice and Exam Questions
Required Practicals Mastery Booklet - PAG 7 - Flame and Precipitate Tests.pdfPAG 7: Mastery Question Booklet
PAG 7: Mastery Question Booklet
This Mastery Booklet is filled with questions and exam questions to help you master this core practical. All answers and mark schemes are attached at the end.
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