Testing for gases + cations Flashcards
Describe how to test for the gas hydrogen
The test for hydrogen:
hold a burning splint at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
why do we hear a loud pop sound for hydrogen test
The noise comes from he hydrogen burning quickly in the oxygen in the air to form water
Describe how to test for the gas oxygen
Insert a glowing splint into a test tube of the gas
The splint relights in oxygen
If the gas is oxygen then the splint relights (bursts into flames)
Describe how to test for the gas carbon dioxide
Bubble a gas (or shake it with) an aqueous solution of calcium hydroxide (lime water).
If carbon dioxide is present, the limewater will turn milky (cloudy)
Describe how to test for the gas chlorine
Use litmus paper
When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white
To test for chlorine, we insert damp litmus paper into the mouth of the test tube of the gas.
Chlorine bleaches the litmus paper and turns it white
if you use blue litmus paper it may turn red for a moment first- that is because a solution of chlorine is acidic
when testing for gases - what is the method of collecting these gases dependent on
The method you use to collect a gas will depend on whether it is lighter or heavier than air
If it heavier (Like chlorine), you have the test tube the right way up and the gas will sink to the bottom
If it is lighter (like hydrogen), you have the test tube up-side down, and the gas will rise to fill it
Describe how to use flame tests to identify five different metal ions
DESCRIBE HOW TO SET UP/carry out A FLAME TEST
Place a small amount of our chemical onto the wire mounted in a handle
We then place the end of this into a blue Bunsen Burner flame
The colour of the flame can be used to work out the metal ion present
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A nichrome wire loop should be dipped in concentrated hydrochloric acid and then heated to clean it first
Then it should be dipped in the acid again before dipping it into the metal compound that is being tested
Then hold the loop in the roaring blue flame of a Bunsen burner
Use the colour of the Bunsen burner flame to identify the metal ion in the compound
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clean a nichrome or platinum wire loop by rubbing it with fine emery paper and then holding it in a blue flame from a Bunsen burner
The Bunsen flame might change colour for a bit, but once it is blue again, the loop is clean
Then, dip the loop into the sample you want to test and put it back in the flame
record the colour of the flame.
Flame test for lithium
Lithium (compounds result) produces a crimson flame (test)
Flame test for sodium
Sodium compounds results in/produces a yellow flame test
Flame test for potassium (ion)
The potassium ion/compounds results in/produces a lilac flame test
Flame test for calcium ion
Calcium compounds results in/produces an orange-red flame test
Flame test for copper ion
The copper ion compoundsresults in/produces a green flame test
Problems with using flame tests to identify a metal ion
The colour of a flame test can be difficult to distinguish
That is especially true if there is only a low concentration of the metal compound
Sometimes a sample contains a mixture of metal ions which can mask the colour of the flame
e.g. if the sample contains the sodium ion, then the intense yellow colour can mask the colour of any other metal ion present
What is an alternative to doing flame tests
Instead of doing flame tests, scientists often use another technique called flame emission spectroscopy
describe flame emission spectroscopy
In flame emission spectroscopy, a sample (of the metal ion in solution) is placed into a flame
The light given out is then passed into a machine called a spectroscope
The spectroscope converts the light into a line spectrum (the output is a line spectrum)
T(he line spectrum can then be analysed to identify the metal ions in the solution and measure their concentrations)
The positions of the lines in the spectrum are specific for a given metal ion
We can use this to identify the metal ion in the sample
Flame emission spectroscopy can also tell us the concentration of the metal ion
This is because the lines become more intense at a higher concentration
