In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
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What is a pure substance in everyday language?
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
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How can you tell if a substance is pure?
-It has a fixed melting and boiling point
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How can you tell if a substance is impure?
-It melts and boils over a range of temperatures
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What is the test for hydrogen?
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
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What is the test for oxygen?
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
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What is the test for carbon dioxide?
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
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What is the test for chlorine?
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
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What can we use to identify metal ions (cations)?
Flame tests
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How do we carry out a flame test?
1. A nichrome wire or damp splint is dipped in the substance to be tested
2. Place end of this into a blue Bunsen burner flame
3. The colour of the flame can be used to work out the metal ion present
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What colour flames do lithium, sodium, potassium, calcium and copper compounds result in?
-The colour can be difficult to distinguish (especially if there is a low conc of the metal compound)
-The sample may contain a mixture of metal ions which can mask the colour of the flame
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What other method is there to identify metal ions?
Flame emission spectroscopy
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What happens in flame emission spectroscopy?
Sample of metal ion in solution is placed into flame. The light given out is passed into a spectroscope, which converts the light into a line spectrum. The position of the lines in the spectrum are specific for a given metal ion.
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What else can flame emission spectroscopy tell us and how?
The concentration of the metal ion as the lines become more intense at a higher conc
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What type of method is flame emission spectroscopy and what does this mean?
It is an instrumental method, as it is carried out by a machine
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Name three advantages of using instrumental methods
-They are rapid (faster than flame tests)
-They are sensitive (will work on tiny sample)
-They are more accurate (↑ likely to be correct)
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Other cards in this set
Card 2
Front
What is a pure substance in everyday language?
Back
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
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