Chemical analysis Flashcards
Define cations and give examples:
positively charged ions which are attracted to negative charges
metals
Define anions and give examples:
negatively charged ions which are attracted to positive charges
non-metals
Explain how we can test for halide ions in a substance: (include what you would expect for different halides)
-put substance in question in a test tube
-add dilute nitric acid and silver nitrate
-if precipitate forms, a halide is present.
white = chloride
cream = bromide
yellow = iodide
Explain how we can test for sulphate ions in a substance:
-put substance in question in a test tube
-add dilute hydrochloric acid and barium chloride
-if white precipitate forms, sulphate is present
Explain how we can test for carbonate ions in a substance:
-add dilute HCL acid into a test tube
-drop in the ionic compound and capture the gas produced from the reaction
-bubble the gas through a test tube into limewater
-if limewater turned cloudy then carbonate ions were present.
Explain how we can test for metal ions in a substance:
-in a test tube dissolve the ionic substance in deionised water
-add sodium hydroxide solution and stir
-if no precipitate appears, continue with flame tests
-if white precipitate forms, continue to add excess sodium hydroxide solution: if the precipitate dissolves then it is either magnesium of calcium (do flame test to fully distinguish). if the precipitate remains then it is aluminium.
-if the precipitate is coloured then it is a transition metal: brown= Fe³⁺. Light green turning to brown= Fe²⁺. Light blue= Cu²⁺
Explain a flame test:
a flame test is used to identify what type of cation an ionic substance contains.
-using a damp wooden stick, pick up some ionic compound
-hold the coated stick over a blue bunsen burner flame.
results:
crimson-lithium
yellow-sodium
orange/red-calcium
green-copper
lilac-potassium
no colour-magnesium
How do you test for carboxylic acids?
add sodium carbonate, if it fizzes then a carboxylic acid was present
State why may we use instrumental methods include what are their disadvantages:
+ more accurate/sensitive
+ quicker
+ we can analyse smaller samples
- expensive
- needs training to operate
- can only analyse substances by comparison to other known substances
Describe flame emission spectroscopy
Flame emission spectroscopy is a technological method of identifying metal ions in solutions. When we burn the metal substance, electrons get excited and jump into higher energy levels, when they return back, they give off certain patterns of radiation for each type of metal (its line spectrum) which a spectroscopy can analyse. These patterns can be compared to those in data bases of a computer. these can also determine the concentration of these metals in the substance, therefore can be used by water companies to figure out levels of metals in water.
