St Hilda’s Secondary School Secondary Four Express / Five Normal Academic Science (Chemistry) 5116 / 5118
~ Qualitative Analysis & YOU ~ Part I – I – An An overview…
Qualitative Analysis investigation is to test your ability to carry out a number of simple experiments and record correct observations. Most of your marks are awarded for the observations.
The unknowns in qualitative analysis can be pure substances or mixtures. The unknowns are usually common chemicals that you have learnt about through the years. Hence they can be salts, salts, elements, acids, alkalis, metal oxides or even organic compounds. o
However, the unknown can also be unusual substances that you have never heard of, and of course there is no need to identify them. If they require you to identify the unknown, more guiding questions will be provided for you.
Before coming for the practical examinations, you are expected to be able to identify gases that may be produced in the chemical tests and record the name of the gas. You are also required to write the test you did to identify the gas, as part of your observations observations.. (You may refer to QA Manual for reference and accurate reading of the process.)
You are also expected to test for and identify cations and anions. The results can help us answer questions in the conclusion segment.
You are expected to know tests for oxidising and r educing educing agents such as KI (RA), acidified KMnO4 (OA) and acidified K2Cr 2O7 (OA).
Titles you can't find anywhere else
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
Titles you can't find anywhere else
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
Part II – II – Recording Recording Observations (ensure maximum marks obtained)
Observations are changes which are seen in the tests. These changes can include: o Colour changes in solids or liquids Formation of precipitates o o Bubbles of gas produced, and if they are coloured or have characteristic smells o Flame test Distinctive smells produced o
When carrying out an experiment, always leave the test tube to stand for one or two minutes as some changes take longer to happen. For instance, most precipitates form instantly but some take a longer. If the precipitate forms slowly, describe this also in your record of observations.
There is an observation for every test. If you do not observe any change, then you should record “no visible change” or “no observable change ”.
Please note that gases produced by reagents alone are NOT observations. For example, when ammonia is added to a substance, you are bound to smell ammonia.
Observations must be recorded opposite the instructions. This is important so that it is clear which test your observations apply to, especially when test instructions consist of two or more separate steps.
You must carry out the tests in the practical instructions. You must not do extra tests of your own choice (unless asked to). to). You will not get any extra marks – but you might even lose marks!
Titles you can't find anywhere else
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
Part III – III – Experimental Experimental Techniques (A) Using test tubes and boiling tubes.
Most reactions are carried out in test tubes.
Use test tube holder if test tube or boiling tube is going to be heated.
Liquid/solution should be added to test tube to a depth of about 2 cm. This is sufficient to see any changes during reactions and also leave room for other reagents to be added without overflowing.
No more than 1 spatula heap of solid should be added into test tube. This should only fill test tube to a depth of about 1 cm. (If too much is added, not all of it may dissolve or react when water or a reagent r eagent is added.)
A boiling tube must not be more than one-third full of liquid when it is being heated, as the contents will most likely splash out of the tube during boiling.
(B) Adding liquid reagents
Liquid reagents must be added a little at a time. time. Between each addition of reagent, you must shake the test tube to make sure contents are well-mixed. Note: If a large amount of reagent is added all at once, important observation may be missed out. For example, precipitate may dissolve in excess reagent so quickly that you may not see it!
If practical instructions say “add reagent X until no further change”, then Reagent X must be added little at a time, with shaking until the test tube is almost full (about two-thirds in depth).
Some reagents such as dilute NaOH and dilute ammonia solution (ammonium hydroxide) must be added in excess to find out if any precipitate produced will dissolve in excess reagent. Reagents must be added a little at a time with shaking until precipitate dissolves or until test tube is full.
If practical instruction say “acidify the solution in the test tube, tube, then add Reagent X”, the acid must be added to the test tube a little at a time, with shaking, until the solution turns moist blue litmus paper red (acidic). Note: Shake the test tube thoroughly before doing the acidity test to make sure the liquid is homogenous.
Titles you can't find anywhere else
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
(C)Heating (C) Heating of solids
Always heat the solid gently at first. Then steadily increase the temperature.
Watch out for water for water vapour condensing on the cooler upper part of the test tube at the beginning of the heating. This could suggest solid is a hydrated salt (with water molecules attached). o
Be prepared to test for any for any gases being produced before the heating strongly.
When recording colour changes, allow the test tube to cool down to room temperature before recording any changes in colour. Note: Some solids change colour on cooling. For example, zinc oxide is yellow when hot and white when cold.
(D)
Testing for gases
Detection and identification of a gas produced in a reaction is an important observation in QA.
Do not waste your time testing for gases unless you know that a gas is being produced.
Sometimes it is stated in the practical instruction that a gas is being produced in an experiment. You must then specially look out for it. Gases can be detected by: o o o
Colour Characteristic Characteristic smell (pungent-smelling) (pungent-smelling) Effervescence (bubbles of gas)
The type of gas produced can help you decide which gas is the most likely gas being produced. For example, o A colourless gas with a pungent smell is produced when dilute NaOH is added to substance. This gas is most likely to be ammonia. o Bubbles of gas observed when acid is added to solid X. Since the gas is produced as a result of reaction with acid, we know the gas can be either carbon dioxide (from metal carbonate) or hydrogen (from reactive metal).
The identity of a gas must be confirmed by a test. test . The correct test for each gas
