Identification of gases and important information for qualitative tests PDF

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Identification of Gases and Important Information for Qualitative Tests...

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Chemistry 2210

Important Information for Qualitative Tests (Experiments 1 – 3) Introduction The object of these test tube reactions is to illustrate some general trends in chemical behaviour. While the work is not quantitative, it does not mean that you can be sloppy. Carry out the tests as described. In particular, do not use larger quantities than mentioned in the appropriate experiment outline. Observe carefully and note all changes, writing them down right away on the sheets provided in the experiment outline, even if you cannot explain them for the moment.

Procedure Each test should be carried out on the smallest possible quantity of material. In practice this means 1 mL of a 1% solution or 10 mg of solid (a small spatula end). When you are performing a number of tests on the same solution, you can make up a stock solution by dissolving about 100 mg (0.1 g) in 10 mL of water. All tests are designed to work for the amounts of reagents noted above. They are generally reliable in the concentration range 0.2%–5% (m / v) but if you use much greater concentrations you may find precipitates where none are expected. The only time that increasing quantities may help is when a gas is evolved and you want to be able to identify it by performing the appropriate gas tests. When you are told to make a solution, make sure you really do have a solution and not a suspension. Solutions are clear (but not necessarily colourless) without any "milkiness". Sometimes warming or addition of a drop of acid or base will remove the milkiness. Ask an instructor if you are not sure which to try. 5% solutions of reagents for experiments 1, 2 and 3 are generally provided for you. Sometimes you may be required to make a saturated solution. This is a solution in which no more solid will dissolve. To make a saturated solution, place some solid in a test tube and add a couple of millilitres of water. If the solid dissolves completely with shaking, add more solid. Continue adding more solid until some solid remains undissolved, even after thorough mixing. Decant (pour off) the solution into a second clean dry tube, leaving the excess solid behind. You now have a saturated solution which you can use to perform tests. You may use small test tubes for all your reactions. Even though these are considered disposable, they should be rinsed and reused when possible. It is wasteful to do otherwise. Always clean out these test tubes once you have made and recorded your observations. The longer they are allowed to stay dirty, the more difficult they are to clean. If you know what the solid or precipitate is, you are more likely to know whether a drop of acid or base will help you to remove it.

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Chemistry 2210

Qualitative Tests (Experiments 1 – 3) (continued) Observe the following safety points: 1.

Wear safety goggles / glasses at all times.

2.

Do not put any of the substances (solid or solution) near your mouth.

3.

If you have to smell a gas, do not put your nose over the test tube but waft the vapour cautiously toward you with your hand.

4.

When you have to heat a test tube, hold it with a test tube holder. Boil a liquid or solution by heating the tube on a slant at a point above the liquid level and shake it gently to move the liquid regularly over the hot spot. This should avoid bumping. NEVER point a hot test tube at yourself or anyone else in the lab! Do not place hot test tubes into plastic test tube racks! Place them in a beaker to allow them to cool.

5.

Allow a hot solution to cool before deposing of it. Pour the cooled solution into a large amount of cold water, particularly if a corrosive reagent such as concentrated sulfuric acid or 6 mol.L-1 sodium hydroxide has been used.

6.

Do not put dropping pipettes into reagent bottles. When using the same reagent for several test tube tests, pour a little into a clean test tube and take it from there with a dropping pipette rather than repeatedly using the reagent bottle. Use a separate pipette for each different reagent used.

7.

If in doubt about any procedure, ask an instructor to demonstrate it to you.

Report

Record your results directly into the provided tables {experiments 1, 2 and 3}.

Observations Record in INK the appearances of the solutions to be tested and the reagents used where space is provided in the tables. Write what you observe after reagents are mixed or mixtures are heated or gas tests are performed etc. in the observation columns of the table, whether or not you understand what is happening.

Equations When requested, explain your recorded observations using a balanced reaction equation. Identify the precipitate, gas etc. by name or chemical formula in each case.

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Qualitative Tests (Experiments 1 – 3)

Chemistry 2210

(continued)

Report Observations should be recorded on the sheets in ink as the tests are done, but the "Conclusions / Equations" pages should be completed after you leave the lab. Take care to distinguish between observations and conclusions. For instance, if you observe colourless bubbles when reagents are mixed, you would record that colourless bubbles were seen as your observations. Identifying the gas as CO2 would be an appropriate conclusion or deduction if adding the gas to lime water produced an opaque white mixture.

Reference G. Svehla, Vogel's Qualitative Inorganic Analysis, 6th edition or 7th edition. This book can be borrowed from the QEII Library (7th edition) or used in the Chemistry Resource Room, C-2010, (6th edition).

Identification of Gases carbon dioxide, CO2

A colourless acidic gas that turns limewater milky white.

hydrogen, H2

A colourless gas that causes a flaming splint to "pop".

oxygen, O2

A colourless gas that relights a glowing splint.

nitrogen, N2

A colourless inert gas --- no reactions.

(All the above are odourless.)

ammonia, NH3

A colourless, basic, pungent gas. A brown acidic gas that dissolves in water to

nitrogen dioxide, NO2

produce a colourless solution. It will turn moist

Toxic gas: test for it in a fume hood!!! sulfur dioxide, SO2 Toxic gas: test for it in a fume hood!!!

starch-iodide paper navy blue or black when it comes in contact with it. A colourless acidic gas that dissolves in water to produce a colourless solution. It will turn filter paper moistened with acidified potassium dichromate green when it comes in contact with it.

(These three have distinctive smells. Waft these gases to your nose if you are required to smell them. Otherwise --- DON'T smell them!!!)

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Chemistry 2210

Lab Exam The results and knowledge acquired in experiments 1 and 2 will be important for the lab exam at the end of the semester. Example:

Unknown #10: HCl (aq):

a white crystalline solid.

a clear, colourless solution.

Left-hand page Test

Observations

/

Right-hand page Conclusions / Equations

A 2 mL sample of

A colourless gas evolved

An acidic gas was evolved. As it is

3 mol.L-1 HCl was

which turned blue litmus red.

colourless, the gas is likely CO2 or

added to a sample of

SO2 .

unknown # 10

CaCO3 precipitated and therefore CO2 is confirmed. The evolved gas was

The clear, colourless calcium

bubbled into a saturated

hydroxide solution becomes

Ca(OH)2 solution.

an opaque white mixture as

(i.e. limewater)

gas is bubbled into it.

The unknown anion is carbonate or hydrogen carbonate. CO32- (aq) + 2 H+ (aq) → CO2 (g) + H2O (l)...