Salt/Qualitative Analysis for P3 Chemistry

[Muhammmad]

Analysis:
Always write observations by looking at observations mentioned in the salt analysis data given at the back of the question paper. Find a close match to your observations from the sheet, and copy the same wordings of the observations given in the data sheet. There'll be rare cases when you'll be unable to find a match. In that case, write down whatever you see.

Moreover, always write proper bench reagents when you are asked to state certain reagents. Never write like H+1 or Cr2O7-2, rather write names of proper bench reagents like HCl(aq) or HNO3(aq) or K2Cr2O7 etc.

When you stir the boiling tube, don’t let the stirrer touch or strike the bottom of the tube. Whenever heating let the tube give time to heat. First heat gently then strongly. Keep the tube oblique. This way your tube will never break. It’s what my experience had taught me. When adding NaOH or NH3 for ion testing, add a few drops first (DO NOT ADD A DROPPER FULL OF REAGENT AT ONCE). Your eye must be close to the top level of the reagent in test tube. When a precipitate is observed, add the reagent in excess to almost ⅔ of the test tube. Don’t use more than 1 cm3 of the reagent because it will have more precipitate which will take more NaOH or NH3 to dissolve; this can confuse you into thinking that ppt is insoluble.

Distinguishing between Pb+2 / Al+3 :

• NaOH / NH3 reagents when used for identification of these ions give similar results (observations).
• PbCl2, PbI2, PbSO4, PbCr2O7 or PbCrO4, is insoluble. Use the following reagents; HCl, KI, K2Cr2O7 or any other reagent that contains the ions mentioned above. The insolubility of the afore-mentioned lead compounds can help us distinguish between the Pb+2 and Al+3 ions.
• Also, take care about other possible precipitates that might confuse your results. For example, BaCl2 (aq) contains Cl-1 and can be used to test for Pb+2 ions but the presence of Ba+2 ions makes precipitates of its own, so always use reagents which have Na+1, K+1, or H+1 ions which always make soluble compounds, reducing the possibility of any other precipitates, except for the precipitates formed by Pb+2.

Distinguishing between Ba+2 / NH4+1 :

• Both give no precipitate with NaOH or NH3 except that NH3 is produced on warmind NH4+1 with NaOH. Ba+2 can be identified by H2SO4, it will give white precipitate.

Test for Manganese Mn+2

• Mn+2 can identified with NH3 / NaOH. With these reagents, it has a white / pale brown ppt which turns brown when in contact with air and are insoluble in excess of the reagent. There would be brown residues floating on the top surface, and on the sides of the test tube, and white ppt/light brown ppt at the bottom.

Tests for Cu+2

• With NaOH: pale blue ppt insoluble in excess.
• With NH3: Blue ppt which dissolves and forms a dark blue solution in excess. It will be hard to dissolve the precipitate if too much Cu+2 are present in the test tube, so use very small quantity of Cu+2 (less than 1 cm3) or use a lot of NH3 (fill the entire test tube) and shake vigorously to dissolve this precipitate.

Problems with Al+3ions test with NaOH:

• The precipitate formed by Al+3 is very soluble and disappears very quickly. Students can easily make the mistake of not noticing any precipitate, and writing down that no change occurred. Use a very tiny quantity of NaOH at first, just a few drops (put one drop, shake it lightly, then put another and so on), and a small white ppt will form floating on the surface of the solution, which would dissolve very quickly if a very small amount of NaOH is added.

The identification of the other cations and anions is easy. For them just refer to the salt analysis notes given at the end of the paper.

Identification of gases:

Whenever a gas evolves, there is some sort of effervescence (bubbles form) that occurs in the solution. Whenever you notice such a thing, just put your thumb on the top of the test-tube. If the pressure builds up, there'll be definitely some sort of gas evolving.

Now the things is that, how to identify them?

CO3-2 : If an acid is added, or is present in the test-tube and you see vigorous effervescence, then definitely it's CO2 that's evolving. If you have time, just to counter check, test it with lime water, it will definitely turn milky. Effervescence produced is similar to gas bubbles in coke. Generally produced when metal carbonates react with acids

NO2-1 : Whenever an acid is added, put your thumb top of the tube, and allow pressure to build up. The tube will turn pale brown and when you release your thumb and allow gas to escape, then a pale brown gas will release. If it occurs then definitely NO2-1 is present in the solution. This pale brown gas is also very visible if seen in front of a white back ground. The gas is especially very visible when the reactants are thrown in the white sink and you will notice brown vapors in the sink easily.

SO3-2 : Another gas that is produced on addition of dilute acids is SO2 which indicates the presence of SO3-2 ions. SO2 gas is colorless and acidic and is produced when dilute acid is added to sulfite SO3-2 ions. If a damp blue litmus paper is placed at the mouth of the tube, then it will turn red. Damp litmus paper must not touch the test tube itself as it might contain an acid. Note, that damp blue litmus paper will turn red when NO2 gas is produced but NO2 is pale brown and can be distinguished from SO2. Another test for SO2 gas is that it smells of rotten eggs or burnt matches. It can also be distinguished by dipping a paper in K2Cr2O7 and then placing it at the mouth of the test tube. This paper will turn from orange to green.

NO3-1, NO2-1 : To test for these ions, NaOH is added followed by the addition of Al foil and heated. When bubbles start to form (vigorous bubbling), put a damp red litmus paper near the mouth of the test-tube. NH3 is liberated if these ions are present, and it turns damp red litmus paper blue.

Always use damp red litmus paper, by making the litmus paper wet. Nothing happens if the litmus is damp! And make sure that the litmus paper never touches the test tube, because the test tube might contain an alkali which will turn the litmus paper blue. A lot of students make the mistake of allowing the litmus paper to touch the top of the test tube, and in many cases an alkali is present in the test tube which makes the litmus paper blue. So, keep the litmus paper a fair distance (1 cm) away from the test tube.

Students should be able to distinguish between a red litmus paper from a blue litmus paper. Red litmus paper is pale pink, and blue litmus paper is pale blue. Some students also make the mistake of using the cover paper of the litmus paper stack which is also pink (Avoid silly mistakes)

Identification for hydrogen ( H2 ) gas:

Metal + Acid ---> Salt + H2

Use the above equation to detect the hydrogen gas.

If you are adding metal, and a gas is produced, then you don’t necessarily need to test for Hydrogen gas, if you see effervescence, then it is obviously hydrogen.
Just for confirming if you have time, test it. Hydrogen gas produces pop sound when burnt with a lighted splint. The only way it produces a pop sound when enough pressure is built up in the test tube. Put your thumb on top of the test tube and allow pressure to build up and only then light it.


NOTE: Always test for NH3 when the reagent is NaOH. This part of identification of NH3 (thus NH4+1) comes almost every time. So you must not neglect it. Also, you must be familiar with the smell of ammonia. This will save your time for testing. This is a pungent smell, but it doesn't feel that bad once you get used to it. Believe me now i really enjoy it's smell
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Hey can you tell how to make a line of best fit. I mean do we hav to get maximum points in it or equal number of points above and below it?