Chemical Tests Aim To be able to identify positive and negative ions
Context Qualitative analysis – this is used wherever it is important to know what is in a material. This could be the identification of paint sample at the scene of a road traffic accident or trace material found at the scene of a burglary or murder.
METHOD- testing for negative ions Chloride- Dissolve a spatula full of the chloride in 5 cm3 of water in a test tube. Add 2 drops of dilute nitric acid, using a dropping pipette, then 4 drops of silver nitrate solution. Sulphate- Dissolve a spatula full of the sulphate in 5 cm3 of water in a test-tube. Add 2 cm3 of dilute nitric acid. Add 2 cm3 of barium nitrate solution. Carbonate- Place a spatula full of the carbonate in a test-tube. Carefully add 2 cm3 of dilute hydrochloric acid. Hold a second test-tube, containing limewater, close to the mouth of your reaction tube. You may prefer to use a delivery tube, so that the gas bubbles through the limewater.
METHOD- testing for positive ions (flame test) 1. (CARE) Put a 1 cm depth of concentrated hydrochloric acid in a test-tube. 2. Dip a Nichrome wire in the acid and then place it in the upper part of a hot Bunsen flame. 3. Repeat step 2 until there is no change in the flame colour (other than a dull orange) when the wire is put into it. The wire is now clean. If you cannot clean it, see your teacher. 4. Put a few crystals of the salt on a watch-glass. Put 2 drops of concentrated hydrochloric acid on the watch-glass, next to the salt. 5. Move the tip of the Nichrome wire from the acid to the salt. Tap off any crystals which stick to the wire. 6. Move the tip of the wire sideways, just into the edge of the Bunsen flame. Note the colour produced. Repeat steps 5 and 6 if you are in doubt. 7. Heat the wire strongly until the colour disappears from the flame. 8. Wash the watch-glass and dry it with a clean tissue.
Results for negative ions Chloride Sulphate Carbonate Ion Explanation Observation
Results for positive ions Sodium Potassium Calcium Copper Ion Explanation Observation
Analysis The salt contained the negative ion …….. I know this because……………… The salt contained the positive ion ………. I know this because …………………
Evaluation My experiment went… I did not have problems with… because… I had problems with … because… To improve my experiment I could… My results would be more reliable if I… Did you have any results that look out of place? If so, how might this have happened?