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Note: Strictly speaking this should be the [Cu(NH 3) 4(H 2O) 2] 2+ ion. At this level, all you may be expected to know is its deep blue colour.
Remember that your speed on this test is the sum of the reaction time of your brain and the movement of your finger to press the button. In fact, it is easier to write the ionic equation, which is the same for all cases - just choosing the (aq) or (s) as appropriate. The presence of the hydroxide ions means that you might expect ammonia solution to react with metal ions in the same way as sodium hydroxide solution. Calcium ions give a white precipitate insoluble in excess sodium hydroxide solution, but no precipitate or a very slight precipitate in ammonia solution.In each case the formation of the precipitates involves exactly the same chemistry as with sodium hydroxide solution. All that matters is the presence of the hydroxide ions, not where they came from. There is a wide variation between what various syllabuses might want you to know about this, and it is is essential that you find out what level of detail your examiners want. The more you have to learn, the more confusing it can get. This reaction speed test is not very scientific, but just for fun and practice. How to do time reaction test This time, only the zinc hydroxide precipitate redissolves - for the same reason that the copper(II) hydroxide that we have already looked at dissolves in excess ammonia solution.
You start with a solution of your compound and then add a few drops of sodium hydroxide solution or ammonia solution. Note what happens. grades 9–12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic What is happening here is that the excess ammonia solution reacts with the copper(II) ions to give the deep blue soluble ion [Cu(NH 3) 4] 2+ where ammonia molecules have attached themselves to the copper ion.
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They dissolve in excess sodium hydroxide solution to produce soluble complex ions in which hydroxide groups are bound to the metal ion. Ammonia solution contains far fewer hydroxide ions than sodium hydroxide solution of the same concentration. So you don't form enough calcium hydroxide to form a reasonable precipitate. The problem in the calcium case is that calcium hydroxide is slightly soluble - we use its dilute solution as lime water. You should, of course, be able to write full equations if you were asked, but they are more time-consuming, with more chance of making mistakes. The next video shows these two reactions. If you look very closely at the ammonia one, you will see a slight trace of a precipitate.
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