Supporting Practical Science, D&T and Art
- in schools and colleges
Ammonia chemistry in a Petri dish
Purpose/Why do this?
Ammonia dissolves in water to produce an alkaline solution (sometimes referred to as ammonium hydroxide).
Ammonia solution can be used to identify metal ions as it produces precipitates – some of which are highly
coloured if transition elements are present. This activity provides a quick and simple way to explore some of
the chemistry of ammonia and metal ions.
Possible curriculum links: ammonia, qualitative analysis, metal ions, acid-base chemistry.
Suitability
Y7
Personal protective equipment and essential safety measures
Wear eye protection.
Universal indicator
paper
Outline Method and Control Measures
1. Place a 9 cm-diameter plastic Petri dish on a white or grey
background.
2. Dampen the 0-14 pH paper before placing it in the dish.
3. Around the dish place 1 or 2 drops of each of the following
solutions. See diagram right and activity template attached.
·
0.1 M metal salts – various (see activity template),
·
0.1M hydrochloric or 0.05M sulfuric(VI) acid containing
universal indicator.
3
Add 0.5 cm of 2 M ammonia (+
optional calcium chloride) to the
empty blister pack container.
4. Place the ammonia generator in the centre of the dish.
This could be a small watch glass or an empty blister pack
container.
5. Take a photograph of the dish (optional).
3
6. Add 0.5 cm of 2 M ammonia solution to the generator.
Optional: a few granules of anhydrous calcium chloride can
be added - this dissolves exothermically so the ammonia is
liberated more quickly.
Around the Petri dish, add 1
or 2 drops of 0.1 M metal
salts (and other suitable
reagents as are available).
7. Immediately put the cover on the Petri dish.
8. Observe what happens over the next 10 minutes.
Take photographs at intervals (optional) .
Disposal
Place the Petri dish in water in a bowl of water. Remove any solid materials and then pour the solution down
a foul water drain. Paper can be placed in a rubbish bin.
References
See Hazcards 5 and 6.
ASE 01/14_DRAFT
Page 1
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
Chemistry notes
3
If all the ammonia is released (which it isn’t), then 24 cm , or 17 mg, of the gas would be released. Although
–3
we can detect the gas by our sense of smell (odour level is 3.5 mg m ), the ammonia levels will be below the
–3
3
Short Term Exposure Level (STEL) level of 25 mg m in a large room (of 300 m ) averaged over 15 minutes.
Expected results
1.
2.
3.
4.
Indicator paper indicates a pH of 11.
Sulfuric(VI) acid is neutralised and finally becomes alkaline.
Some of the metal salts form precipitates of hydroxides.
Some of the metal salts form hydroxide precipitates which then dissolve in excess ammonia solution as
complex ions are formed.
At any one time in a 1 M aqueous ammonia solution, only about one molecule of ammonia in 240 is converted
+
–
to ammonium, NH4 (aq), and hydroxide, OH (aq) ions. Ammonia is an example of a weak alkali because not
all the ammonia molecules form a hydroxide ion. The hydroxide ions produced neutralise acids and react with
metal ions.
Tags
Ammonia, ammonium hydroxide, weak alkalis, metal ions, indicators
ASE 01/14_DRAFT
Page 2
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
Supporting Practical Science, D&T and Art
- in schools and colleges
Ammonia chemistry
Wear eye protection. Work in a well-ventilated lab.
¦ Place a 9 cm diameter Petri dish on the large solid-line circle (left). Remove lid.
¦ Place the test solutions/papers in the Petri dish in the numbered positions,
1 – 6, as described below.
1
6
1.
2.
3.
4.
5.
6.
2
Universal indicator paper – damp (a 1-2 cm strip).
Hydrochloric acid, 0.1 M (2 drops) containing Universal indicator.
Copper(II) salt solution, 0.1 M (2 drops).
Iron(II) salt solution, 0.1 M (2 drops)
Iron(III) salt solution, 0.1 M (2 drops)
Zinc salt solution, 0.1 M (2 drops)
¦ Also place 1-2 drops of solutions 2-6 in the matching outer-ring circles (i.e.
RV*
outside the Petri dish) - you will then be able to compare with those inside the
Petri dish to see what effect the ammonia has.
3
5
¦ Place an empty ‘reaction vessel’ (eg, small watch glass) in the ‘RV’ circle.
¦ To generate ammonia gas (TOXIC): use tweezers to place 2 granules of
4
anhydrous calcium chloride in the reaction vessel (gas generator).
3
Add 0.5 cm (approximately 10 drops) of ammonia solution (2 M) to the reaction
vessel. Immediately place the lid on the Petri dish*.
¦ Watch carefully and record your observations over the next 5-10 minutes
(eg, take photographs).
¦ Explain as much of the chemistry going on in the Petri dish as you can.
¦ Disposal: refer to notes on page 1.
*RV = reaction vessel/gas generator
ASE 01/14_DRAFT
Page 3
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk