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precipitation reactions

Submitted by sat on 24 June 2019

Have you considered conducting precipitate reactions using microscale techniques? The use of micro quantities of chemicals can enable the demonstration of many chemistry principles with no compromise of outcomes and with minimal impact on the environment.

A simple way to demonstrate precipitation reactions, instead of using test tubes, is to use a laminated A4 page with a grid printed on it, or to place a page with a grid printed on it inside a plastic sleeve. One drop of two different dilute (0.1M) solutions can be added together and observed for the formation of a precipitate.  For an example of this, see ’Microscale reactions of positive ions with sodium hydroxide’, Learn Chemistry website, http://www.rsc.org/learn-chemistry/resource/res00000757/microscale-reactions-of-positive-ions-with-sodium-hydroxide?cmpid=CMP00005906 (July 2016). The chemical waste for these activities on the plastic sheet is not significant and can be wiped up with a paper towel or tissue and disposed of in the general waste.

I also refer you a previous question that we answered regarding microscale chemistry where we provided a number of links, see https://assist.asta.edu.au/question/4483/microscale-chemistry

In particular I draw your attention to the YouTube videos by Bob Worley a CLEAPSS advisor, as he has a number of videos showing very dramatic reactions in a drop of water.  He uses a method which clearly demonstrates dissolving; diffusion and then precipitation. For example:

For details on this method, see ‘Precipitates’, YouTube (1:17 min) https://youtu.be/9zYcE35_Vsw

‘Four ways to do a microscale precipitation reaction’ can be found here

  1. Add one drop of a chemical solution on an acetate sheet and then on top of this drop, add one drop of a different chemical solution and observe if a precipitate forms.
  2. Add one drop each of two different chemical solutions, side by side on an acetate sheet and then mix the two drops together and observe if a precipitate forms.
  3. Add two drops of distilled water side by side on an acetate sheet and then add a tiny amount of two different solid chemicals to each drop. Mix each drop and then combine the two drops
  4. Add one drop of distilled water on an acetate sheet. Using fine pop sticks (The thin coffee type sticks or toothpicks) push a tiny amount of solid chemical into opposite sides of the drop, then wait, after a short time, you should see a line appear, where the two solutions ‘collide’ and form a line where a precipitate has formed

The 4th and last method clearly demonstrates dissolving; diffusion and then precipitation.

An alternate way to place the solid chemicals into the one drop of distilled water for these precipitation activities is to:

Other examples of microscale chemistry in a drop

For an example of a microscale reaction for metal displacement observing silver nitrate reacting with copper wire see ‘ Silver nitrate reacting with copper wire’, YouTube (0.20 sec) https://youtu.be/bj1Bjh0VbvU.

We suggest that you trial this using 1-2 drops of 0.1M silver nitrate on an acetate sheet/plastic sleeve protector and place a bare (not coated) copper wire half-way into the droplet. This could even be filmed using a mobile phone with a magnifying lens added.

The chemical waste for these activities in drops of water is not significant and can be wiped up with a paper towel or tissue and disposed of in the general waste. This assumes safe chemical handling, good laboratory practices and in particular ensuring that hands are washed at the end of the laboratory session.

Safety notes for using lead nitrate: Only solutions 0.5M or lower are recommended for use by year 7-12 students1. Small quantities should be used for short amounts of time, wearing appropriate PPE such as gloves and safety glasses. Good laboratory hygiene such as cleaning up any spills, no eating or drinking in laboratories and washing hands at the end of all laboratory sessions should also be implemented.2

References:

1Science ASSIST. 2017. List of recommended chemicals for science in Australian schools, ‘Notes’ Tab, note C, Science ASSIST website, https://assist.asta.edu.au/resource/3052/list-recommended-chemicals-science-australian-schools

2’Use of lead in ACT schools’, Science ASSIST Q&A, Science ASSIST website, https://assist.asta.edu.au/question/4070/use-lead-act-schools (September 2016)