Sciencemadness Discussion Board

Interesting Double-Displacement Reactions?

nebotmo - 29-3-2015 at 11:52

Hi! I'm new to this forum, but I've snooped around the topics for around half a year now and finally have gotten around to creating an account. I will soon be trying to teach about double-displacement rxns in my class, and was hoping to be able to find a few stimulating ones to demonstrate (aside from the usual mixing of two solutions to form a bland precipitate). As far as interesting goes, I am looking for something visually stimulating or that produces a neat product. The only thing that I have been able to think of off of the top of my head was the formation of yellow Lead(II) Iodide (2KI + Pb(NO3 )2 →PbI2 + 2KNO3). Any suggestions are useful! Especially those with products including water and/or an insoluble gas. Thank you!

Volanschemia - 29-3-2015 at 20:43

Precipitation of Lead(II) Iodide is very visually appealing, just remember that you are handling Lead salts so take proper precautions and dispose of the products properly.

Another one I can think of is CuSO4 + NaOH = Cu(OH)2 + Na2SO4, producing a dark blue precipitate. I'm sure there are more, but I can't think of any at the moment. There are also some pretty nice single displacement reactions that produce metal precipitates.

[Edit] Typo.

[Edited on 30-3-2015 by TheAustralianScientist]

j_sum1 - 29-3-2015 at 21:24

In my memory, Cu(OH)2 is a light blue and has a somewhat gelatenous appearance. I might be wrong.

If by interesting you mean coloured then you should be thinking of insoluble transition metal salts.
Insoluble means that you are probably looking at hydroxides, sulfides and carbonates.
The problem as I see it is that you are going to have coloured solutions to begin with and then the surprise is gone.

Copper might be your main target here. It is really easy to produce all kinds of browns and oranges that come from Cu(I) ions I think. The problem here is that you do also have some redox going on. I admit I have never really looked into it in detail.

Other possibilities include Cr(III) salts, and Ni salts. I have really not produced anything particularly "stunning" with Fe.

One effect that can be quite stunning is to form a bright white precipitate that falls out of a coloured solution. Copper sulfate and zinc chloride for example.

You could also pursue the line of iodide salts. These tend to be yellowish. You might find a combination that works really well.


In my experience, single displacement reactions can actually be better to look at. My favourite is pouring CuSO4 solution through steel wool and seeing it go clear. The reaction is quite exothermic which is interesting. A lot of very visible copper is produced. And for a bit of extra fun, you can use the liquid product to turn potassium permanganate from purple to clear. Of course, this is redox, but if you are fishing for ideas to use in the future...

Amos - 30-3-2015 at 04:07

I would take j_sum1's idea a bit further, and dissolve a pretty decent quantity(25g or more) of copper sulfate pentahydrate in water and add excess steel or iron wool. The solution will go very pale green by the end of it and there will be no more copper in solution; the solution can then be filtered, reduced in volume, and left to evaporate to produce beautiful blue-green crystals of iron(II) sulfate. Afterward, you can treat the steel wool/copper mixture in the filter with more copper sulfate and get a pile of pure copper powder! It's a fun process to carry out, and provides you with a useful iron salt and very high surface area copper.

Dang it! I missed my one-year anniversary on the site!

[Edited on 3-30-2015 by Amos]

Etaoin Shrdlu - 30-3-2015 at 11:29

I like cobalt chloride + sodium hydroxide. A pink solution mixed with a clear solution makes a blue precipitate, and if you let it stand, the precipitate goes pink again.

DraconicAcid - 30-3-2015 at 11:58

Quote: Originally posted by j_sum1  
In my memory, Cu(OH)2 is a light blue and has a somewhat gelatenous appearance. I might be wrong.


It depends on how you make it. If you add ammonia to a solution of copper(II) sulphate, you get a light blue powdery precipitate. If you add sodium hydroxide to copper(II) sulphate, you get a gelatinous mass that my students describe as "cookie-monster blue", because it is exactly the shade of that muppet.

vmelkon - 30-3-2015 at 13:09

Quote: Originally posted by DraconicAcid  
Quote: Originally posted by j_sum1  
In my memory, Cu(OH)2 is a light blue and has a somewhat gelatenous appearance. I might be wrong.


It depends on how you make it. If you add ammonia to a solution of copper(II) sulphate, you get a light blue powdery precipitate. If you add sodium hydroxide to copper(II) sulphate, you get a gelatinous mass that my students describe as "cookie-monster blue", because it is exactly the shade of that muppet.


If you add ammonia, it should dissolve some of the Cu(OH)2 to form cupramine (Cu(NH3)4)2+ which is a deep blue solution. Adding enough should dissolve all of the solid Cu(OH)2.

DraconicAcid - 30-3-2015 at 13:24

Quote: Originally posted by vmelkon  

If you add ammonia, it should dissolve some of the Cu(OH)2 to form cupramine (Cu(NH3)4)2+ which is a deep blue solution. Adding enough should dissolve all of the solid Cu(OH)2.

Depends on how much you add. if you only add a small amount, you get precipitation.

Pasrules - 30-3-2015 at 13:24

I've always thought is it possible to get 2 precipitates from 2 solutions being mixed together. I've always been told no and haven't found anything on it but I thought I would just throw up the question.
I have considered using a solvent other than water which I think is putting me on the right track but no luck.

DraconicAcid - 30-3-2015 at 13:26

If you mix copper(II) sulphate and calcium hydroxide, you will get both calcium sulphate and copper(II) hydroxide (or some basic copper(II) sulphate, or something) precipitating.

BromicAcid - 30-3-2015 at 15:59

Silver fluoride and calcium chloride is a good example of two pretty soluble salts that come together to give two insoluble products.

Amos - 30-3-2015 at 16:12

Quote: Originally posted by DraconicAcid  
If you mix copper(II) sulphate and calcium hydroxide, you will get both calcium sulphate and copper(II) hydroxide (or some basic copper(II) sulphate, or something) precipitating.


Calcium sulfate is actually MORE soluble in water than calcium hydroxide. Copper hydroxide is itself only poorly soluble.

DraconicAcid - 30-3-2015 at 17:23

Quote: Originally posted by Amos  

Calcium sulfate is actually MORE soluble in water than calcium hydroxide. Copper hydroxide is itself only poorly soluble.


Then let's say I said barium hydroxide.

woelen - 30-3-2015 at 23:56

A very nice one is the reaction of a solution of silver nitrate with a solution of potassium dichromate. If you mix them you do not get a precipitate immediately, but quickly after mixing the solutions ruby red glittering crystals are formed, which like glittering red snow are formed in the liquid and slowly settle at the bottom. It is fascinating to watch this process.

[Edited on 31-3-15 by woelen]

nebotmo - 31-3-2015 at 02:37

Those are some great ideas, thank you! When I go over single-displacement I will definitely have to do something along the lines of the CuSO4 and steel wool reaction and the redox of KMnO4. Very neat. And the Copper (ii) Hydroxide should work well for the double-displacement.

j_sum1 - 31-3-2015 at 03:11

woelen! How did I know that you would know something cool like that. I am going to have to try it! Silver dichromate? Whodathunkit?


Not entirely related but possibly a useful one to have on hand.
I have prepared a task designed to assist junior high students in making scientific observations.
A few crystals of sodium thiosulfate are added to a flask of acidified potassium permanganate. (Approx 0.2M H2SO4 and enough KMnO4 to make a deep purple.)
First a redox reaction occurs turning the solution clear and producing S2O82- ions. [Should be S2O62- ions. Thankx woelen.] Then after half a minute or so the solution begins to cloud with a fine sulfur precipitate. A short while later it is yellow in colour with a distinct (but mild) sulfur smell. The effect on the students as they perform this is quite dramatic.

[edit - typo]
[edit - correction]

[Edited on 31-3-2015 by j_sum1]

[Edited on 31-3-2015 by j_sum1]

woelen - 31-3-2015 at 03:40

The redox reaction of S2O3(2-) and permanganate does not give S2O8(2-) ions, but S4O6(2-). There also is a side reaction with formation of sulfate ions. At the low pH, when there is excess acid and thiosulfate, you also get sulfur and sulfur dioxide. The latter gives what you call "sulfur smell".

MrHomeScientist - 31-3-2015 at 05:59

This thread is full of great ideas. The red silver dichromate is something I'd never heard of, and would pair well with my recent video on the golden rain.

What precipitate does copper sulfate and zinc chloride produce? Copper(I) chloride?

I'll have to steal these for my Youtube channel!

PHILOU Zrealone - 31-3-2015 at 08:38

Speaking of chromate and dichromate...lead provide also colorful precipitate...
Pb(NO3)2 + K2CrO4 --> PbCrO4 (yellow precipitate) + 2 KNO3
Pb(NO3)2 + K2Cr2O7 --> PbCr2O7 (orange precipitate) + 2 KNO3

Colored chemical trees with sodium silicate saturated solution and various soluble salts of transition metal...

Colored borates precipitate of various transition metal with borax saturated solutions...

[Edited on 31-3-2015 by PHILOU Zrealone]

Amos - 31-3-2015 at 10:05

Quote: Originally posted by nebotmo  
Those are some great ideas, thank you! When I go over single-displacement I will definitely have to do something along the lines of the CuSO4 and steel wool reaction and the redox of KMnO4. Very neat. And the Copper (ii) Hydroxide should work well for the double-displacement.


Keep in mind that if you're producing copper hydroxide, the product will differ based on which hydroxide you use to precipitate it. Sodium hydroxide produces anywhere from a deep blue to black precipitate, and when prepared this way copper(II) hydroxide is notoriously unstable, degrading to copper(II) oxide in air. The product formed when using ammonia is instead a light blue that actually dissolves just a bit in water. It's important to stir and to not use too much ammonia, or some of the copper hydroxide will complex with the ammonia to form the brilliant dark blue tetraammine copper(II) complex. Yes, it's a pretty sight, but if your goal is to make the copper(II) hydroxide and use it for something, this complexation will reduce your yield.

j_sum1 - 31-3-2015 at 10:19

Quote: Originally posted by woelen  
The redox reaction of S2O3(2-) and permanganate does not give S2O8(2-) ions, but S4O6(2-). There also is a side reaction with formation of sulfate ions. At the low pH, when there is excess acid and thiosulfate, you also get sulfur and sulfur dioxide. The latter gives what you call "sulfur smell".

Um yeah. :embarrassed: That's what I meant.

nezza - 31-3-2015 at 13:11

A pretty one is the reaction of Ferrous salts with silver nitrate. It is an equilibrium reaction as the Fe(II) is oxidised to Fe(III) and the silver reduced to metallic silver, which appears as fine crystals of silver in the solution.

Mercuric iodide is pretty as it precipitates and then redissolves in excess iodide, but again uses toxic reagents.

Most of the visually appealing precipitation reactions use toxic materials. A number of chromates are interesting colours.

Silver nitrate plus hydroxide giving brown silver oxide.

j_sum1 - 29-4-2015 at 04:48

I tried the potassium dichromate and silver nitrate today in a spare few minutes.
I got the red precipitate and while interesting, it was not quite I imagined. Precipitate was instantaneous, quite fine and more of a ruddy brown than I anticipated.
I will try another time with more dilute solutions.

Amos - 29-4-2015 at 19:42

Quote: Originally posted by j_sum1  
I tried the potassium dichromate and silver nitrate today in a spare few minutes.
I got the red precipitate and while interesting, it was not quite I imagined. Precipitate was instantaneous, quite fine and more of a ruddy brown than I anticipated.
I will try another time with more dilute solutions.


I would imagine that temperature, concentration, and pH all tie in heavily with the color of the precipitate. I've seen the same wide variation in color with lead chromate.

j_sum1 - 29-4-2015 at 20:02

Quote: Originally posted by Amos  
Quote: Originally posted by j_sum1  
I tried the potassium dichromate and silver nitrate today in a spare few minutes.
I got the red precipitate and while interesting, it was not quite I imagined. Precipitate was instantaneous, quite fine and more of a ruddy brown than I anticipated.
I will try another time with more dilute solutions.


I would imagine that temperature, concentration, and pH all tie in heavily with the color of the precipitate. I've seen the same wide variation in color with lead chromate.

Paging woelen.
Are there any particular conditions required to give the prettiest, sparkliest reddest precipitates? (I am assuming at least slightly acidic for dichromate.)

george76904 - 1-5-2015 at 06:20

The reaction of Fe2O3+Al=Al2O3+Fe is always interesting and the reagents can be made at home or bought fairly cheaply.