Recently I have watched a video demonstration with a sponge and mercury and the sponge can't absorb mercury like it does with water.
Why is that?
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3$\begingroup$ I'm a bit puzzled at the usage of the “everyday chemistry” tag, because mercury is quite toxic. I don't think many households still have mercury in them, nor should they! $\endgroup$– F'xOct 23, 2013 at 15:12
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1$\begingroup$ @F'x : LOL! then you should remove it from post. $\endgroup$– ashuOct 23, 2013 at 15:48
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2$\begingroup$ @F'x That explains this persistent twitch I have all of the time! $\endgroup$– jonscaOct 23, 2013 at 16:07
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3$\begingroup$ @jonsca You'd be amazed… Until 1950's, people used mercury-based teething powders for babies, because the mercury made their gums softer (this book is a very interesting history of poison) $\endgroup$– F'xOct 23, 2013 at 16:53
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2$\begingroup$ @F'x have you ever heard about thing that measure temperature? $\endgroup$– Senad MeškinOct 24, 2013 at 15:40
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2 Answers
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I think the key feature is the surface tension of mercury-air which is extremely high (about 7 times water-air which is already high), combined with the chemical nature of the sponge, which is typically cellulose.
Since cellulose is a big polysaccharide it has a reasonably favorable interaction with both water (due to the OH groups) and alcohols and alkanes (due to the carbon chain). This causes the contact angle of water, alcohol and alkanes with the sponge to be low (at least lower than 90 degrees) thus allowing capillary action to suck the liquid into the sponges pores. The metallic nature of mercury has an unfavorable interaction with the sponge thus resulting in a contact angle above 90 degrees. This will not allow capillary action because the cosine of angles above 90 degrees is negative, therefore the sponge cannot soak up the mercury.
A sponge made out of metal (silver is common I believe) would be able to soak up the mercury because it has a favorable interaction in terms of surface energy.
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$\begingroup$ A simple way to summarise this would be to observe that mercury doesn't wet cellulose but water does. $\endgroup$ Jul 20, 2017 at 8:43
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You are asking wrong way. Sponge does not absorb almost anything. Apples, sand, dust, wires,...
The point is: Why can sponge absorb water?
Because the interaction of water and the sponge are similarly or even more favorable than interaction inside water. The nature of this effect is capilary action.
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1$\begingroup$ I don't see how “apples, sand, dust and wires” can be compared with water and mercury, as they are not liquids… A sponge adsorbs water, ethanol, alkanes, acetone, and most other liquids. Mercury does stand out. $\endgroup$– F'xOct 23, 2013 at 15:52
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$\begingroup$ You exactly got the point. Molten metal is technically a liquid, but stands out. As will probably Gallium, if somebody could try. Maybe also ionic liquids? All and all, capillary action is the answer. I probably overexaggerated around the important point, which is why it works for water, not why it does not work for mercury. $\endgroup$– ssavecOct 24, 2013 at 8:50
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$\begingroup$ It's not about capillary action. It is about whether mercury will wet the surface. Only then is capillary action important. $\endgroup$ Jul 20, 2017 at 8:44