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The density of powdered sugar is about 20% lower than that of granulated sugar. There is small composition difference between the two - about 3% cornstarch and 0.5% water in powdered sugar, but that's it.

It seems this is an issue of packing then. Why does powdered sugar pack worse than granulated sugar?

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  • $\begingroup$ same reason snow packs worse than a solid block of ice $\endgroup$ – Jim Jul 8 '14 at 19:02
  • $\begingroup$ @Jim bad analogy - I'm not comparing powdered sugar to a solid sugar block $\endgroup$ – eddi Jul 8 '14 at 19:06
  • $\begingroup$ maybe not one solid sugar block, but many smaller solid sugar blocks. The larger gaps in granulated sugar is offset by the denseness of the solid crystals. $\endgroup$ – Jim Jul 8 '14 at 19:21
  • $\begingroup$ @Jim huh? and that offset does what/how/why exactly? $\endgroup$ – eddi Jul 8 '14 at 19:37
  • $\begingroup$ lower density gaps are compensated for in the average density of the granulated sugar by the higher density of the solid crystals. In powder sugar, the gaps are smaller but there is significantly less solid crystal and so the end result is that the average density of granulated sugar is higher $\endgroup$ – Jim Jul 8 '14 at 19:41
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I've been having a play with some granulated and some icing suger (I think "icing sugar" is the same as "powdered sugar") and the thing that strikes me is that icing sugar is less free flowing than granulated sugar. I would guess this is the reason for the density difference.

You mention in a comment that the packing fraction for spheres does not depend on the size of the spheres. This is true, but spheres will only get anywhere near the theoretical packing fraction if they can slide over each other freely and rearrange themselves into a close packed array. If the spheres stick together your get a flocculate that will have a much lower packing fraction.

So my suggestion is that in icing sugar the grains have a tendancy to stick together rather than flow freely over each other. I'd guess this is just down to particle size. Assuming the adhesion between grains is a surface phenomenon then the adhesive strength won't increase with grain size, so the increased mass and size of larger grains makes them easier to pull apart mechanically. The adhesion might be due to Van der Waals forces, or it could be due to an adsorbed water layer making the grain surface slightly sticky.

Response to comment:

The relationship between sediment density and flocculation is well known in the colloid science world (I was a colloid scientist in a previous life) and indeed it's used in industrial processes. For example this patent describes using flocculation to stabilise zeolite slurries. Although it covers zeolite grains in water the principle is exactly the same. If the slurry is not flocculated all the zeolite grains settle into a close packed sediment at the bottom of the tanker and you can't get them out. If you make the zeolite grains stick together they for a less dense sediment (just like the icing sugar forms a less dense powder).

In industry at least most colloid scientists work with fluid suspensions, and sugar while technically still a suspension, is a suspension of solids in air. The way to probe the effect of particle adhesion on powder density would be to control the grain-grain adhesion and show that changes the density. However I don't know how you would do that for a system in air. In fluids it's easy because you can adsorb surfactants and polymers onto the grain surfaces.

It would be interesting to see what density powders were formed in vacuum. If an adsorbed water layer is responsible for the stickiness it should be reduced in vacuum so the powder density will increase. Also you could try vibrating the powder. If particle adhesion has caused formation of a less dense aggregate then vibrating the powder should increase the density because it has will separate adhered grains.

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    $\begingroup$ What is "medium" sugar? Whatever it is, you've just shown it to be less dense than powdered sugar, but "normal" granulated sugar is more dense than powdered. $\endgroup$ – user10851 Jul 9 '14 at 11:54
  • $\begingroup$ This link measures that powdered sugar has 10x more water than granulated sugar. It also has cornstarch which might help your stickiness theory. Do you have any ideas how your hypothesis could be tested/falsified? $\endgroup$ – eddi Jul 9 '14 at 15:11
  • $\begingroup$ @eddi: if you wanted to do this properly you'd use a shear test but that requires specialist kit. The simplest approach is simply to make a conical pile by pouring out the sugar and measure the angle of repose. If I'm correct the icing sugar should have a greater angle of repose. $\endgroup$ – John Rennie Jul 9 '14 at 15:18
  • $\begingroup$ @JohnRennie sorry, perhaps I wasn't clear on what hypothesis I want a test for. I believe that powdered sugar is more sticky. What I want a test/proof for is for your hypothesis of the relationship between stickiness and density (and don't forget your previous answer btw - how would that factor in?). $\endgroup$ – eddi Jul 9 '14 at 15:23
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    $\begingroup$ @eddi: I've updated my answer to respond to your comment $\endgroup$ – John Rennie Jul 9 '14 at 15:40

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