• Question: If salt is made up of alternating positive and negative ions, why don't the crystals of salt in a salt shaker all stick together?

    Asked by freddie to Adam, Geoff, Rob, Sheila, Suzie on 23 Mar 2011 in Categories: .
    • Photo: Suzie Sheehy

      Suzie Sheehy answered on 21 Mar 2011:

      A crystal of salt is much much larger than the ions it’s made up of – so a grain of salt is neutral on average as the negative and positive ions cancel each other out. So they don’t stick together. Unless you put water in there… I’ve had steam from boiling pots get into my salt shaker and that makes it stick together!! (because it dissolves and re-crystallises I guess!)

    • Photo: Adam Tuff

      Adam Tuff answered on 22 Mar 2011:

      The atomic bonds in a crystal of salt aren’t particularly strong – they hold the crystals together, but it’s not a case of putting one crystal next to another and they’d stick together – you’d need to get the bonds almost perfectly aligned to do that, and that wouldn’t be possible. Good question!

    • Photo: Sheila Kanani

      Sheila Kanani answered on 22 Mar 2011:

      I’m not sure the forces would be strong enough for this to happen, but I’m not actually sure!

    • Photo: Robert Simpson

      Robert Simpson answered on 23 Mar 2011:

      Within the crystals, the ions bond because of these forces – but the crystals themselves don’t stick together because a crystal of salt is neutral – it has no charge. The crystals are all different shapes and sizes so they would find it hard to align perfectly and stick together – if they did that would form a bigger crystal – but on the scale of the salt-shaker that is still very small.

Comments

  • Photo: freddie

    freddie commented on :

    But why don’t the edges match up positive to negative? I’ve drawn a diagram which might make my question clearer. http://i.imgur.com/3JMYp.jpg

  • Photo: doppler

    doppler commented on :

    My guess (and it’s completely a guess) would be that the Na+ and Cl- ions don’t form a nice straight line at the edge where the whole structure of a single salt crystal stops, and that the structure isn’t as orderly as it would be within the crystal, because at the edges the ions are left without as many other ions surrounding them as they would “like”. In some places the surface must be left with some ions on a higher layer than other edge ions are – after all, when the salt lattice structure is forming, it wouldn’t form a whole smooth “sheet” of ions to build up its structure in the same instant. Hence some ions jutting out from the surface would get in the way of two salt crystals bumping together close enough for this attraction to be strong enough, let alone bumping together in the right orientation, for the molecules to bond in this way. The structure needs to be rearranged for two crystals to become one, for example through dissolving and then recrystallising, or melting the salt then cooling it again. I hope that makes sense – ’twas very fun thinking about it even my answer isn’t correct. It would be much appreciated if (one of) the scientists could give me their view(s) on this, please 🙂 – thank you for the question, freddie!

  • Photo: Rob

    Rob commented on :

    Wow! i just answered this and found a whole discussion going on here about it. Good work guys 🙂