[cliverlong]

Hi,

   Four salt (NaCl) questions.

1. Why are the surfaces of salt flat (at least at a macroscopic scale)? If you build a stick and ball or sphere packing mode of NaCll, this suggests the growth of a salt crystal from an evaporating, saturated aqueous solution could occur in any direction. So the resulting crystal could have peaks and troughs on its surface as the ions bonded randomly with the surface of the crystal. However, one sees a smooth surface on the crystals formed. Why is this?

2. Growing large NaCl crystal. I tried a saturated distilled water solution in a clean jam jar and dangled a piece of string in the solution tied to to a pencil and covered the exposed surface with filter paper. The string dangled through a hole in the filter paper. My thinking was the filter paper would help the water to evaporate and the crystals would form on the rough string as the solution passed its saturation point. What's happened is the salt solution has passed up the string and crystallised on the edge of the filter paper rather than forming a crystal on the string. So I have lots of small crystals - that's where I started.

3. How can I explain pyramidal salt flakes (I think they are from "sea salt")? The pyramids are up to 3mm across the base and have ridges a bit like those Mesopotamian or Mexican pyramids. Is the shape a result of the presence of iodide ions? Can a salt crystal form which is a mixture of NaCl and NaI thereby affecting the resulting crystal shape?

4. Where does the energy come from to transform Na(s) and Cl(s) to NaCl(s)? One reads as the overall salt-forming reaction is energetically favourable / exothermic that determines the direction of the reaction. When one talks about formation of salt one says the Na is ionised to Na+ and donates its electron to Cl to form Cl- and the electrostatic attraction between the ions forms the ionic bonds. However, when one explains ionisation of an atom one says the cause of the ionisation is the absorption of an e.m. quantum of appropriate energy  by the outer shell electron. I haven't worked out what the photon energy needs to be and hence the frequency of the quantum - whether it would be a light photon or an Xray or a microwave or whatever. Now, when one reacts sodium with chlorine it is described as burning and the energy given off "drives" the reaction. Now, the energy given off by the burning is heat and light. Is the energy from the heat or light as e.m. radiation or is it the increased temperature from the atoms "banging together" that causes the ionisation? If it is the "banging together" that provides the activation / ionisation energy, where are the e.m. photons to make the ions that react?

Thanks


Clive

[Borek]

However, when one explains ionisation of an atom one says the cause of the ionisation is the absorption of an e.m. quantum of appropriate energy  by the outer shell electron.

Cl to Cl^- is not an ionisation. Besides, to make things more complicated, ions are stabilized in water thanks to hydration.

As to the first question - there is an energy barrier associated with adding new ion on the already existing flat surface. However, once this ion sits there, it is much easier for other ions to occupy adjacent positions. Thus once it starts, next layer is laid giving flat crystal surface.

When you were groving crystals air in the jar (between solution surface and filter paper) was water vapor saturated, so no drying was possible. At the same time air above was much more dry so that's where the action (evaporation/crystallization) took place.

[Arkcon]

Hi,

   Four salt (NaCl) questions.

1. Why are the surfaces of salt flat (at least at a macroscopic scale)?

When is it not flat?  This is, is it flat at a microscopic scale(scanning electron micrographs are all over the net)?  Is that enough to explain the accumulated errors?

[cliverlong]

However, when one explains ionisation of an atom one says the cause of the ionisation is the absorption of an e.m. quantum of appropriate energy  by the outer shell electron.

Cl to Cl^- is not an ionisation.

Thanks for replying

I didn't actually write that  Cl to Cl^- is an ionisation what I wrote was

When one talks about formation of salt one says the Na is ionised to Na+ and donates its electron to Cl to form Cl- 
.

... but I may easily have mixed up the ideas in my mind so thanks for pointing out the difference.

Besides, to make things more complicated, ions are stabilized in water thanks to hydration.

I didn't know about that. I will read more on that topic.

As to the first question - there is an energy barrier associated with adding new ion on the already existing flat surface. However, once this ion sits there, it is much easier for other ions to occupy adjacent positions.

I don't understand why that is true. Is there an explanation of that effect?

Thus once it starts, next layer is laid giving flat crystal surface.

When you were groving crystals air in the jar (between solution surface and filter paper) was water vapor saturated, so no drying was possible. At the same time air above was much more dry so that's where the action (evaporation/crystallization) took place.

Ah ... I think I see why the crystals formed on the paper. But I remember making copper sulphate crystals by dangling bits of string in a solution in a jam jar. I will do some googling on techniques to grow larger sodium chloride crystals. I read about tying thread around "seed" crystals but I can't understand how that would hold the seed in place.

Thanks for your help.

Clive

[cliverlong]

Hi,

   Four salt (NaCl) questions.

1. Why are the surfaces of salt flat (at least at a macroscopic scale)?

When is it not flat?  This is, is it flat at a microscopic scale(scanning electron micrographs are all over the net)?  Is that enough to explain the accumulated errors?

Thanks for your reply.

I'm not entirely sure I understand your answer. I took your suggestion and searched for electron micrographs of sodium chloride. Could I rephrase to,

"Not all, in fact few, sodium chloride crystals are perfect cubes with flat surfaces - that takes carefully controlled conditions to grow such a regular crystal. However. the right angles between the plane surfaces on a salt crystals are seen much more often as in this micrograph

http://www.denniskunkel.com/product_info.php?products_id=10067

which show the flat plane surfaces creating hollows and depressions in the surface of the sodium chloride crystal. However, the right angles between the planes are still clearly visible"



So the school-text statement that salt crystals are cubic is simplistic to a bit misleading ??

Thanks

Clive

[Arkcon]

So the school-text statement that salt crystals are cubic is simplistic to a bit misleading ??

Yes, but the image I was thinking of was more like this one: