A Moment of Science

Why Doesn’t Table Salt Stick Together?

Salt crystals are made of sodium and chlorine atoms linked together like Tinkertoys to form a three-dimensional framework.

close up of table salt

Photo: parl (flickr)

Table salt does not stick together.

Our last program talked about table salt, technically called sodium chloride, which is available in iodized form–that means that it includes a small amount of potassium iodide or sodium iodide added to the sodium chloride.

Iodized salt is intended to supply us with the chemical element iodine, an essential nutrient. Iodized salt has been available in the United States since 1924.

Sticky Situation

But a table-salt additive even more common than iodine is some anti-caking agent, to keep the salt crystals from sticking together as they absorb moisture from the air.

Salt crystals are made of sodium and chlorine atoms linked together like Tinkertoys to form a three-dimensional framework. Because the atoms are evenly spaced and the connections between atoms are at right angles, sodium chloride crystals take the form of cubes.

Those cubes can pack closely together, like children’s blocks. As those closely-packed cubes absorb water from the air, they partially dissolve and fuse together.

Sodium Silicoaluminate and Magnesium Carbonate

To prevent this, salt manufacturers add a compound such as sodium silicoaluminate or magnesium carbonate, among other possibilities.

These anti-caking agents absorb moisture more readily than salt and also coat the salt crystals to keep them from touching.

An old-fashioned way to fight salt from caking is to add sugar or rice to the salt shaker. These absorb moisture but don’t coat the crystals as well as commercial additives do.

Star-Shaped Crystals

Some manufacturers also attack the caking problem by adding a small amount of the compound commonly known as yellow prussiate of soda to the brine from which the salt is made.

This additive causes the salt to form star-shaped crystals than can’t pack so closely together as cubical crystals.

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