If you were stranded on an island, could you use seawater to make salt and eat it?

Being shipwrecked and stranded on an island is a classic plot in literature and film and television works, but the probability of it happening in real life is not very high. However, who can predict what will happen in the future? If you were stranded on an island one day, would you be able to directly use seawater to make salt for consumption?

If you want to survive on an isolated island, the first thing you need to solve is food. Assuming you have a certain ability to survive in the wild, you can still get food, such as catching some small animals on the island. If you are good at swimming, you can also go fishing. However, food alone is not enough. People cannot live without salt, so how can you get salt? There is no shortage of salt on the island, because the sea water is full of salt. The problem is how to get the salt out of the sea water. The simplest way is to dig a pit, dry the sea water, and precipitate the sea salt.

The salt precipitated from directly drying seawater is coarse salt, which contains many other impurities in addition to the main ingredient sodium chloride. Such salt not only tastes bitter, but long-term consumption will also have adverse effects on health. Considering that we don’t know when we can be rescued if we are stranded on an isolated island, we still need to purify these coarse salts under limited conditions.

There are three mainstream seawater salt production technologies, namely, salt field sun drying, heat pump compression and electrodialysis. Among the three methods, the salt field sun drying is the most traditional. However, for us who are stranded on an isolated island, there is no way to do it completely. We can only purify the sea salt appropriately under limited conditions to improve the taste and eliminate the harm. How to do it? First, we need to circle a piece of beach, then take seawater and pour it on the circled beach. After the sun dries the sand, we continue to pour seawater on the sand, and repeat this process.

Repeatedly splashing the beach with sea water is to increase the salt content of the beach.

The second step is to dig a pit and try to find stones on the island to fill the bottom of the pit. Make sure to pile it tightly to minimize the amount of seawater that seeps down into the pit. After the pit is dug, we also need to dig a drainage ditch for the pit. This is to facilitate the collection of concentrated salt seawater later. If there is no such ditch, we can only use containers to scoop it out. After all this is done, we can move on to the third step. Put the piece of sand with a high salt content that was repeatedly splashed with seawater into the newly dug pit, and then pour seawater into the pit and stir it vigorously. The purpose of this is to dissolve the salt in the sand into the seawater, turning ordinary seawater into saturated concentrated salt seawater.

If we use the traditional salt field sun-drying method to make salt, we should first treat the impurities in the saturated brine. The simple method is to add potassium carbonate to react with the calcium and magnesium ions in the sea salt water to precipitate them. However, this is impossible for us who are stranded on a desert island, so let's skip it and move on to the fourth step.

The fourth step is to remove the solid impurities in the saturated brine. The basic method is physical filtration. You can use the rocks and sand on the island to lay the filter materials from top to bottom from coarse to fine, and then pour the saturated brine on it, and use a container to catch it. In this way, we can get relatively pure saturated brine. Solid impurities are relatively easy to remove, but there are still many impurities dissolved in the brine itself. Only by removing these impurities can we get real refined salt.

Next, we proceed to the fifth step, pour out about one-fifth of the concentrated brine and set aside, and heat and boil the rest until it is dried up.

After the water is boiled dry, we will find a large number of crystals left at the bottom of the container, which are coarse salt. There are many impurities in coarse salt, which not only tastes bitter, but is also harmful to health, so it needs to be purified into refined salt, so the last step, the sixth step is purification. Heat the small amount of concentrated brine that was previously served to boiling, and then pour the coarse salt into the boiling concentrated brine and stir and blend it thoroughly. After it is fully blended, let it stand for a while, and then pour out the hot concentrated brine. At this time, you will find that there are some very white crystals left in the container. They are refined salt. Although it is just a simple purification, these refined salts no longer have a bitter taste, and there are very few impurities in them. Long-term consumption will not affect health.

What is the principle of purifying refined salt?

Different substances have different solubility in water, and the solubility of most substances will increase significantly with the increase of water temperature. For example, the solubility of potassium nitrate at 0℃ is only a little more than 10g, while its solubility can reach 100g at around 55℃. But sodium chloride is special. The increase in temperature has a very limited effect on its solubility. At 0℃, the solubility of sodium chloride is 35.7g, and at 100℃, the solubility is only 39.8g. Therefore, after the crude salt is fully stirred and blended with the boiling saturated brine, most of the impurities in the crude salt are dissolved into the water due to the increase in solubility, while the solubility of sodium chloride is very limited, so it remains, and we get refined salt.

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