Secondary Science – Salt

Some of us like to put a little salt on our food to enhance the flavour. While there is no doubt that many of us have too much salt in our diets there is a lot more to salt. Salt is truly one of the chemical “superstuffs” that makes an appearance in many aspects of the KS3 and KS4 science curriculum. The topics below could be used as projects and research ideas in specific areas of the curriculum, or the questions developed into starter or plenary discussions.

Salt and diet
Salt or to be precise, the sodium in the sodium chloride is an essential element for all living things and is involved in transport across cell membranes and the function of muscle and nerve cells.
• How are sodium (and potassium) ions moved across cell membranes?
• Why do nerves and muscles depend on sodium ions?
• What effect does too little or too much salt have on health?

Salt and food
Salt is used for flavouring and as a preservative.  Before canning, refrigeration and vacuum packing became common, salting was an important way of keeping food for long periods.
• What foods have been preserved using salt?
• How does salt preserve food?  (investigate osmosis)

Salt and trade
Prehistoric people living away from the sea found it difficult to get enough salt so it became one of the earliest traded commodities reflected in the names of places where salt was found and traded.
• Where is salt found in the UK?  (look for place-names with “-wich” in their name)
• Why does the city of Salzburg in Austria have salt in its name?
• What places are named after salt?  (look for “sal” and “hal” in names)

Salt in words
Salt was such an important and useful commodity that it has had a big impact on language.
• What words are derived from salt?
• How are words like “salary” and “salad” linked to salt?

Salt and geology
The presence of salt deposits tells us a lot about past climates and earth movements.
• How did the sea become salty?
• How did rock salt deposits come to be buried underground?

Salt and climate
Ocean currents have a big effect on our climate and weather. Some scientists worry that the warming effect of the Gulf Stream could be affected by the melting of arctic ice.
• How does the density of salt water compare to pure water?
• What causes the ocean currents to circulate around the world?

Salt and roads
Every year thousands of tonnes of rock salt are spread on roads to keep them free of ice.
• Where does the rock salt used on icy roads come from?
• How does salt reduce the amount of ice on roads?

Salt and water
Salt is extracted from seawater and rock salt by driving off the water from salt solutions. Other salts in sea water have a bitter taste and must not be mixed with table salt. Table salt has other substances added to keep it dry and free-flowing.
• What happens if pure salt is left in the air?
• How is salt extracted from sea water?
• What substances are added to table salt and why are they added?

Salt and elements
In the Middle Ages salt was thought to be one of the three “principles” or elements (along with sulfur and mercury) from which other substances were derived. Lavoisier suspected that salt contained a metallic element, but it was Davy who isolated sodium.
• Who was Paracelsus and why did he think that salt was a special substance?
• Who was Lavoisier and how did he revolutionise chemistry?
• Why did Lavoisier think that salt contained oxygen?
• How did Davy isolate sodium?

Salt and crystals
The cubic shape of sodium chloride crystals has intrigued people for centuries but it took the discovery of X-rays to work out how the sodium and chloride ions were arranged.
• What is the arrangement of sodium and chloride ions in salt crystals and why are they arranged in this pattern?
• How did W.H.Bragg and W.L.Bragg discover the structure of salt?
• How has X ray crystallography proved useful?

Salt and industry
Salt has found many uses apart from in food, for example in making glass and soap. In the nineteenth century the use of salt in manufacturing processes led to the growth of the chemical industry.
• How is salt used to make soap?
• How did the Leblanc process cause the development of the chemical industry? (for example in the north-west of England)
• What are the industrial uses of salt today?

Peter Ellis

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