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Why is the sea salty?

Professor Rachel Mills explains why water is such a powerful solvent and how this contributes to making our oceans salty.
PROFESSOR RACHEL MILLS: So water is a really powerful solvent. The water molecule has some really useful properties that keep it liquid on this planet. One is that it takes up a lot of energy without vaporising, the other is that it floats when it freezes. But what we’re going to talk about this week is that water is actually very good at dissolving salt. And so a lot of salt can dissolve in the oceans which makes them have their characteristic taste and properties. So on average, seawater contains something like 35 grams of salt for every litre of water. And if we evaporate that water, we find we have salt crystals.
And it was the Romans that actually worked out how to evaporate tasty salt crystals from seawater to get rid of the chalky calcium carbonate and the really bitter magnesium salts that are the minor components of seawater and produce crystals which we can sprinkle on our food. So every element that is present on this planet, in the solar system indeed, is present in seawater at some concentration. At the higher end, we have chloride, which is present at 19 grams in every litre of seawater. And we can measure this relatively easily by titrating the chloride against chemicals to work out what the concentration is.
But at the lower end, we have isotopes such as thorium 234, which we have to measure by radioactive decay, and there’s only something like 100,000 atoms of thorium 234 present in every litre of seawater. So luckily, the major constituents in seawater are present in the same relative proportions. So this allows us to actually make measurements of one constituent and estimate the salinity of seawater wherever we are in the oceans. So in the early days, oceanographers evaporated seawater and weighed the salts that precipitated out. That gave us an estimate of how much salt there was in a litre of seawater. And then we moved on to doing chemical titrations where we measured one component chemically to estimate the salinity of seawater.
But nowadays, actually, we just measure how well an electric current is passed through the solution, and we compare this with the electric current going through standard electrolytes. And this allows us to estimate the salinity content with great precision wherever we are in the oceans. So let’s go back to Jon and Verity aboard Calista to see how they’re measuring temperature and salinity in the oceans.
VERITY: So now, we’re going to measure the salinity and temperature of the sea right here with the T and S probe.
JON: To measure the salinity, what we actually measure is the conductivity of the seawater passing around the probe. And if we know the temperature of that water, from that we can calculate the salinity, how much salt is in that water. So let’s pop the probe in and see how salty the sea is right at the surface here.
OK, I’ve got the probe at the surface.
VERITY: I’m getting a reading of 13.8 degrees centigrade and 31.4 for the salinity.
JON: OK, so let’s see how the salinity changes if we go a little bit deeper.
OK, I’ve gone down about five metres.
VERITY: And the salinity reading is 33.2.
JON: So we’ve got saltier water here underneath a layer of fresher water at the surface. And that’s because we’re in an estuary where a river meets the sea. So we’ve got fresh water sitting on top of salty water. The fresh water is less dense, and the salty water is more dense, so it’s on the bottom.
Professor Rachel Mills explains why water is such a powerful solvent* and how this contributes to making our oceans salty. Over the next few steps we will explore the question ‘why is the sea salty?’ by considering where the salt comes from, how much there is in the ocean and where does it go?
Rachel describes how early oceanographers measured the saltiness of the ocean and, with the help of Jon Copley and Dr Verity Nye (a former Postgraduate Research Fellow of the university), demonstrates the modern day method using technology.
What units are used to measure salinity?
Often we think in terms of g per kg of water and we can express this as % or as parts per mille (‰). But when Verity measures conductivity of seawater she actually is using the Practical Salinity Scale and this doesn’t have units (i.e. it is dimensionless). You can read more about salinity here.
You will see in the following sections how temperature and salt content both affect the density of the water in the estuary just like they do in the open ocean. By measuring salinity we track the mixing of the fresh water (zero salinity) with the seawater (salinity of around 35). In general the saltier water is denser so sits below the fresher water which is less dense. The temperature differences depend on the temperature of the river water which is colder than the oceans in the winter but is a similar temperature to the oceans in the summer months. The relationship between saltiness, temperature and density is a complicated, non-linear one but in an estuary, as we have seen in the video, the dominant control on density is the salinity of the water. Colder fresh water flowing in from the land flows over the top of warmer saltier seawater and is mixed up in the estuary as the tide comes in and out.
A solvent is something into which a solute will dissolve in order to make a solution. Water is a solvent for anything that is soluble in it, salt is a solute because it can be dissolved in a solvent and salty water is a solution.
Titration is a means of determining what sort of or how much of a solute is dissolved in a solvent.
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Exploring Our Ocean

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