Salinity is the dissolved salt in water. Ocean water has a salinity that is about 3.5% salt. About 90% of sea salt is sodium chloride (or table salt) the rest is made up of chlorine, sodium, magnesium, sulfur, calcium, and potassium.
Saltwater Circuit
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Students investigate the conductivity of saltwater, and develop an understanding of how the amount of salt in a solution impacts how much electrical current flows through the circuit.
Argo Floats measure salinity in a very similar way. Water passes through a cell on the float that records the conductivity and then converts this into a salinity reading.

What you need:
masking tape
1.5 volt battery
2.5 volt Light bulb and holder
Wire (and dog clips)
2 ice block sticks
aluminium foil
water in a small container
salt
a stirrer
Instructions

  • First, cover two ice block sticks with aluminium foil. These are now your probes.
  • Connect one wire to each probe.
  • Connect the opposite end of one wire to one terminal of the light bulb socket.
  • Connect a wire to the opposite terminal of the light bulb socket to the battery.
  • Connect the wire from the other probe to the battery
  • You can see if your tester is working by touching the metal together. This will complete the circuit and make the light bulb glow. If it doesn't glow, check your connections to make sure everything is taped together in the right way.

Now to use your saltwater tester, put just the tips of the metal in saltwater, about an inch apart. Make sure the two metal parts don't touch. The saltwater will act like a wire, connecting the metal sticks, completing the circuit, and making the light bulb glow.

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How this works.
The light bulb glows in saltwater because the saltwater acts like an invisible wire to connect the circuit. That's because when you add salt to water, the salt molecules dissolve in the water and break into smaller parts called ions. The ions carry electricity through the water.

Fresh water doesn't have these ions. So it's harder for the electricity to move through the water. It doesn't complete the circuit, and the light bulb doesn't glow.


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A few hands- on experiments

Measure 1 cup of cold water into each of the Styrofoam cups. Measure 1 tbsp. of salt into one cup, then mix well with spoon. Don’t add anything to the other cup. Put both cups in the freezer for 10 minutes

Put an ice cube on a plate. Sprinkle salt on the ice cube and watch it melt. Salt lowers the temperature or freezing point of water. It will freeze, but it has to be colder than the freezing point of unsalted fresh water

Pull out the two cups in the freezer once 10 min is up. You will find the cup with the plain water has started to freeze. The cup with the salt will not freeze because it has lowered the freezing point of the water.


Salt to float objects in water

Fill a bowl with 2 cups of cold water. Put an egg, marble, apple and a rock in the bowl on separate occasions. Add salt to the bowl one teaspoon at a time to find out how much salt is needed to make the object float. Start with the egg because it takes about 9 tsp. of salt to make it float

Use other items such as pencils, pens and balls. The density of the item will determine how much salt you need to float it. Adding salt happens to make the water denser, so an item floats because water becomes denser than the item.