How Salt Affects Floating
D. Crowley, 2007
How Salt Affects Floating

To understand how salt affects floating
Floating

The boat below floats, whilst the rock sinks

Draw out the pictures below, adding force arrows to them - think
about the size of the arrows and their direction (can you name
the forces)?
Reaction Force - up thrust of water
Reaction Force - up thrust of water
Both arrows are the same size
- meaning the boat remains
where it is (i.e. it doesn’t sink
or lift out of the water)
Gravity
Gravity force arrow is longer meaning the rock will
accelerate towards the bottom
of the ocean
Gravity
Density

Remember, we said that things float due to density

If an object is more dense than the liquid it is in, then it will sink

If an object is less dense than the liquid it is in, then it will float

So what is going on with the Dead Sea?
Dead Sea

The Dead Sea is called “Dead” because nothing can grow in it - it is too salty

There is 300g of salt per 1kg of water - 10x more salt than found in Oceans

The Dead Sea also contains 21 minerals,
12 of which cannot be found in any other
sea or ocean

The Dead Sea is so salty because rivers
only flow into it, not out of it. This means
the only way water removed is from
evaporation, leaving the salt behind…

For some reason all this salt seems to help
things float in the Dead Sea - why is this
so?
Salt versus Unsalted!

Look at the force arrows - if we have the same object in normal water
and salted water the gravity force pulling it down is the same

But - the force pushing it back up from the water (the reaction / up
thrust) is bigger in the salt water than in normal water

This means something which sinks in normal water, can float in salty
water (such as the Dead Sea)
Reaction Force - up thrust of water
Reaction Force - up thrust of water
Gravity
Gravity
Experiment

You are going to see if you can make an object float in your own
miniature Dead Sea

How can you devise an experiment to make our object float,
bearing in mind you cannot change the object in any way

Hint: Density of liquid
Density = mass / volume

So by adding salt, we are increasing the mass of the water meaning its
density increases, helping our object float (bigger up-thrust force)

You have a 250ml beaker of water - this has a start mass of 250g and
start volume of 250ml

This means the original density of the water is
Density = mass / volume
Density = 250 / 250
Density = 1

Adding salt to the experiment will increase the mass, but the volume
will stay the same, so you can work out what density of liquid you need
to keep the object afloat

So what density do we need to reach to keep the object afloat?
Experiment

The crucial thing in this experiment is to accurately record the mass of salt
you add to the water (using the salt balances)

You will need to weigh exactly how much salt you are putting in - perhaps
2g salt per time is a good amount

Record how much salt you needed to put into the beaker to make the
object float

Work out the new density of the water + the salt (remember mass = the
water (250) + the salt you added. The volume remains 250)
Density = mass / volume
Salt Added
Object Floating?
0g
Yes / No
2g salt
Yes / No
4g salt
Yes / No
Force diagrams

Adding more salt to the water increased the density of the water

This meant that the force arrows changed

Originally the reaction (up-thrust) force of the water was smaller than
that of the gravity force pulling the object down

But by adding the salt, we increased the up-thrust force, so they
became balanced, and the object floated…
Reaction Force - up thrust of water
Reaction Force - up thrust of water
Gravity
Gravity