How Salt Affects Floating

D. Crowley, 2007

How Salt Affects Floating

To understand how salt affects floating

Wednesday, April 19, 2023

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)?

Gravity

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)

Reaction Force - up thrust of water

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)

Gravity

Reaction Force - up thrust of water

Reaction Force - up thrust of water

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 object in any way

Hint: -Density of liquid

Density = mass / volume So by adding salt, we are increasing the mass of the medium 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 medium is

Density = mass / volumeDensity = 250 / 250

Density = 1

Adding salt to the experiment will increase the mass, but the volume will remain very similar, 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 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 medium

The density of the object remained the same, and having a smaller density than the medium meant it floated

Gravity

Reaction Force - up thrust of water

Reaction Force - up thrust of water

Gravity

Force diagrams Adding more salt to the water increased the density of the medium

This meant that the force arrows changed

Originally the reaction (up-thrust) force of the medium 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 as the medium’s density increased

Gravity

Reaction Force - up thrust of water

Reaction Force - up thrust of water

Gravity