How Salt Affects Floating D. Crowley, 2007

How Salt Affects Floating To understand how salt affects floating Saturday, August 01, 2015

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 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 / volume Density = 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 AddedObject Floating? 0gYes / No 2g saltYes / No 4g saltYes / 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 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 Gravity