Floating in Salt Water

These are assorted internet articles on the reality that it is easier to float in salt water compared to fresh water:

 From: http://www.andybrain.com/sciencelab/2008/01/13/why-is-it-easier-to-swim-in-the-ocean-learn-about-buoyancy/

 Why is it easier to swim in the ocean? Learn about buoyancy.

Description:

Learn about buoyancy. This lab answers the question: Why is it easier to swim in the ocean than in a lake?

Equipment needed:

An egg (it can be either raw or hard-boiled. It needs to be fresh.)

A tall, wide glass

Water

A long-handled spoon for stirring in the glass

Salt (you’ll need a lot of salt, more than what’s contained in a salt shaker. If you’re purchasing for a group or class, shop for salt in bulk.)

The Digital Bits Science Lab Experiment:

Carefully place your egg in the empty glass. Then fill the glass with water, leaving an inch or so of space at the top. You’ll see that the egg sinks, and rests happily at the bottom of the glass. (If your egg floats in the fresh water, that’s an indicator of an old egg. Use a fresh egg instead.)

Now that we’ve seen the egg sink, use the spoon to carefully lift the egg out of the glass.

Next, pour salt into the water.

Keep pouring.

And pour a little more.

Stir it.

The goal is to get so much salt in the water, that you can’t dissolve any more. Stir the mixture for a while. If you lift the spoon out after stirring, and still see a few salt grains clinging to it, your salt-water mixture is ready.

Using the spoon, carefully lower the egg into the water. If you’ve mixed enough salt in the water, the egg will now float!

The egg floats in the salt water because it has more buoyancy in salt water than in fresh water. Buoyancy is determined by the density of the water. Fresh water is not very dense. Things will sink easier in fresh water. Salt water consists of water mixed with a LOT of salt. That salt adds density to the water. So when you put the egg in salt water, the heavier density of the water causes the egg to float.

This is why it’s easier to swim in the ocean than in a lake: the ocean is salt water, a freshwater lake is not. Your body is more buoyant in the higher-density salt water, and you can more easily float.

Regie, not sure what you’re asking, so I’m going to rephrase your question:

 “How does salt increase the density of water?”

In order to understand this question, you need to understand the concept of a solution: a solution is a mixture of two different substances – one often being a liquid. In our solution of water and salt, the water dissolves the salt. But that doesn’t mean the salt is gone. The salt is converted into a solution with the water. You can’t see it, but it’s still there, taking up space in the water. The more salt we add, the more space the salt takes up. The more space the salt takes up, the more dense the solution becomes.

The short version: Adding salt to water makes the solution more dense.

https://deepstop.wordpress.com/2009/02/26/buoyancy-salt-water-vs-fresh-water/

 Buoyancy: Salt Water vs. Fresh Water February 26, 2009

 Apparently this is a great source of confusion – or perhaps my previous writings about the subject have attracted many searches, and so biases my view of what divers are interested in. Anyway, I’m going to attempt to write something clear on the subject. Here are some of the searches on the topic that have found their way to my blog:

bouyancy of sea water
bouyancy salt
buoyancy salt water vs fresh scuba
buoyancy calculations salt water
buoyancy difference between salt water a
buoyancy fresh salt
buoyancy of salt & freshwater
buoyancy of sea water calculators
salt kg required for tank foot
salt water buoyancy calculation
salt water fresh water buoyancy
which is more buoyant fresh or salt water
why is bouancy more in salt water than fresh
why salt water is more buoyant than fresh
sea water density bar msw
why is salt water more buoyant
is salt or fresh water more buoyant
buoyancy salt water fresh water

Obviously people want to know. The answer is fundamentally simple – objects (including divers) are more buoyant in salt water than in fresh water because salt water is denser than fresh water.

Why is salt water denser than fresh? Because salt is denser than water and so if you add it to water the resulting solution is denser than pure water. By how much? The salinity (saltiness) of the ocean varies, but the generally accepted average amount is 2.5%. So  salt water weighs 2.5% more than the same volume (a gallon or litre, for example) of fresh water.

Buoyancy is an upward force equal to the weight of water displaced by the object. A cubic foot of fresh water weighs 62.4 pounds, so an object with a volume of 1 cubic foot would experience 62.4 pounds of upward force due to buoyancy when immersed in fresh water. Gravity will exert an opposite (downward) force equal to the object’s weight, so if the object weighs less than 62.4 pounds it will float. If the object weighs more than 62.4 pounds it will sink. If the object weighs exactly 62.4 pounds it will be neutrally buoyant, and will stay where it is unless pushed by something (current, turbulence, a diver, etc.).

In salt water, that same 1 cubic foot will displace 64 pounds, because that’s what a cubic foot of sea water (which you recall is heavier by 2.5 percent) weighs. So there is 1.6 pounds more buoyancy in salt water than in fresh. That means if an object with a volume of 1 cubic foot weighs 63 pounds it will float in salt water and sink in fresh water. So objects in salt water are more buoyant than objects in fresh water because salt water is denser than fresh.

Note that it is incorrect to say that salt water is more buoyant than fresh water. Objects in salt water are more buoyant than objects in fresh water. The buoyant force is exerted on an object, not the water itself.

Hope that clears things up!

P.S. The density of salt is 2.16 grams per cubic centimetre vs. the maximum density of fresh water at 1 gram per cubic centimetre.  Sea water is about 3.5% salt by weight. A kilogram of sea water will have 35 grams of salt and 965 grams of fresh water (I’m ignoring the stuff that’s in sea water which isn’t water and salt). The volume of the water will be 965 cubic centimetres, while the volume of the salt will be 35 grams divided by the density of 2.16 which is 16.2 cc. The total volume of the components of this kilogram of sea water is 981.2 cc, versus a kilogram of fresh water at 1000 cc, so the combined density is almost 2% more with the salt than with fresh. This is a little less than the 2.5% real world difference – what’s going on?

My chemistry knowledge runs out at this point but my guess is that when salt dissolves in water, the salt molecules pack a little bit closer together with the water molecules than they do with each other, making the volume a bit smaller than the sum of the volumes of the salt and the water separately. If I find out the real answer I’ll post it here, unless someone beats me to it.

Hi Shawn,

To lift 100 lb you need to displace 100 lb of water, plus a little bit. For fresh water, which weighs 62.4 lb/cubic foot, you need 100 divided by 62.4 cubic feet of air, or just over 1.6 cubic feet. Remember though, that at depth, the volume of air from the tank that is required will increase with depth, so you need to multiply by the number of absolute atmospheres of pressure to get the volume used from the tank.

For instance, with our 100lb lift, if we were starting from 102′, which is 4 ata (absolute atmospheres), we then need 4×1.6=6.4 cubic feet of air from the tank. As the object ascends, the air will expand and either spill from the lift bag or increase the lift (and therefore the rate of ascent) if the bag can hold the extra volume. But a 100lb lift bag will hold 1.6 cubic feet of air and work at any depth, the difference is how much air you consume from your tank to fill it.

In salt water, which weighs 64lb/cubic foot, the volume is slightly less at 100/64 or 1.56 cubic feet. If you wanted to take the buoyancy of the object you are lifting into account, you would just subtract the volume of the object from that volume. So if the object’s volume was .46 cubic feet, you would only need 1.1 cubic feet of air.

Chris

Hm, not sure why you ask, but…

100 cu ft of fresh water is 6,242.8 pounds and 2,831.68 kg

100 cu ft of sea water is 6,398.87 pounds and 2,902.48 kg

100 tons of water (or 100 tons of anything) is 200,000 pounds or 90,718.4 kg

100 tons of salt water is 200,000 pounds or 90,718.4 kg

100 tons of fresh water is 3,203.69 cubic feet or 90.7184 cubic metres

100 tons of salt water is 3,125.55 cubic feet or 88.5058 cubic metres

This is all very easy to calculate using calchemy.

Chris

There’s a very similar question on the PADI open water exam. Goes something like “A neutrally buoyant object in salt water will ____ when in Fresh Water”

(a) sink (b) float (c) stay the same (d) impossible to tell.

Because salt water is denser, it provides more buoyancy for an object of a given volume. So in fresh water there will be less buoyancy, so the upward force is less. So the object will sink in fresh water. We know this intuitively I think because it is easier for a swimmer to float in the sea than in a fresh water lake, on in the Dead Sea, which has more salinity than regular sea water, people float really well, almost like they’re on a air mattress.

Sorry it took so long to respond. Had some issues to deal with.

Chris

I’m currently taking a scuba class, and on a recent test I got an answer wrong that I disagreed with; the instructor was able to tell me it was wrong, but didn’t really address my reason for disagreement… the question was:

“If an object is neutrally buoyant in salt water, it would mostly _______ in fresh water.”
a. Float (positively buoyant)
b. Sink (negatively buoyant)
c. Neither float nor sink (neutrally buoyant)
d. The question lacks enough information

-I chose d. based on the fact that the question didn’t address what the ‘object’ was… is it not at all possible for an object to be both neutrally buoyant in salt water AND neutrally buoyant in fresh water? Aren’t there several different levels of neutral buoyancy? Does the weight of the object in relation to the difference in weight/density of salt water not play a part?

Thanks
Thomas Houston

Hi Thomas,

The correct answer is (b) sink. Buoyancy is equal to the weight of the water displaced by the object, so as the volume is the same in both cases, but that volume of fresh water weighs less than the same volume of salt water, the buoyancy is less and the formerly neutrally buoyant object will now sink. The object is question has exactly the same density as salt water, which is why it’s neutrally buoyant. This explains why we need less weight when diving in fresh water.

Chris

P.S. A physicist might argue the point. Some “objects” may have special properties, like individual atoms for instance, or stuff that dissolves.

From: http://geography.about.com/library/misc/ucghyben.htm

Salt Water vs. Fresh Water – Ghyben-Herzberg Lens
Guest Column by Muncel Chang, Department of Geography, Butte College (California)

Dateline: 09/19/00

Fresh water is lighter than salt water. Therefore, fresh water “floats” on top of salt water. This principle becomes extremely important when considering the drilling of a well in order to tap into the ground water of any island. The weight of the rain water that percolates into the ground depresses the salt water beneath it forming a profile that has the appearance of a lens. This is called the Ghyben-Herzberg lens. The principle of this relationship was discovered independently by a Dutch scientist named Baden-Ghyben and a German scientist named Herzberg.

The underground boundary that separates the fresh water layer from the salt water is not a sharp boundary line. In reality, this boundary is a transition zone of brackish water (fresh/salt mixture). This is caused by seasonal fluctuations in rainfall, tidal action, and the amount of water being withdrawn either by humans or by natural discharge.

Fresh water has a density of 1.0 while salt water has a density of 1.025. From this, you can see that salt water is slightly heavier than fresh water. The ratio between the two is 41:40. The formation of the Ghyben-Herzberg lens has a profound effect upon the availability of fresh water on an island. This principle essentially states that for every foot of ground water above sea level there are forty feet of fresh water below sea level! The mathematical formula for the fresh to salt water relationship is:

hs = hf   /  es – ef

where hs is the depth of fresh water below sea level, hf is the depth of fresh water above sea level, es is the density of salt water, and ef is the density of fresh water. Using the common density figures for fresh and salt water the formula can thus be simplified into

hs = hf  /  .025

http://www.h2ouniversity.org/html/3-5_sci_salt_fresh.html

Salt Water vs . Fresh Water
How can you tell the difference between salt water and fresh water? Try the experiment below and find out.

You will need:
  • 1 egg
  • A glass of water
  • 12 tablespoons of salt
Begin the experiment:
  1. Fill a glass with water until it is half full.
  2. Put the egg in the glass. What happens? Does it sink or float?
  3. Now add the salt, one tablespoon at a time. Watch what happens to the egg.
Making discoveries
  • The egg sinks in fresh water. Why do you think this happens?
  • As salt is added to the water, the egg starts to float to the top.
  • The salt makes the water denser. The denser a liquid is, the more likely it is that an object will float.
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