Why does ice float in liquid water?

Water is less dense in its frozen form, ice, than it is in its liquid form. This is because the molecules in ice are further apart than the molecules in liquid water. The molecules in ice are held further apart by the hydrogen bonds between the oxygen and hydrogen atoms.

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• But why is the density of ice less than water?
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• Test your Understanding:

From giant icebergs to tiny cubes, ice – the frozen form of water – always floats on its liquid form. Isn’t that weird?

We’re not the only ones who think it’s unusual; the entire world finds it rather surprising that a solid should float on its liquid form. Do a quick Google search and you’ll find dozens of pages discussing this queer tendency of ice.

As it turns out, like everything else, there is a scientific reason behind this phenomenon. But first off, let’s be clear about what makes stuff sink or float.

The singular rule of thumb, when it comes to the ability of an object to float in water (or any other liquid), concerns the density of the object in question. Have you ever heard of Archimedes’ Principle?

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Archimedes’ Principle states that for an object to float on water, it must displace an equal amount of water. In other words, you can say that the fate of an object in a body of water is decided by Archimedes’ Principle.

It’s common knowledge that solid objects have more density than their liquid counterparts. Chemically, this makes sense too. Molecules are more closely bound with one another in a well-defined manner in a solid, which makes them rigid and gives them more weight. All common substances that we see and observe in daily life follow this basic principle: solid objects are denser and have more weight than liquids.

Given that, why does ice – which is a solid – float on water? Shouldn’t it sink, as a solid, and according to general convention, also have more density?

Water is a wonderful liquid, and full of unusual behaviors and chemical structures, which is why it presents an interesting exception to the general behavior of solids floating over their liquid forms.

If you keep cooling a liquid, its density continues to increase until it becomes solid, where it attains maximum density. However, in the case of water, this trend is slightly different, which is the root cause of this whole discussion.

Water’s density increases as you continue to cool it; but opposed to other liquids, which have their highest density when they freeze, water achieves maximum density when its temperature reaches 4 degrees Celsius (39.2 Degrees Fahrenheit).

A graph showing how density varies with changing temperature

If you continue to cool water past 4 degrees Celsius, its density starts to plummet (you can see this in the graph). At zero degrees, i.e., the temperature at which water turns into ice, the density of water is actually quite low.

It turns out that ice has a lower density than water, and any object that has a lower density than the liquid form on which it’s kept (in this case, water) will be able to float!

But why is the density of ice less than water?

To answer that, you’ll have to look at the chemical structure of water.

A water molecule is made of two hydrogen atoms and one oxygen atom

The negatively-charged oxygen atoms bind strongly with hydrogen atoms, forming a strong hydrogen bond.

When a liquid is cooled, more and more molecules are brought closer together and need to be accommodated in a smaller area. This gives most solids more density than their liquid form. However, in the case of water, the negatively-charged oxygen atoms repel each other (when brought together in a smaller space) to prevent the ice from becoming any denser. This is the reason that density actually decreases as temperature continues to fall below 4 degrees Celsius.

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Ice floats because it is less dense than water. Water has a density of 1.0 gm/cubic cm.

The density of ice Ih is 0.931 gm/cubic cm.

But, why is ice less dense than water if both are made up of molecules of H2O?

-------------->spin on -------->- spin off

To Rotate the Molecule--->Left Click and Drag

To Zoom-->>Left Click + hold Shift button and Drag Vertically

Jsmol Menu --->>Right-Click

Try this!! 

Change to wireframe first:

Click on right mouse button over box

Style --> Schemes --> wireframe

Label atoms:

Style -->Label ---> atom number

Measure the following angles:

<415 <512 <213

<314 

To measure angles: hold left mouse bottom over atom and double click on atom 1, drag to second atom (center atom) single click center atom, drag and double click atom 3.

Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up.

In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules.

In ice each each molecule is hydrogen bonded to 4 other molecules.

Compare the two structures below. Notice the empty spaces within the ice structure.

In ice Ih, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. Because of ordered structure in ice there are less H20 molecules in a given space of volume.

Try this --

Try this --

1) Measure the O-O distances between any two adjacent oxygen atoms in ice shown in the above structure..

Please enter your answer in the space provided:

2) Measure the O-O-O angle formed between adjacent oxygen atoms in ice.

3) What is the length of the hydrogen bond H-O in ice?

Unlike ice -- water at room temperature is in constant motion -- hydrogen bonds are constantly formed and being broken-- click here to see water molecules in motion

Page 2

NEW -- WHY SOAP IS SO EFFECTIVE IN DEACTIVATING THE CORONAVIRUS?

Soaps are mixtures of sodium or potassium salts of fatty acids which can be derived from oils or fats by reacting them with an alkali (such as sodium or potassium hydroxide) at 80°–100 °C in a process known as saponification. 

fat + NaOH ---> glycerol + sodium salt of fatty acid 

CH2-OOC-R - CH-OOC-R - CH2-OOC-R (fat) + 3 NaOH ( or KOH) 

both heated --->

CH2-OH -CH-OH - CH2-OH (glycerol) + 3 R-CO2-Na (soap) R=(CH2)14CH3

-------------->spin on -------->- spin off

Try this!! 

Click the right mouse button with the cursor over the image-->

Style -->Labels --> Element Symbols

Click on the left mouse button and rotate the soap structure.

Notice that one end of the molecules is made up of a hydrocarbon chain -- the other end is a very polar structure containing oxygen and sodium.

Soap molecules have both properties of non-polar and polar at opposite ends of the molecule.

How does Soap Work?

Nearly all compounds fall into one of two categories: hydrophilic ('water-loving') and hydrophobic ('water-hating'). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic compounds just don't mix. 

The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).

When grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution, the soap molecules work as a bridge between polar water molecules and non-polar oil molecules. Since soap molecules have both properties of non-polar and polar molecules the soap can act as an emulsifier. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. The soap will form micelles (see below) and trap the fats within the micelle. Since the micelle is soluble in water, it can easily be washed away.

-------------->spin on -------->- spin off
------>space fill/cpk -------->stick ----> ball-and-stick

When you mix soap into the water the soap molecules arrange themselves into tiny clusters (called 'micelles'). The water-loving (hydrophilic) part of the soap molecules points outwards, forming the outer surface of the micelle. The oil-loving (hydrophobic) parts group together on the inside, where they don't come into contact with the water at all. Micelles can trap fats in the center.

Try this ---> click right mouse button over image --> spin --> on

Test your Understanding:

Page 3

Drugs generally work by interacting with receptors on the surface of cells or enzymes (which regulate the rate of chemical reactions) within cells. Receptor and enzyme molecules have a specific three-dimensional structure which allows only substances that fit precisely to attach to it. This is often referred to as a lock and key model.

Most drugs work because by binding to the target receptor site, they can either block the physiological function of the protein, or mimics it's effect. If a drug causes the protein receptor to respond in the same way as the naturally occurring substance, then the drug is referred to as an agonist. Examples of agonists are morphine, nicotine, phenylephrine, and isoproterenol. Antagonists are drugs that interact selectively with receptors but do not lead to an observed effect. Instead they reduce the action of an agonist at the receptor site involved. Receptor antagonists can be classified as reversible or irreversible. Reversible antagonists readily dissociate from their receptor. Irreversible antagonists form a stable chemical bond with their receptor (eg, in alkylation). Examples of antagonist drugs are: beta-blockers, such as propranolol.

Instead of receptors, some drugs target enzymes, which regulate the rate of chemical reactions. Drugs that target enzymes are classified as inhibitors or activators (inducers). Examples of drugs that target enzymes are: aspirin, cox-2 inhibitors and hiv protease inhibitors (see below).

Many drug companies will design structural variants for compounds that bind receptor sites in hope of making a compound that is more effective. Until recently design of new drugs was very difficult. Scientists had no way to know what the binding site of the protein looked like. Scientist now have a powerful new tool. Molecular modeling allows researchers to view the 3-D structure of proteins and their binding sites using data from X-ray crystallography and NMR spectroscopy . The synthesis of several recent drugs (including HIV Protease Inhibitors for treatment of AIDS) have been assisted using the 3-D structure of protein. 

CASE I: HOW ASPIRIN AND OTHER NONSTEROIDAL ANTI-INFLAMMATORY INHIBITORS WORKS

Nonsteroidal anti-inflammatory drugs work by interfering with the cyclooxygenase pathway. The normal process begins with arachidonic acid, a dietary unsaturated fatty acid obtained from animal fats. This acid is converted by the enzyme cyclooxygenase to synthesize different prostaglandins. The prostaglandins go on to stimulate many other regulatory functions and reactionary responses in the body including: inflammation and increased sensitivity to pain . Aspirin and other NSAID's work by inhibiting this pathway.

Recent research has shown that there are two types of cyclooxygenase, denoted COX-1 and COX-2. Each type of cyclooxygenase lends itself to producing different types of prostaglandins. COX-1 is located in the stomach wall.

pdb fle: 1CVU (shown using Jsmol) By selectively binding the arachidonic acid site NSAID inhibit the COX-2 enzyme. Shown to left: Cyclooxygenase-2 complexed with a non-selective inhibitor, indomethacin (only the A chain is shown with heme and inhibitor molecule

To view Backbone model with hetero atoms (heme and indomethacin). 

to Color Backbone by Amino Acid

How do NSAID get to the active site?
Access to the COX-2 catalytic site is through the membrane lipid. Celecoxib intercalates into the membrane core and then diffuses along a path to gain access to the hydrophobid binding site Other drugs may use a slightly different mechanism providing evidence for the flexible nature of cyclooxygenase.

What causes side effects in the case of aspirin? Why does aspirin cause cause stomach upset but the newer NSAID drugs do not? 

The two forms of cyclooxygenase have equal molecular weights and are very similar in structure. However, the binding active site of COX-1 (located in the stomach walls) is smaller than the similar site of COX-2, so it accepts a smaller range of structures as substrates. In the stomach COX-1 makes prostaglandin that seems to keep your stomach lining nice and thick by stimulating mucous production; inhibiting this enzyme can cause irritation of the stomach lining.

CASE II: HOW DO AIDS ANTI-VIRAL DRUGS WORK? 

Protease inhibitors inhibit the activity of protease, an enzyme used by HIV to cleave nascent proteins for final assembly of new HIV virons, and so prevent viral replication. This was the second class of antiretroviral drugs developed. Indinavir -- Trade name: Crixivan® was FDA approved March 13, 1996. It was the eighth approved antiretroviral drug. Indinavir was much more powerful than any prior antiretroviral drug; using it with dual NRTIs set the standard for treatment of HIV/AIDS and raised the bar the design and introduction of subsequent antiretroviral drugs. Protease inhibitors changed the very nature of the AIDS epidemic from one of a terminal illness to a somewhat manageable one.

CASE IIII: HUMAN GROWTH HORMONE RECEPTOR

Human growth hormone (pink) binds two receptor molecules (gold) and thereby induces signal transduction through receptor dimerization. Growth hormone is naturally produced by the pituitary gland and is necessary to stimulate growth in children. .

CASE IV: G -PROTEIN RECEPTOR 

G proteins are molecular switches that use GDP to control their signaling cycle. The G protein system plays a central role in many signaling tasks, making it a sensitive target for drugs and toxins. Many of the drugs that are currently on the market, such as Claritin and Prozac, as well as a number of drugs of abuse, such as heroin, cocaine and marijuana, act at G-protein-coupled receptors in these signaling chains. 

GPCRs are also involved in aging, cancer, cell growth stimulation, controlling metabolism ....

For more information see G Proteins.