Water has unusual properties: the key is that the electrons of each covalent bond are not shared equally between oxygen and hydrogen atoms. Oxygen pulls electrons much more strongly than does hydrogen.

This sharing of electrons causes the oxygen end of the molecule to have a slight negative charge, while the end with the two hydrogen atoms is slightly positive.

A molecule in which opposite ends have opposite electric charges is called a polar molecule. Water is a compound consisting of polar molecules.

Water molecules are attracted to one another in a specific way. The slightly negative oxygen end of one molecule attracts the slightly positive hydrogen ends of adjacent water molecules, causing the molecules to become arranged.

A type of weak attraction between the hydrogen atom of one molecule and a slightly negative atom within another molecule is a type of chemical bond called a hydrogen bond.

Cohesion and Adhesion ► Each hydrogen bond between molecules of liquid water lasts for a short time. ► Most of the molecules are involved in hydrogen bonding with other molecules because new hydrogen bonds form as fast as old ones break.

The tendency of molecules of the same kind to stick to one another is called cohesion. Cohesion is much stronger for water than for most other liquids. Water molecules are also attracted to certain other molecules. The type of attraction that occurs between unlike molecules is called adhesion.

Water's Ability to Dissolve Other Substances ► When you stir table salt into a glass of water, you are forming a solution, a uniform mixture of two or more substances. ► The substance that dissolves the other substance and is present in the greater amount is the solvent (in this case, water).

Acids and Bases

► Sometimes atoms give their electrons up altogether instead of sharing them in a chemical bond. This process is known as disassociation. ► Water, for instance, dissociates by the following formula: ► H 2 O H + + OH – ► The hydrogen atom gives up a negatively charged electron, gaining a positive charge, and the OH compound gains a negatively charged electron, taking on a negative charge. ► The H + is known as a hydrogen ion and OH – ion is known as a hydroxide ion.

► The disassociation of water produces equal amounts of hydrogen and hydroxide ions. ► However, the disassociation of some compounds produces solutions with high proportions of either hydrogen or hydroxide ions. ► Solutions high in hydrogen ions are known as acids, while solutions high in hydroxide ions are known as bases. ► Both types of solutions are extremely reactive—likely to form bonds—because they contain so many charged particles.

► The technical definition of an acid is that it is a hydrogen ion donor, or a proton donor, as hydrogen ions are consist of only a single proton. ► Acids put H + ions into solution.

► The definition of a base is a little more complicated: they are H + ion or proton acceptors, which means that they remove H + ions from solution. ► Some bases can directly produce OH – ions that will take H + out of solution. NaOH is an example of this type of base: ► NaOH Na + + OH –

► A second type of base can directly take H + out of an H 2 O solution. Ammonia (NH 3 ) is a common example of this sort of base: ► NH 3 + H 2 O NH OH –

The pH Scale ► The pH scale, which ranges from 0 to 14, measures the degree to which a solution is acidic or basic. If the proportion of hydrogen ions in a solution is the same as the proportion of hydroxide ions or equivalent, the solution has a pH of 7, which is neutral.

The pH Scale ► The most acidic solutions (those with a high proportion of H + ) have pHs approaching 0, ► while the most basic solutions (those with a high proportion of OH – or equivalent) have pHs closer to 14.

► Water has a pH of 7 because it has equal proportions of H + and OH – ions. ► Some acids are more acidic than others because they put more H + ions into solution. Stomach fluid, for example, is more acidic than saliva.

► When sodium hydroxide (NaOH) disassociates, it forms only hydroxide ions, making it a base and giving it a pH above 7. ► Like acids, bases can be strong or weak depending on how many hydroxide ions they put in solution or how many hydrogen ions they take out of solution.

Temperature Moderation ► Because of hydrogen bonding, water has a better ability to resist temperature change than most other substances.

Thermal energy ► Thermal energy is the total amount of energy associated with the random movement of atoms and molecules in matter.

Temperature ► Temperature is a measure of the average energy of random motion of the particles in a substance. When two substances differ in temperature, thermal energy in the form of heat is transferred from the warmer substance to the cooler one

When you heat a substance—such as a metal pan or water—its temperature rises because its molecules move faster. But in water, some of the thermal energy that is absorbed goes to break hydrogen bonds.

As a result, the water absorbs the same amount of thermal energy but undergoes less temperature change. This releases thermal energy in the form of heat, so there is less of a drop in temperature than in metal.

Low Density of Ice ► Density is the amount of matter in a given volume. ► A high-density substance is more tightly "packed" than a low-density substance. ► In most substances, the solid state is more dense than the liquid state. Water is just the opposite—its solid form (ice) is less dense than the cold liquid form.

The substance that is dissolved and is present in a lesser amount is the solute. When water is the solvent, the result is called an aqueous solution

Acids, Bases, and pH ► In aqueous solutions, a very small percentage of the water molecules themselves break apart into ions. ► The ions formed are positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-). ► A hydroxide ion is a combination of an oxygen atom and a hydrogen atom that carries a 1- charge..

For the chemical processes of life to work correctly, the right balance of H+ ions and OH- ions is critical

Some chemical compounds contribute additional H+ ions to an aqueous solution while others remove H+ ions from it. A compound that donates H+ ions to a solution is called an acid.

A compound that removes H+ ions from an aqueous solution is called a base.

The pH Scale ► The pH scale describes how acidic or basic a solution is. ► The scale ranges from 0 (most acidic) to 14 (most basic). ► Each pH unit represents a tenfold change in the concentration of H+ ions.

Buffers ► Because the molecules in cells are very sensitive to concentrations of H+ and OH- ions, even a slight change in pH can be harmful to organisms. ► Many biological fluids contain buffers, substances that cause a solution to resist changes in pH.