Polystyrene, The Density of Ice And Water As A Function of Temperature

The Phases of Water

Similar to many other substances, water can take numerous forms. Its liquid phase, the most common phase of water on Earth, is the form that is generally meant by the word "water."

3.9 Solid Phase (Ice)

The solid phase of water is known as ice and commonly takes the structure of hard, amalgamated crystals, such as ice cubes, or of loosely accumulated granular crystals, such as snow. Unlike most other substances, water's solid form (ice) is less dense than its liquid form, as a result of the nature of its hexagonal packing within its crystalline structure. This lattice contains more space than when the molecules are in the liquid state.

FIGURE 3.2 The hexagonal structure of ice

As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. It possesses a regular crystalline structure based on the molecular structure of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms: H-O-H.

The fact the density of ice is less than that of liquid water's has the important consequence that ice floats.

3.10 The density of ice and water as a function of temperature

The solid form of most substances is denser than the liquid phase; therefore, a block of a given solid will generally sink in its corresponding liquid. However, a block of ice floats in liquid water because ice is less dense than liquid water. The inset shows the curve in more detail in the range of 0-10 degrees Celsius. Liquid water is most dense at 4 degrees Celsius.

3.11 Liquid Phase (Water)

Water is primarily a liquid under standard conditions (25 degrees Celsius and 1 atm of pressure). This characteristic could not be predicted by its relationship to other, gaseous hydrides of the oxygen family in the periodic table, such as hydrogen sulfide. The elements surrounding oxygen in the periodic table – nitrogen, fluorine, phosphorus, sulfur, and chlorine – all combine with hydrogen to produce gases under standard conditions. Water forms a liquid instead of a gas because oxygen is more electronegative than the surrounding elements, with the exception of fluorine. Oxygen attracts electrons much more strongly than does hydrogen, resulting in a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. The presence of such a charge on each of these atoms gives a water molecule a net dipole moment.

The electrical attraction between water molecules caused by this dipole pulls individual molecules closer together, making it more difficult to separate the molecules, and therefore raising the boiling point. This type of attraction is known as hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at intervals briefer than 200 femto seconds (200 x 10^-15 seconds). Arrangement of water molecules in the liquid phase. Water molecules align based on their polarity, forming hydrogen bonds (signified by "1").

Many of the physical and chemical properties of water (including its capacity as a solvent) are partly to the acid-base reactions it can be part of. Water can be described as an amphoteric molecule, meaning that it can react as both a Brønsted-Lowry acid or base. This can be shown in the reaction between two water molecules that produces the hydronium ion (H₃O⁺) and a hydroxide ion (OH^-):

H2O(l)+H2O(l)H3O+(aq)+OH−(aq)