Properties of substances are divided into two categories: physical and chemical. Physical properties are those which are measurable and can be seen without changing the chemical makeup of a substance. In contrast, chemical properties are those that determine how a substance will behave in a chemical reaction. These two categories differ in that a physical property may be identified just by observing, touching, or measuring the substance in some way; however, chemical properties cannot be identified simply by observing a material. Rather, the material must be engaged in a chemical reaction in order to identify its chemical properties.
When substances are combined without a chemical reaction to bond them, the resulting substance is called a mixture. Physical changes can be used to separate mixtures. For example, heating salt water until the water evaporates, leaving the salt behind, will separate a saltwater solution.
In a mixture, the components can be unevenly distributed, such as in trail mix or soil. These mixtures are described at heterogeneous. Alternatively, the components can be homogeneously, or uniformly, distributed, as in salt water.
A solution is a special type of stable homogenous mixture. The components of a solution will not separate on their own, and cannot be separated using a filter. The substance being dissolved is the solute, and the substance acting on the solute, or doing the dissolving, is the solvent.
Though it is one of the most common and biologically essential compounds on Earth, water is chemically abnormal. Its chemical formula is H2O, which means that water consists of one oxygen atom bound to two hydrogen atoms. The shape of this molecule is often described as looking like Mickey Mouse, with the oxygen atom in the middle as Mickey’s face and the two hydrogen atoms as his ears.
This imbalanced shape means that oxygen has a slightly positive charge localized on the two hydrogen atoms, and a slightly negative charge on the lone oxygen. Because of this polarity, water molecules attract each other and tend to clump together, a property called cohesion. Water is also extremely adhesive, meaning it clings to other substances. These attractive forces account for a number of water’s unique properties.
Water has a high surface tension, meaning the bonds between water molecules on the surface of a liquid are stronger than those beneath the surface. Surface tension makes it more difficult to puncture the surface of water. Combined with adhesion, it also helps cause capillary action, which is the ability of water to travel against gravity. Capillary action moves blood through vessels in the body and water from the roots to the leaves of plants.
Water is an efficient solvent for ionic compounds because of its hydrogen bonds and associated polarity. When ionic compounds like NaCl (table salt) are placed in water, the individual ions are attracted to the opposite ends of the dipole moment in water, causing compound to separate into Na+ and Cl− ions. The readiness with which ionic compounds dissolve in water is why so many minerals and nutrients are found naturally in water.
Water also has a low molecular weight. Most low-weight compounds exist in a gaseous form at room temperature, but water is a liquid at room temperature. Though water molecules have a relatively low weight, the boiling point and freezing point of water are abnormally high. This is because water’s strong hydrogen bonds require high amounts of heat to break. These properties of water make it the only compound found naturally in all three phases—solid, liquid, and gas—on Earth.
Consistent with its high boiling point, water also has an unusually high specific heat, meaning that water needs to absorb a lot of heat before it actually gets hot. This property allows the oceans to regulate global temperature, as they can absorb a large amount of energy.
Ice, or frozen water, is also abnormal. Normally molecules are tightly packed in the solid state, but water’s hydrogen bonds form a crystalline lattice structure, placing molecules far apart. This extra space makes ice less dense than liquid water, which is why ice floats.
Molecules and atoms have a tendency to spread out in space, moving from areas of high concentration to areas of lower concentration. This net movement is called diffusion. When solutions of differing concentrations are separated from each other by a porous membrane, the solvent molecules will flow across the membrane in order to equalize these different concentrations. This net movement of solvent particles is called osmosis. Osmosis is especially important in biological contexts, as cell and organelle membranes are semipermeable. Osmosis provides the main means by which water is transported in and out of cells.
When two solutions are separated by a semipermeable membrane, their relative concentrations (which determine the direction of the movement of solute molecules) are called tonicity. This chemical property is typically used to describe the response of a cell when placed in a solvent.
Three types of tonicity are relevant in biological situations. Hypertonic solutions are those which have a higher concentration of a given solute than the interior of the cell. When placed in such solutions, the cell will lose solvent (water) as it travels to areas of higher solute concentration.
Hypotonicity refers to a solution that has a lower concentration of a given solute than the cell. Water will enter the cell, causing it to swell in response to hypotonic solutions.
Isotonic solutions are those in which solute concentration equals solute concentration inside the cell, and no net movement of solvent will occur between the cell and an isotonic solution.