Water
Water’s Polarity
Water's polarity is responsible for many of its properties including its attractiveness to other molecules.Learning Objectives
Describe the actions that occur due to water's polarityKey Takeaways
Key Points
- The difference in electronegativities between oxygen and hydrogen atoms creates partial negative and positive charges, respectively, on the atoms.
- Water molecules attract or are attracted to other polar molecules.
- Molecules that do not dissolve in water are known as hydrophobic (water fearing) molecules.
Key Terms
- hydrophilic: having an affinity for water; able to absorb, or be wetted by water
- hydrophobic: lacking an affinity for water; unable to absorb, or be wetted by water
- polarity: The intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule.
Nonpolar Molecules: Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound.
Water’s States: Gas, Liquid, and Solid
The orientation of hydrogen bonds as water changes states dictates the properties of water in its gaseous, liquid, and solid forms.Learning Objectives
Explain the biological significance of ice's ability to float on waterKey Takeaways
Key Points
- As water is boiled, kinetic energy causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor).
- When water freezes, water molecules form a crystalline structure maintained by hydrogen bonding.
- Solid water, or ice, is less dense than liquid water.
- Ice is less dense than water because the orientation of hydrogen bonds causes molecules to push farther apart, which lowers the density.
- For other liquids, solidification when the temperature drops includes the lowering of kinetic energy, which allows molecules to pack more tightly and makes the solid denser than its liquid form.
- Because ice is less dense than water, it is able to float at the surface of water.
Key Terms
- density: A measure of the amount of matter contained by a given volume.
Water's States: Gas, Liquid, and Solid
The formation of hydrogen bonds is an important quality of liquid water that is crucial to life as we know it. As water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids, and since living things have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are constantly formed and broken as the water molecules slide past each other. The breaking of these bonds is caused by the motion (kinetic energy) of the water molecules due to the heat contained in the system. When the heat is raised as water is boiled, the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor). On the other hand, when the temperature of water is reduced and water freezes, the water molecules form a crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds). This makes ice less dense than liquid water, a phenomenon not seen in the solidification of other liquids.Water's lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther apart compared to liquid water. With most other liquids, solidification when the temperature drops includes the lowering of kinetic energy between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid.
The low density of ice, an anomaly, causes it to float at the surface of liquid water, such as an iceberg or the ice cubes in a glass of water. In lakes and ponds, ice forms on the surface of the water creating an insulating barrier that protects the animals and plant life in the pond from freezing. Without this layer of insulating ice, plants and animals living in the pond would freeze in the solid block of ice and could not survive. The detrimental effect of freezing on living organisms is caused by the expansion of ice relative to liquid water. The ice crystals that form upon freezing rupture the delicate membranes essential for the function of living cells, irreversibly damaging them. Cells can only survive freezing if the water in them is temporarily replaced by another liquid like glycerol.
Ice Density: Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice makes it less dense than the freely flowing molecules of liquid water, enabling it to (b) float on water.
Water’s High Heat Capacity
Water is able to absorb a high amount of heat before increasing in temperature, allowing humans to maintain body temperature.Learning Objectives
Explain the biological significance of water's high specific heatKey Takeaways
Key Points
- Water has the highest heat capacity of all liquids.
- Oceans cool slower than the land due to the high heat capacity of water.
- To change the temperature of 1 gram of water by 1 degree Celsius, it takes 1.00 calorie.
Key Terms
- heat capacity: The capability of a substance to absorb heat energy
- specific heat: the amount of heat, in calories, needed to raise the temperature of 1 gram of water by 1 degree Celsius
Water's High Heat Capacity
The capability for a molecule to absorb heat energy is called heat capacity, which can be calculated by the equation shown in the figure. Water's high heat capacity is a property caused by hydrogen bonding among water molecules. When heat is absorbed, hydrogen bonds are broken and water molecules can move freely. When the temperature of water decreases, the hydrogen bonds are formed and release a considerable amount of energy. Water has the highest specific heat capacity of any liquid. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie, or 4.184 Joules. As a result, it takes water a long time to heat and a long time to cool. In fact, the specific heat capacity of water is about five times more than that of sand. This explains why the land cools faster than the sea.The resistance to sudden temperature changes makes water an excellent habitat, allowing organisms to survive without experiencing wide temperature fluctuation. Furthermore, because many organisms are mainly composed of water, the property of high heat capacity allows highly regulated internal body temperatures. For example, the temperature of your body does not drastically drop to the same temperature as the outside temperature while you are skiing or playing in the snow. Due to its high heat capacity, water is used by warm blooded animals to more evenly disperse heat in their bodies; it acts in a similar manner to a car's cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even temperature.
Water’s Heat of Vaporization
Evaporation of water requires a substantial amount of energy due to the high heat of vaporization of water.Learning Objectives
Explain how heat of vaporization is related to the boiling point of waterKey Takeaways
Key Points
- The dissociation of liquid water molecules, which changes the substance to a gas, requires a lot of energy.
- The boiling point of water is the temperature in which there is enough energy to break the hydrogen bonds between water molecules.
- Water is converted from its liquid form to its gaseous form (steam) when the heat of vaporization is reached.
- Evaporation of sweat (mostly water) removes heat from the surface of skin, cooling the body.
Key Terms
- heat of vaporization: The energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure (often atmospheric pressure).
Water's Heat of Vaporization
Water in its liquid form has an unusually high boiling point temperature, a value close to 100°C. As a result of the network of hydrogen bonding present between water molecules, a high input of energy is required to transform one gram of liquid water into water vapor, an energy requirement called the heat of vaporization. Water has a heat of vaporization value of 40.65 kJ/mol. A considerable amount of heat energy (586 calories) is required to accomplish this change in water. This process occurs on the surface of water. As liquid water heats up, hydrogen bonding makes it difficult to separate the water molecules from each other, which is required for it to enter its gaseous phase (steam). As a result, water acts as a heat sink, or heat reservoir, and requires much more heat to boil than does a liquid such as ethanol (grain alcohol), whose hydrogen bonding with other ethanol molecules is weaker than water's hydrogen bonding. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), the heat is able to break the hydrogen bonds between the water molecules, and the kinetic energy (motion) between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point, water's individual molecules acquire enough energy from each other such that some surface water molecules can escape and vaporize; this process is known as evaporation.
Humidity, Evaporation, and Boiling: (a) Because of the distribution of speeds and kinetic energies, some water molecules can break away to the vapor phase even at temperatures below the ordinary boiling point. (b) If the container is sealed, evaporation will continue until there is enough vapor density for the condensation rate to equal the evaporation rate. This vapor density and the partial pressure it creates are the saturation values. They increase with temperature and are independent of the presence of other gases, such as air. They depend only on the vapor pressure of water.
Water’s Solvent Properties
Water's polarity makes it an excellent solvent for other polar molecules and ions.Learning Objectives
Explain why some molecules do not dissolve in water.Key Takeaways
Key Points
- Water dissociates salts by separating the cations and anions and forming new interactions between the water and ions.
- Water dissolves many biomolecules, because they are polar and therefore hydrophilic.
Key Terms
- dissociation: The process by which a compound or complex body breaks up into simpler constituents such as atoms or ions, usually reversibly.
- hydration shell: The term given to a solvation shell (a structure composed of a chemical that acts as a solvent and surrounds a solute species) with a water solvent; also referred to as a hydration sphere.
Water's Solvent Properties
Water, which not only dissolves many compounds but also dissolves more substances than any other liquid, is considered the universal solvent. A polar molecule with partially-positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water.When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na+ and Cl– ions, and spheres of hydration form around the ions. The positively-charged sodium ion is surrounded by the partially-negative charge of the water molecule's oxygen; the negatively-charged chloride ion is surrounded by the partially-positive charge of the hydrogen in the water molecule.
Dissociation of NaCl in water: When table salt (NaCl) is mixed in water, spheres of hydration form around the ions.
Water’s Cohesive and Adhesive Properties
Cohesion allows substances to withstand rupture when placed under stress while adhesion is the attraction between water and other molecules.Learning Objectives
Describe the cohesive and adhesive properties of water.Key Takeaways
Key Points
- Cohesion holds hydrogen bonds together to create surface tension on water.
- Since water is attracted to other molecules, adhesive forces pull the water toward other molecules.
- Water is transported in plants through both cohesive and adhesive forces; these forces pull water and the dissolved minerals from the roots to the leaves and other parts of the plant.
Key Terms
- adhesion: The ability of a substance to stick to an unlike substance; attraction between unlike molecules
- cohesion: Various intermolecular forces that hold solids and liquids together; attraction between like molecules
Water's Cohesive and Adhesive Properties
Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (the mass per unit volume) than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. It's even possible to "float" a needle on top of a glass of water if it is placed gently without breaking the surface tension.
Surface Tension: The weight of the needle is pulling the surface downward; at the same time, the surface tension is pulling it up, suspending it on the surface of the water and keeping it from sinking. Notice the indentation in the water around the needle.
Adhesion: Capillary action in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves.
Cohesion & Adhesion: Water's cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat.
pH, Buffers, Acids, and Bases
Acids dissociate into H+ and lower pH, while bases dissociate into OH- and raise pH; buffers can absorb these excess ions to maintain pH.Learning Objectives
Explain the composition of buffer solutions and how they maintain a steady pHKey Takeaways
Key Points
- A basic solution will have a pH above 7.0, while an acidic solution will have a pH below 7.0.
- Buffers are solutions that contain a weak acid and its a conjugate base; as such, they can absorb excess H+ ions or OH- ions, thereby maintaining an overall steady pH in the solution.
- pH is equal to the negative logarithm of the concentration of H+ ions in solution: pH = - log[H+].
Key Terms
- alkaline: having a pH greater than 7; basic
- acidic: having a pH less than 7
- buffer: a solution composed of a weak acid and its conjugate base that can be used to stabilize the pH of a solution
Self-Ionization of Water
Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. The hydroxide ions remain in solution because of their hydrogen bonds with other water molecules; the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules and form hydronium ions (H30+). By convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. The pH is calculated as the negative of the base 10 logarithm of this concentration:
pH = -log[H+]
The negative log of 1 × 10-7 is equal to 7.0, which is also known as neutral pH. Human cells and blood each maintain near-neutral pH.
pH Scale
The pH of a solution indicates its acidity or basicity (alkalinity). The pH scale is an inverse logarithm that ranges from 0 to 14: anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is basic (or alkaline ). Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH in cells (6.8) and the blood (7.4) are both very close to neutral, whereas the environment in the stomach is highly acidic, with a pH of 1 to 2.
The pH scale: The pH scale measures the concentration of hydrogen ions (H+) in a solution.
An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by dissociating one of its hydrogen atoms. A base provides either hydroxide ions (OH–) or other negatively-charged ions that react with hydrogen ions in solution, thereby reducing the concentration of H+ and raising the pH.
Strong Acids and Strong Bases
The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) is highly acidic and completely dissociates into hydrogen and chloride ions, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids; conversely, strong bases readily donate OH– and/or react with hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly basic and give up OH– rapidly when placed in water; the OH- ions react with H+ in solution, creating new water molecules and lowering the amount of free H+ in the system, thereby raising the overall pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral that well-adapted marine organisms thrive in this alkaline environment.Buffers
How can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers usually consist of a weak acid and its conjugate base; this enables them to readily absorb excess H+ or OH–, keeping the system's pH within a narrow range.Maintaining a constant blood pH is critical to a person's well-being. The buffer that maintains the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, excess carbonic acid can be converted into carbon dioxide gas and exhaled through the lungs; this prevents too many free hydrogen ions from building up in the blood and dangerously reducing its pH; likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body's pH would fluctuate enough to jeopardize survival.
Buffers in the body: This diagram shows the body's buffering of blood pH levels: the blue arrows show the process of raising pH as more CO2 is made; the purple arrows indicate the reverse process, lowering pH as more bicarbonate is created.