Chapter 3 Textbook Flashcards
Polar covalent bonds in water molecules result in ___
Hydrogen Bonding
Water is a polar molecule. A hydrogen bond forms when the slightly negatively charged oxygen of one water molecule is attracted to the slightly positively charged oxygen of another water molecule. Hydrogen bonding is the basis for water’s properties.
What are the four emergent properties of water that make earth a suitable environment for life?
Cohesive behaviour
Ability to moderate temp
Expansion upon freezing
Versatility as a solvent
Explain the cohesion of water molecules
Water molecules stay relatively close together as a result of hydrogen bonding. These linkages cause water to be more structured than other liquids, with this being referred to as cohesion.
Additionally, this contributes to water and dissolved nutrients moving against gravity in plants. As the pull is transmitted all the way down to the water molecules in the soil when water evaporates from a leaf. Adhesion, the clinging of one substance to another plays another part as water clings to the cell walls of the plants to counter gravity as well.
Water also has an unusually high surface tension due to the hydrogen bonding.
Moderation of Temp by water
Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler. Water is effective as a heat bank because it can absorb or release a relatively large amount of heat with only a slight change in its own temperature.
Water has a high specific heat: Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form. This helps keep temperatures relatively steady, within limits that permit life. Evaporative cooling is based on water’s high heat of vaporization. The evaporative loss of the most energetic water molecules cools a surface.
Explain expansion upon freezing
Water is one of the few substances that are less dense as a solid than as a liquid. In other words, ice floats on liquid water. While other materials contract and become denser when they solidify, water expands.
As the temperature falls from 4°C to 0°C, water begins to freeze because more and more of its molecules are moving too slowly to break hydrogen bonds. At 0°C, the molecules become locked into a crystalline lattice, each water molecule hydrogen-bonded to four partners. The hydrogen bonds keep the molecules at “arm’s length,” far enough apart to make ice about 10% less dense (10% fewer molecules for the same volume) than liquid water at 4°C.
The ability of ice to float due to its lower density is an important factor in the suitability of the environment for life. If ice sank, then eventually all ponds, lakes, and even oceans would freeze solid, making life as we know it impossible on Earth.
Water: The Solvent of Life
Water is a very versatile solvent due to the polarity of the water molecule. This is because liken dissolves like, so ionic and nonionic compounds all dissolve in water. For ions specifically, the sphere of water molecules around each dissolved ion is called a hydration shell.
Substance that has an affinity for water is called ____
hydrophilic
Any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philos, loving). In some cases, sub- stances can be hydrophilic without actually dissolving. For example, some molecules in cells are so large that they do not dissolve. Another example of a hydrophilic substance that does not dissolve is cotton, a plant product.
Substances that do not have an affinity for water
hydrophobic
Substances that are nonionic and nonpolar (or other- wise cannot form hydrogen bonds) actually seem to repel water; these substances are said to be hydrophobic (from the Greek phobos, fearing). An example from the kitchen is vegetable oil, which, as you know, does not mix stably with water-based sub- stances such as vinegar. The hydrophobic behaviour of the oil molecules results from a prevalence of relatively nonpolar cova- lent bonds, in this case bonds between carbon and hydrogen, which share electrons almost equally. Hydrophobic molecules related to oils are major ingredients of cell membranes.
Acids
An acid is a substance that increases the hydrogen ion concentration of a solution. This source of H+ (dissociation of water is the other source) results in an acidic solution—one having more H+ than OH-.
Bases
A substance that reduces the hydrogen ion concentration
of a solution is called a base. Some bases reduce the H+ concentration directly by accepting hydrogen ions.
Other bases reduce the H+ concentration indirectly by disso- ciating to form hydroxide ions, which combine with hydro- gen ions and form water.
pH scale
The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration
Buffers
The presence of substances called buffers allows biological fluids to maintain a relatively constant pH despite the addition of acids or bases. A buffer is a substance that minimizes changes in the concentrations of H+ and OH- in a solution. It does so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions to the solution when they have been depleted. Most buffer solutions contain a weak acid and its corresponding base, which combine reversibly with hydrogen ions.
Example of buffer within body
There are several buffers that contribute to pH stability in human blood and many other biological solutions. One of these is carbonic acid (H2CO3), formed when CO2 reacts with water in blood plasma. As mentioned earlier, carbonic acid dissociates to yield a bicarbonate ion (HCO3-) and a hydrogen ion (H+).
Why are Scientists worried about Ocean Acidification
Among the many threats to water quality posed by human activities is the burning of fossil fuels, which releases CO2 into the atmosphere.
25% of human-generated CO2 is absorbed by the oceans. In spite of the huge volume of water in the oceans, scientists worry that the absorption of so much CO2 will harm marine ecosystems.
The data to support
Recent data have shown that such fears are well founded. When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH. This process, known as ocean acidification, alters the delicate balance of conditions for life in the oceans . Based on measurements of CO2 levels in air bubbles trapped in ice over thou- sands of years, scientists calculate that the pH of the oceans is 0.1 pH unit lower now than at any time in the past 420 000 years. Recent studies predict that it will drop another 0.3–0.5 pH unit by the end of this century.
