Summary
Key terms
Polar molecule: A molecule where opposite ends of the molecule have opposite charges. (Ex– Water molecule)
Non–polar molecule: A molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a diatomic molecule or because of the symmetrical arrangement of polar bonds in a more complex molecule.
Cohesion: The intermolecular force that holds together the molecules in a solid or liquid; for example, oxygen and hydrogen molecules in water.
Adhesion: The clinging of one substance to another; for example, water to cell walls (by the force of hydrogen bonds).
Surface tension: A measure of how difficult it is to stretch or break the surface of a liquid.
(Ex: Water has a greater surface tension that most other liquids because of the hydrogen–bonded water molecules on and below the surface).
Heat: It is a measure of the total amount of kinetic energy due to molecular motion in a body of matter.
Temperature: It measures the intensity of heat due to the average kinetic energy of the molecules.
Specific heat: The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C.
Heat of vaporization: The quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.
Evaporative cooling: The property of a liquid whereby the surface becomes cooler during evaporation, owing to a loss of highly kinetic molecules to the gaseous state.
Acid: A substance that increases the hydrogen ion concentration of a solution.
Base: A substance that reduces the hydrogen ion concentration of a solution.
Buffer: A substance that consists of acid and base forms in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
pH scale: A measure of hydrogen ion concentration equal to –log [H+] and ranging in value from 0 to 14.
Molecular mass: The sum of the masses of all the atoms in a molecule.
Mole: The molecular weight of a substance expressed in grams.
Molarity: The number of moles of solute per liter of solution.
Key concepts
- Water is the elixir of life: Water is the molecule that supports all of life. All organisms familiar to us are made mostly of water and live in an environment dominated by water. Water is the biological medium here on Earth, and possibly on other planets as well.
- The polarity of water molecules : This results in hydrogen bonding. A hydrogen bond forms when the oxygen of one water molecule is electrically attracted to the hydrogen of a nearby molecule. Hydrogen bonding between water molecules is the basis for water's unusual properties.
- Properties of water : Four emergent properties of water contribute to Earth's fitness for life.
- Cohesion : Hydrogen bonding keeps water molecules close to each other, and this cohesion helps pull water upward in the microscopic vessels of plants. Hydrogen bonding is also responsible for water's surface tension. Adhesion, the clinging of one substance to another, also plays a role. Adhesion of water to the walls of the cells helps counter the downward pull of gravity.
- Moderation of temperature : Hydrogen bonding give water a high specific heat. Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form, helping minimize temperature fluctuations to within limits that permit life. Evaporative cooling is based on water's high heat of vaporization. Water molecules must have a relatively high kinetic energy to break hydrogen bonds. The evaporative loss of these energetic water molecules cools a surface.
- Insulation of bodies of water by floating ice : Ice is less dense than liquid water because its more organized hydrogen bonding causes expansion into a crystal formation. The lower density causes ice to float, which allows life to exist under the frozen surfaces of lakes and polar seas.
- The Solvent of life : Water is an unusually versatile solvent, because its polar molecules are attracted to charged and polar substances. Ions or polar substances surrounded by water molecules dissolve and are called solutes. Hydrophilic substances have an affinity for water; hydrophobic substances do not. Molarity, the number of moles of solute per liter of solution, is used as a measure of solute concentration in solutions. A mole is a certain number of molecules of a substance. The mass of a mole of the substance in grams is same as the molecular mass in daltons.
- Dissociation of water molecules : The water molecules dissociate to form acids and bases. Water can dissociate into H+ and OH−. The concentration of H+ is
expressed as pH, where pH= − log [H+]. Acids donate additional H+ in aqueous solutions; bases donate OH−or accept H+.
In a neutral solution at 25°c, [H+] = [OH−] = 10 −7M, and pH = 7.
In an acidic solution, [H +] is greater than [OH−], and pH is less than 7.
In a basic solution, [H+] is less than [OH−], and the pH is greater than 7.
Buffers in biological fluids resist changes in pH. Buffers are defined as the solutions which resist change in pH by the addition of small amounts of acids or bases. - Threat of acid precipitation: Acid precipitation is rain, snow, or fog with a pH below 5.6. Acid rain, also known as acid deposition, is caused by emissions of sulfur dioxide (SO 2) and nitrogen oxides (NOx) from power plants, cars and factories.
Water
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