CH.3
Adding a base tends to lower the hydrogen ion concentration and increase the pH of a solution.
A substance that reduces the hydrogen ion concentration is a base. Reducing the hydrogen ion concentration increases the pH. Some bases reduce the H+ concentration directly by accepting hydrogen ions. Ammonia (NH3), for instance, acts as a base when the unshared electron pair in nitrogen's valence shell attracts a hydrogen ion from the solution, resulting in an ammonium ion (NH4+): NH3+ + H+ NH4+.
Hydrophilic molecules are charged molecules that are attracted to the partial charge of the water molecule
Any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philos, loving). In some cases, substances 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. Cotton consists of giant molecules of cellulose, a compound with numerous regions of partial positive and partial negative charges that can form hydrogen bonds with water. Water adheres to the cellulose fibers. Thus, a cotton towel does a great job of drying the body, yet it does not dissolve in the washing machine. Cellulose is also present in the walls of water-conducting cells in a plant; the adhesion of water to these hydrophilic walls helps water move up the plant against gravity.
When ice forms, the hydrogen bonds are farther apart than in liquid water, allowing the ice to form an organized crystal structure and float
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. When ice absorbs enough heat for its temperature to rise above 0°C, hydrogen bonds between molecules are disrupted. 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.
The partial changes on a water molecule occur because of the unequal sharing of electrons between the hydrogen and oxygen atoms of the water molecule
Atoms in a molecule attract shared electrons to varying degrees, depending on the element. The attraction of a particular atom for the electrons of a covalent bond is called its electronegativity. The more electronegative an atom is, the more strongly it pulls shared electrons toward itself. Oxygen is one of the most electronegative of all the elements, attracting shared electrons much more strongly than hydrogen does. In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than they do near the hydrogen nucleus. Because electrons have a negative charge and are pulled toward oxygen in a water molecule, the oxygen atom has a partial negative charge (indicated by the Greek letter δ with a minus sign, δ-, or "delta minus"), and each hydrogen atom has a partial positive charge (δ+, or "delta plus"). Because the oxygen atom in the water molecule is more electronegative than the hydrogen atoms the oxygen, the electrons of the bond are not shared equally. This type of bond is called a polar covalent bond
Hydrophobic molecules are uncharged, nonionic substances that seem to repel water
Hydrophobic substances do not have an affinity for water. Substances that are nonionic (no charge) and nonpolar (or otherwise cannot form hydrogen bonds) actually seem to repel water (that cluster away from water molecules); these substances are said to be hydrophobic (from the Greek phobos, fearing).
Glucose has a molecular mass of 180 daltons. To make a 2-molar (2 M) solution of glucose, stir 360 g of glucose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution up to 1 L
If we wanted to make a liter (L) of solution consisting of 1 mol of glucose dissolved in water, we would measure out 180 g of glucose and then gradually add water, while stirring, until the sugar was completely dissolved. We would then add enough water to bring the total volume of the solution up to 1 L. At that point, we would have a 1-molar (1 M) solution of glucose. In this problem we want a 2 M solution. That means that we must double the amount of grams that we weigh out and measure out 360 g of glucose and then gradually bring the total volume of the solution up to 1 L.
A mole of methanol (CH3OH) weighs 32 g. The number of grams of methanol that are needed to produce 1 L of a 1 millimolar solution is 0.032 g
In order to solve this problem, convert 1 millimolar to molar units. A millimolar unit is a unit of 1/1,000 molar units. Since we have 1/1,000 molar units, this gives us the decimal 0.001. To calculate the amount of grams needed for this solution, multiply 46 g × 0.001 = 0.032 g.
You can fill a glass of water to just slightly above the rim without it spilling over the glass. The property of water best explains this phenomenon is surface tension
Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than most other liquids. At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below. This makes the water behave as though coated with an invisible film. You can observe the surface tension of water by slightly overfilling a drinking glass; the water will stand above the rim. In a more biological example, some animals can stand, walk, or run on water without breaking the surface.
All of the following are hydrogen bonding properties except nonpolar covalent bonding
Surface tension, adhesion, partial changes on the oxygen and hydrogen, and cohesion are hydrogen bonding properties, but nonpolar covalent bonding is not. Water's polar nature is responsible for these properties. The extraordinary qualities of water are emergent properties resulting from hydrogen bonding. The properties in question refer to the electrical attraction between something and a water molecule or between water molecules.
Adhesion
The attraction of unlike substances.
Evaporative cooling
The cooling of a liquid's surface as some of the liquid evaporates.
Water moves up a plant because of hydrogen bonds by a process called cohesion and adhesion
The hydrogen bonds hold the substance together, a phenomenon called cohesion. Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants. Water from the roots reaches the leaves through a network of water-conducting cells. As water evaporates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots. Adhesion, the clinging of one substance to another, also plays a role. Adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity.
A solution at pH 10 contains 100 times less H+ than the same amount of solution at pH 8
The pH scale compresses the range of H+ and OH- concentrations by employing logarithms. The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH log [H+]. For a neutral aqueous solution, [H+] is 10-7 M, giving us log 10-7 = -(-7) = 7. Notice that pH declines as H+ concentration increases. Remember that each pH unit represents a tenfold difference in H+ and OH-concentrations. It is this mathematical feature that makes the pH scale so compact. A solution of pH 3 is not twice as acidic as a solution of pH 6, but a thousand times (10 × 10 × 10) more acidic. When the pH of a solution changes slightly, the actual concentrations of H+ and OH- in the solution change substantially. Since the difference between pH 10 and pH 8 is "10 × 10," this pH 10 solution is 100 times more alkaline than the pH 8 solution.
The property of water that is most directly responsible for the ability to sweat to lower temperatures by mammals is the absorption of heat by the breaking of hydrogen bonds.
This is explained by the definition of the "heat of vaporization" which is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. As a liquid evaporates, the surface of the liquid that remains behind cools down. This evaporative cooling occurs because the "hottest" molecules, those with the greatest kinetic energy, are the most likely to leave as gas. Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. High humidity on a hot day increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body. In order to convert liquid water to a gas hydrogen bonds must be broken a process that requires the absorption of heat. Therefore the process that allows mammals to sweat to lower temperatures requires the absorption of heat so that the liquid water can vaporize.
The ability of water molecules to form hydrogen bonds with other water molecules and water's ability to dissolve substances that have charges or partial charges are both caused by water's partial charges.
Two molecules of water are held together by a hydrogen bond formed between the slight positive charge of the hydrogen and the slight negative charge of the oxygen. Water is a very versatile solvent, a quality traced to the polarity of the water molecule.
An acid is a compound that donates hydrogen ions to a solution
When acids dissolve in water, they donate additional H+ to the solution. An acid is a substance that increases the hydrogen ion concentration of a solution. For example, when hydrochloric acid (HCl) is added to water, hydrogen ions dissociate from chloride ions: HCl → H+ + Cl-. This source of H+ (dissociation of water is the other source) results in an acidic solution—one having more H+ than OH-.
A substance that reduces the hydrogen ion concentration of a solution is called a base.
When bases dissolve in water, they donate additional OH+ to the solution, thereby increasing the hydroxyl ion concentration of a solution. For example, when sodium (NaOH) is added to water, hydroxyl ions dissociate from sodium ions: NaOH Na+ + OH- . This source of OH- (dissociation of water is the other source) results in an alkaline or basic solution—one having more OH- than H+.