Water and Solutions

Water is the most abundant liquid on the earth and is necessary for all life. Because of water's great dissolving properties, any sample is a solution containing solids, other liquids, and gases from the environment. This stream also carries suspended, ground-up rocks, called rock flour, from a nearby glacier.

Solutions

Introduction A solution is a homogeneous mixture of ions or molecules of two or more substances. Two parts Solvent is the component that is in the largest quantity Solute is the component that is dissolved in the solvent. If one of the components of a solution is a liquid it is usually the solvent. If the solvent is water then the solution is identified as an aqueous solution.

Above this city there are at least three kinds of solutions Above this city there are at least three kinds of solutions. These are (1) gas in gas-oxygen dissolved in nitrogen, (2) liquid in gas-water vapor dissolved in air, and (3) solid in gas-tiny particles of smoke dissolved in the air.

Concentration of Solutions The concentration of a solution describes the relative amounts of the solute and solvent. Concentrated solution contain large amounts of solute. Dilute solutions contain little solute. Different ways to express concentration Parts per million (ppm) Parts per billion (ppb) Percent by volume – the volume of solute per 100 volumes of solution Molarity – number of moles of solute per liter of solution

There are different ways to express the concentration of a solution There are different ways to express the concentration of a solution. How many different ways can you identify in this photograph?

Three ways to express the amount of solute in a solution: (A) as parts (e.g., parts per million), this is 2 parts per 100; (B) as a percent by volume, this is 2 percent by volume; (C) as percent by weight, this is 2 percent by weight.

Salinity is a measure of the amount of salts dissolved in 1kg of solution. If 1,000 g of seawater were evaporated, 35.0 g of salts would remain as 965.0 g of water leave.

Solubility A saturated solution is an equilibrium mixture that exists between solute and solvent where the rate of dissolving is equal to the rate at which the solute is coming out of solution. Solubility is the concentration of solute that can be dissolved to make a saturated solution at a given temperature. It is important to note that solubility varies with temperature Solubility of most ionic solids increases with increasing temperature Solubility of most gases decreases with increasing temperature.

Approximate solubility curves for sodium nitrate, potassium nitrate, potassium chloride, and sodium chloride.

Water Solutions

(A) A tetrahedron is a four-sided pyramid (A) A tetrahedron is a four-sided pyramid. (B) Molecules with four bonding electron pairs have a tetrahedral shape like that of the CCI4 molecule illustrated here. Note that the carbon atom is in the center of the pyramid, with chlorine atoms at each of the four corners. (C) A water molecule has two bonding pairs and two lone pairs in a tetrahedral arrangement. (D) The water molecule has an angular arrangement called bent. If something were attached to the two lone pairs, it would be a tetrahedral arrangement.

Properties of Water Molecules Water is a polar molecule since the oxygen has a greater electronegativity than the hydrogen. The electrons spend more time around the oxygen, giving it a partial negative charge and giving the hydrogen a partial positive charge. It is this charge differential that allows most water soluble compounds to dissolve in water.

The polarity of the water molecule allows it to set up intermolecular forces with other molecules. The positive end of the water molecule can attract the negative part of another molecule The negative end of water molecule can attract the positive end of another molecule. These types of attractions are called van der Waals forces. The intermolecular forces between a hydrogen atom and an electronegative atom such as oxygen is called a hydrogen bond. A hydrogen bond is a weak bond between a partial positive hydrogen and a partial negative atom on another molecule.

(A) The water molecule is polar, with centers of positive and negative charges. (B) Attractions between these positive and negative centers establish hydrogen bonds between adjacent molecules.

The hexagonal structure of ice The hexagonal structure of ice. Hydrogen bonding between the oxygen atom and two hydrogen atoms of other water molecules results in a tetrahedral arrangement, which forms the open, hexagonal structure of ice.

The density of water from OOC to 10O C The density of water from OOC to 10O C. The density of water is at a maximum at 4O C, becoming less dense as it is cooled or warmed from this temperature. Hydrogen bonding explains this unusual behavior.

The Dissolving Process There is a limit to the solubility of any molecule in a solution Some substance do not dissolve to any appreciable amount in a solution. Fluids that can mix freely with one another are called miscible Fluids that cannot mix freely are called immiscible

Water is polar, and carbon tetrachloride is nonpolar Water is polar, and carbon tetrachloride is nonpolar. Since like dissolves like, water and carbon tetrachloride are immiscible.

Soap cleans oil and grease because one end of the soap molecule is soluble in water, and the other end is soluble in oil and grease. Thus, the soap molecule provides a link between two substances that would otherwise be immiscible.

An ionic solid dissolves in water because the number of water molecules around the surface is greater than the number of other ions of the solid. The attraction between polar water molecules and a charged ion enables the water molecules to pull ions away from the crystal, a process called dissolving.

Properties of Solutions Electrolytes Ionic substances dissolved in an aqueous solution create a charge differential in the solution Since there is a charge differential there is a potential difference and electrical charge can flow between unlike charged areas. A solution which can conduct an electrical charge is called an electrolyte A solution which cannot conduct an electrical charge is called a nonelectrolyte. Ionic compounds ionize in water, disassociate to form their independent, charged particles. Acids ionize in water to protonate the water molecule and form a hydronium ion H3O+

(A) Water solutions that conduct an electric current are called electrolytes. (B) Water solutions that do not conduct electricity are called nonelectrolytes.

Three representations of water and hydrogen chloride in an ionizing reaction. (A) Sketches of molecules involved in the reaction. (B) Electron dot equation of the reaction. (C) The chemical equation for the reaction. Each of these representations shows the hydrogen being pulled away from the chlorine atom to form H3O+, the hydronium ion.

Boiling Point The boiling point of a solution is the point at which enough energy has been added to overcome the intermolecular forces that hold the solute in the solution. At this point, the molecules gain enough kinetic energy to produce a pressure that is greater than the atmospheric pressure keeping them in solution Once this point is reached, the solution vaporizes (becomes a gas)

The rate of evaporation, and thus the vapor pressure, is less for a solution than for a solvent in the pure state. The greater the solute concentration, the less the vapor pressure.

Freezing Point The freezing point of a solution is the point where enough energy has been removed from the solution to slow the molecules down and increase intermolecular forces so the solution becomes a solid

Since ionic compounds dissolve by separation of ions, they provide more particles in solution than molecular compounds. (A) A mole of sugar provides Avogadro's number of particles. (B) A mole of NaCl provides two times Avogadro's number of particles. (C) A mole of CaCl2 provides three times Avogadro's number of particles.

Acids, Bases, and Salts

Properties of Acids and Bases The properties of acids are: Sour taste Change blue litmus paper to red. React with metals to release hydrogen gas. Neutralize bases, forming water and salts The properties of bases are: Bitter taste Bases turn red litmus paper blue. Feel slippery to the skin. Neutralize acids, forming water and salts.

(A)Acid solutions will change the color of blue litmus to red (A)Acid solutions will change the color of blue litmus to red. (B) Solutions of bases will change the color of red litmus to blue.

Explaining Acid-Base Properties Acid base properties are explained by looking at how they ionize in water. Equilibrium occurs when two opposing reactions happen at the same time and rate. H2O + H2O  H3O+ + OH- This reaction can occur in both directions and chemical equilibrium is reached when the rate of the reaction to the right (the forward reaction) is equal to the rate of the reaction to the left (the reverse reaction) An acid is defined as a proton donor A base is defined as any substance that is a proton acceptor

Strong and Weak Acids and Bases Strong acids ionize completely in water Weak acids ionize only partially in water Strong bases ionize completely in water. Weak bases ionize only partially in water.

Hydrochloric acid (HCl) has the common name of muriatic acid Hydrochloric acid (HCl) has the common name of muriatic acid. Hydrochloric acid is a strong acid used in swimming pools, soil acidifiers, and stain removers.

The pH Scale Acid base strength is a measure of relative H3O+ and OH- concentrations. Acidic solutions have concentrations of H3O+ above 1 X 10-7 Neutral solutions have H3O+ concentrations equal to 1 X 10-7 Basic solutions have H3O+ concentrations less than 1 X 10-7 A neutral solution has a pH of 7, acidic solutions have pH below 7, and basic solutions have pH above 7. Each division of the pH scale is an increase or decrease in concentration to a power of 10, which makes the scale a logarithmic scale

The pH scale.

The pH increases as the acidic strength of these substances decreases from left to right. Did you know that lemon juice is more acidic than vinegar? That a soft drink is more acidic than orange juice or grapefruit juice?

Properties of Salts A salt is produced by a neutralization reaction between an acid and a base. A salt is any ionic compound, except for hydroxide or oxides Hard and Soft Water A solution of Ca2+ and Mg2+ ions is called hard water because soap does not lather up very well in it. The Ca2+ and Mg2+ replace the sodium or potassium ions in the soap and become insoluble

A water softener exchanges sodium ions for the calcium and magnesium ions of hard water. Thus, the water is now soft, but it contains the same number of ions as before.

Buffers A buffer is a mixture of a weak acid with the salt of the acid. A buffer has the ability to resist changes in pH when small amounts of acids or bases are added to the solution If small amounts of acid is added, hydronium ions are neutralized by reacting with the salt in solution If small amounts of base is added, hydroxide ions are neutralized by reacting with the acid.