Physical Science - Ch 11

PSCI 170: Physical Science I

Spring 2004

Notes for Chapter 13 Applying the Concepts Questions 1 - 3, 9 - 11,15,16, 19 -21, 24 - 26, 28. These are questions that you should be able to answer after reading the text and lab manual pages and conducting the activities. Questions for Thought: You were assigned to look at 1-4, 6, 9, 10 and 12. However, all of the answers are given here. 1.How is a solution different from other mixtures? Answer: A solution is a homogeneous mixture of ions or molecules of two or more substances. Not all mixtures are homogeneous and not all are mixtures of ions or molecules. 2. Explain why some ionic compounds are soluble while others are insoluble in water. Answer: If the attraction between the ions is greater than the hydration energy the ionic compound is insoluble in water. Ionic compounds whose attraction to the polarized water molecules is greater than those between ions are soluble. 3. Explain why adding salt to water increases the boiling point. Answer: The presence of the dissolved salt decreases the number of water molecules at the surface. This occurs since the sodium ions and chlorine ions are taking up some of the space near the surface. Since there are fewer water molecules near the surface to escape, the vapor pressure is lowered. This increases the temperature necessary to obtain the vapor pressure required to boil the water and salt solution. 4. A deep lake in Minnesota is covered with ice. What is the water temperature at the bottom of the lake? Explain your reasoning. Answer: The greatest density of water occurs at a temperature of 4oC. A lake covered with ice (0oC) would probably have the greatest density water—with a temperature of 4oC—at the bottom of the lake. 5. Explain why water has a greater density at 4oC than at 0oC. Answer: Water at very close to freezing has hydrogen bonds present between many of the molecules. These bonds spread out the water molecules. As the temperature of the water increases above the freezing point (from 0oC to 4oC), the bonds break and allow the molecules to collapse together. Therefore there end up being more molecules per unit volume – it is more dense. 6. What is hard water? How is it softened? Answer: A solution of Ca2+ or Mg2+ ions in water is called hard water (sometimes Fe2+ is included in this list of hard water ions). Removing the calcium and magnesium ions softens the water. This is done by exchange of the Ca2+ and Mg2+ ions with Na+ ions. Because sodium salts of soaps and detergentsare much more water soluble than the Ca2+ or Mg2+ salts of soaps or detergents.

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7. According to the definition of an acid and the definition of a base, would the pH increase, decrease, or remain the same when NaCl is added to pure water? Explain. Answer: NaCl does not donate protons to form hydronium ions, and it does not accept protons to form hydroxide ions when it is dissolved in water. Since the pH scale is a measure ofthehydronium ion concentration, the pH does not change because the hydronium ion concentration does not change. 8. What is a hydrogen bond? Explain how a hydrogen bond forms. Answer: A hydrogen bond is a weak to moderate strength bond between the hydrogen end (δ+) of a polar molecule and the negative end (δ-) of a second polar molecule. The bond forms when the positively charged end of a polar molecule, which is usually a hydrogen atom, is attractedto the negative end of another polar molecule. This negative end is usually an oxygen, fluorine or nitrogen atom. If the two ends come close enough together, this electrical force of attractions forms a bond. 9. What feature of a soap molecule gives it cleaning ability? Answer: A soap molecule has two parts, a part that is polar, which allows the soap to be soluble inwater, and a nonpolar part, which is absorbed into oil. Soap molecules attach onto oil molecules and carry the oil away with the rinse water. 10. What ion is responsible for (a) acidic properties? (b) for basic properties? Answer: (a) Acids are associated with the presence of hydronium ions in a water solution. (b) Bases are associated with the presence of hydroxide ions in a water solution. 11. Explain why a pH of 7 indicates a neutral solution – why not some other number? Answer: The pH scale is based upon the reciprocal of the hydronium ion concentration in water. Pure water has a hydronium ion concentration of 1 X 10-7, the reciprocal of which is 1 X 107. The pH is the exponent of the ten, so 7 is defined as neutral. If any other number were used, the scale would make no sense and have to be redefined. 12. What is a buffer solution? Answer: A buffer solution consists of a weak acid and salt that has the same negative ion as the acid. Ifan acid is added to this solution, the hydronium ions react with the negative ions. If a base is added, the hydroxide ions react with the acid. In this way the pH of the solution is maintained relatively constant. There are definitions that you need to know from chapter 13: Acid: a substance that dissolves in water to produce hydronium ions, H3O+. Another definition is that it is a proton (H+) donor. Then you need to look at the properties of acids as they are found in your lab manual and in your text. Also note some of the examples of acids. acid-base indicator: a chemical which changes color in the presence of acid or base. In other words, a chemical that is one color in the presence of acid and another color in the presence of base. 2

base: a substance that dissolves in water to produce hydroxide ions, OH-. Another definition is that a base is a proton acceptor. Then you need to look at the properties of bases as they are found in your lab manual and in your text. Also note some of the examples of bases. boiling point : Boiling occurs when the pressure of the vapor escaping from a liquid is equal to the atmospheric pressure on the liquid, so molecules in liquid form are changing to the gaseous form. The normal boiling point is defined as the temperature at which the vapor pressure is equal to the average atmospheric pressure at sea level. For pure water, this temperature is 100oC. Boiling is a purely physical process. Adding some solute to a solvent decreases the vapor pressure, and therefore the boiling point temperature increases. Need to raise the temperature more to get boiling. Boiling point elevation. buffer solution: a mixture of weak acid and its conjugate base. Contains a weak acid and a salt. Buffer solutions do not change pH significantly with addition of strong acid or base. chemical equilibrium: occurs when two opposing reactions happen at the same time and at the same rate. When the rate of the forward reaction equals the rate of the reverse reaction. , concentration: indicates the amount of solute per given volume of solution. Some ways of calculating concentrations you conducted in lab. For example, when you calculated the amount of sodium acetate per mL of solution, g/mL, that was a concentration. Sometimes concentrations are given in percentages (parts per hundred). Sometimes in parts per million and parts per billion. The chemistry unit of concentration is Freezing Point: Freezing occurs when the kinetic energy (motion) of molecules has been reduced sufficiently so the molecules can come together, forming the crystal structure of the solid. Reduced kinetic energy of the molecules, that is, reduced temperature, results in a specific freezing point for each pure liquid. The presence of solute molecules in a solvent interferes with the ability of the molecules to line up in the crystal structure. Therefore, it has to be colder to get the solution to freeze than it has to be to get a pure solvent to freeze. Freezing point depression. molarity: The number of moles of solute dissolved in one liter of solution. For example, a 2M HCl solution means that 2 moles of HCl are dissolved in each 1 liter of solution. electrolytes: Ionic substances that dissolve in water to produce a conductive solution. The solution containing ions is also called an electrolyte. , hard water: It is hard to get soap to lather in hard water. Hard water has relatively high concentrations of calcium ions, Ca2+, and magnesium ions, Mg2+. Also, there is often iron Fe2+ in hard water. , hydronium ion: the ion produced when acids are dissolved in water. H3O+. miscible fluids: fluids that can mix in any proportion – liquids that are soluble in each other. Think of examples – such as water and alcohol, coffee and water, etc. The opposite is immiscible. Two fluids that are immiscible do not dissolve in each other. One will layer on top of the other, with the most dense of the two solutions ending up on the bottom and the less dense solution floating on top of the other one. Examples include oil plus water, gasoline plus water, etc. neutralized: when acids and bases are added together in equal molar amounts, the resulting solution no longer has properties of either the acid or the base. An acid-base neutralization is the reaction of an acid and a base to produce a salt and water. HCl (acid) + NaOH (base) Æ H2O + NaCl (a salt), nonelectrolytes: Substances that dissolve in water but do not result in a conductive solution. Polar, covalent compounds are examples of nonelectrolytes. Also, the resulting solution that does not conduct electricity is sometimes called a nonelectrolyte. pH scale: The pH scale is based on the concentration of the hydronium ion in the solution. This scale shows the relative acidity of different solutions. If pH’s are less than 7, the solution is acidic. If the pH’s are more than 7, the solution is basic. If the pH of the solution is equal to 7, the solution is said to be neutral. salt: is any ionic compound except those with hydroxide or oxide ions. Look at examples of salt that are found in table 13.6 on page 315 of your text. 3

saturated solution: a state of equilibrium that exists between dissolving solute and solute coming out of solution. Means that the maximum amount of solute has been dissolved into a given volume of solution at a given temperature. The maximum concentration that can be attained at a given temperature under usual conditions. solubility: the concentration of solute dissolved in solution resulting in a saturated solution at a particular temperature. Therefore, the greatest amount of solute that can be dissolved per given volume of solution at a given temperature. solute: in a solution, the component present in the lesser amount is the solute. solution: a homogenous mixture of ions or molecules of two or more substances. Note especially that a solution is a homogeneous mixture. The amounts of the components of a solution are identified by the solvent and the solute. Know some examples of these. For example, room air is a solution composed of 78% nitrogen, 21 percent oxygen, 0.9 percent argon and traces of other gases including CO2 and water vapor. solvent: in a solution, the component present in the larger amount is called the solvent. strong acids (and know some examples): Acids that ionize completely in water, with all acid molecules dissociating into ion. HCl (hydrochloric acid) and HNO3 (nitric acid), are examples of strong acids. Look at more in your lab manual. strong bases (and know some examples): Bases that ionic completely in water. A strong base is completely ionic in solution and has hydroxide ions. weak acids (and know some examples): acids that ionize only to a small extent in water. Examples are acids such as acetic acid, CH3COOH. weak bases (and know some examples.) bases that ionize only to a small extent in water. Examples are bases such as ammonia, NH3. You do not have to know how to do the calculations that are discussed in this chapter. From class – note the properties of acids and bases and be able to give examples of acids and bases. Know the different properties of acids and bases. Know what the hydronium ion is, H3O+, and the hydroxide ion, OH-, and how they relate to acids and bases. Remember that foods are usually acidic and that cleaning products are usually bases (but not always) Know how an acid-base indicator works and how you would test for an acid-base indicator. Know that pH scale is used to rank the acidities of different solutions. Solutions with pH’s below 7 are considered acids. Solutions with pH’s above 7 are bases. Solutions with pH equal to 7 are neutral. The lower the pH, the more acidic the solution The high the pH, the more basic the solution. Remember the three methods you used in class to measure acidities and basicities of different solutions. Remember the experiments that you developed to do this.

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