Chapter 15 - Water and Aqueous Systems
What is the percent by mass of water in CuSO₄∙5H₂O?
36.1% H₂O
69. A hydrate has the following percent composition: 75.5% CaCl₂ and 24.5% H₂O. What is the formula for this hydrate?
CaCl₂ → 2H₂O
53. What are two circumestances that help keep colloidal particles in suspension?
Brownian motion and repulsion between like-charged ions adsorbed on colloidal particles' surfaces.
The chaotic movement of colloidal particles, which was first observed by the Scottish botanist Robert Brown (1773-1858)m us called _________ _________?
Brownian motion. Brownian motion is caused by collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles. These collisions help prevent the colloidal particles from settling.
Water can be driven by a hydrate by ______________?
Heating. See figure 15.10.
The water contained in a crystal is called the water of hydration or water of _______________.
crystallization.
__________ have particles smaller than those in suspensions and larger than those in solutions.
Colloids.
What are compounds called that are so hygrscopic that they become wet when exposed to normally moist air?
Deliquescent, which means that they remove sufficient water from the air to dissolve completely and form solutions. Figure 15.3 shows that pellets of sodium hydroxide are deliquescent. For this reason, containers of NaOH and other chemicals should always be tightly stoppered and chemicals should never be touched by fingers. The solution formed by a deliquescent substance has a lower vapor pressure than that of the water in the air.
Polar molecules are attracted to one another by ___________ interactions.
Dipole. The negative end of one molecule attracts the positive end of another molecule. Figure 15.3 shows how this intermolecular attraction among water molecules results in the formation of hydrogen bonds. Many unique and important properties of water - including its high surface tension and low vapor pressure - result from hydrogen bonding.
FYI Some familiar hydrates are listed in Table 15.2 and shown in Figure 15.12.
Each one contains a fixed quantity of water and has a definite composition.
What is surface tension?
The inward force, or pull, that tends to minimize the surface area of a liquid.
51. Why don't solutions demonstrate the Tyndall effect?
The molecules or ions are too small to have reflective surfaces.
41. What is meant by a substance's water of hydration?
water in the crystal structure of a substance.
Ice melts at what temperature?
0°C. This is a high melting temperature for a molecule such a lower molar mass. A considerable amount of energy is required to return water molecules in the solid state to the liquid state.
95. The temperature of 1 L of steam at constant volume and 1.00 atm pressure is increased from 100⁰C to 200⁰C. Calculate the final pressure of the steam in atmospheres, assuming the volume does not change.
1.27 atm
89. The decomposition of hydrogen peroxide is given by this equation. 2H₂O₂ (l) → 2H₂O (l) + O₂ (g) Calculate the mass of water (in grams) and the volume of oxgyen at STP formed when 2.00 x 10⁻³ mol of hydrogen peroxide is decomposed.
3.60 x 10⁻² g H₂O, 2.24 x 10⁻² L O₂
Calculate the percent by mass of water in calcium chloride hexahydrate *CaCl₂∙6H₂O).
49.3% H₂O
91. Acetaldehyde (C₂H₄O) is produced commercially by the reaction of acetylene (C₂H₂) with water, as shown by this equation. C₂H₂ + H₂O → C₂H₄O How many grams of C₂H₄O can be produced from 2.60 x 10² g H₂O, assuming sufficient C₂H₂ is present?
636 g C₂H₄O
Calcium chloride is used as a desiccant in the laboratory. A desiccant is a ______________________?
A desiccant is a substance used to absorb moisture from the air and create a dry atmosphere. When a desiccant has absorbed all the water it can hold, the salt can be returned to its anhydrous state by heating.
The fact that ice floats has important consequences for organisms. What are they?
A layer of ice on the top of a pond acts as an insulator for the water beneath, preventing it from freezing solid except under extreme conditions. Because the liquid water at the bottom of an otherwise frozen pond is warmer than 0°C, fish and other aquatic life are better able to survive. If ice were denser than liquid water, bodies of water would tend to freeze solid during winter months, destroying many types of organisms.
What is a strong electrolyte?
A solution in which a large portion of the solute exists as ions. Not all electrolytes conduct an electric current to the same degree. In the simple conductivity test shown in Figure 15.9, a bulb glows brightly when electrodes attached to it are immersed in a sodium chloride solution. The bright glow shows that sodium chloride is a strong electrolyte because nearly all the dissolved sodium chloride exists as separate Na⁺ and Cl⁻ ions. The ions move in solution and conduct an electric current. Most soluble salts, inorganic acids, and inorganic bases are strong electrolytes. The bulb glows dimly when the electrodes are immersed in a mercury (II) chloride solution because mercury (II) chloride is a weak electrolyte.
What is a weak electrolyte?
A solution that conducts electricity poorly because only a fraction of the solute exists as ions. Other weak electrolytes are ammonia (NH₃) and organic acids and bases.
77. A problem for firefighters is that much of the water they spray on a fire doesn't soak in but runs off carrying debris and pollution into the environment. Explain how the additions of a surfactant to water used to fight fires could help put out the fire more rapidly and protect the environment.
A surfactant helps to wet the burning material, so less water is needed to put out the fire. Thus less water carries pollutants into the environment.
25. What is a sufactant? Explain how it works.
A surfactant is a wetting agent such as a soap or detergent. A surfactant interferes with hydrogen bonding between water molecules and reduces surface tension.
What is an emulsion?
An emulsion is a colloidal dispersion of a liquid in a liquid. An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion's stability. For example, oils and grease are not soluble in water. However, they readily form a colloidal dispersion if soap or detergent is added to the water. Soaps and detergents are emulsifying agents. One end of a large soap or detergent molecule is polar and is attracted to water molecules. The other end of the soap or detergent molecule is nonpolar and is soluble in oil or grease. Soaps and other emulsifying agents thus allow the formation of colloidal dispersions between liquids that do not ordinarily mix. Figure 15.6 shows a familiar example of an emulsion-mayonnaise. Mayonnaise is a heterogeneous mixture of oil and vinegar. Such a mixture would quickly separate without the presence of egg yolk, which is the emulsifying agent. Other foods such as milk, margarine, and butter are also emulsions. Cosmetics, shampoos, and lotions are fomulated with emulsifiers to maintain consistent quality. Table 15.4 summarizes the properties of solutions, colloids, and suspensions.
Why is ice less dense than liquid water?
As you can see in Figure 15.5, hydrogen bonds hold the water molecules in place in the solid phase. The structure of ice is a regular open framework of water molecules arranged like a honeycomb. When ice melts, the framework collapses and the water molecules pack closer together, making liquid water more dense than ice.
29. What would be some of the consequences if ice were denser than water?
Bodies of water would freeze from the bottom up. This would kill many forms of aquatic life. Also, ice floats.
47. Some hydrates are efflorescent. Explain what that means. Under what conditions will a hydrate effloresce?
Efforescence is the loss of water of hydration that occurs when the hydrate has a higher vapor pressure than that of the water vapor in air.
True or false. Solid water has a higher density than liquid water.
False. Solid water has a LOWER density than liquid water. This is not usual for liquids. As a typical liquid cools, it begins to contract and its density increases gradually. Increasing density means that the molecules of the liquid move closer together so that a given volume of the liquid contains more molecules and thus more mass. If the cooling continues, the liquid eventually solidifies with a density greater than the density of the liquid. Because the density of a typical solid is greater than that of the corresponding liquid, the solid sinks in its own liquid. As water begins to cool, it behaves initially like a typical liquid. In contracts slightly and its density gradually increases, as showing in Table 15.1.
What do you see when colloids are studied under a microscope?
Flashes of light or scintillations. Colloids scintillate because the particles reflecting and scattering the light move eractically.
Name a nonelectrolyte.
Glucose (C₆H₁₂O₆). It is a molecular compound that does not form ions.
A compound that contains water of hydration is called a _______________________.
Hydrate. In writing the formula of a hydrate, use a dot to connect the formula of the compound and the number of water molecules per formula unit. Crystals of copper (II) sulfate is treated with water, the blue pentahydrate is regenerated. CuSO₄∙5H₂O(s) +heat/-heat CuSO₄(s) + 5H₂O(g) Another compound that changes color in the presence of moisture is cobalt (II) chloride.
____________ bonding between water molecules also explains water's unusually low vapor pressure.
Hydrogen. Remember that the vapor pressure of a liquid is the result of molecules escaping from the surface of the liquid and entering the vapor phase. Because hydrogen bonds hold water molecules to one another, the tendency of these molecules to escape is low and evaporation is slow.
Hydrates and other compounds that remove moisture from the air are called _________________?
Hydroscopic. Hydrated salts that have a low vapor pressure remove water from moist air to form higher hydrates.
45. Explain why a hydroscopic substance can be used as a disccant.
Hygroscopic substances absorb water vapor from the air and create a dry environment in a sealed container.
What is an electrolyte?
It is a compound that conducts an electric current when it is in an aqueous solution or in the molten state. Conduction of electricity requires ions that are mobile and thus able to carry an electrical current. All ionic compounds are electrolytes because they dissociate into ions.
Define suspension.
It is a mixture from which particles settle out upon standing. A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely. The particles in a typical suspension have an average diameter greater than 1000 mm. By contrast, the particle size in a solution is usually about 1 nm. The larger size of suspended particles means that gravity plays a larger role in causing them to settle out of the mixture. Suspensions are heterogeneous because at least two substances can be clearly identified.
What is a surfactant?
It is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension. Soaps and detergents are surfactants. Adding a detergent to beads of water on a greasy surface reduces surface tension, causing the beads of water to collapse and spread out.
What is an aqueous solution?
It is water that contains dissolved substances.
39. Why does molten sodium chloride conduct electricity?
Its ions are free to move toward an electrode.
How do the properties of colloids differ from those of suspensions and solutions?
Like suspensions, many colloids are milky in appearance when they are concentrated. Like solutions, colloids may look clear or almost clear when they are dilute. The important difference between colloids and solutions and suspensions is in the size of the particles. Colloids have particles smaller than those in suspensions and larger than those in solutions. These intermediate-sized particles cannot be retained by filter paper as are the larger particles of a suspension, and they do not settle out with time. Colloids can be distinguished by the Tyndall effect and by the observation of Brownian motion. They are also subject to coagulation, or clumping together, and they can be emulsified or made stable.
71. When ethyl alcohol (C₂H₆O0 dissolves in water, the volume of the final solution is less than the separate volumes of the water and alcohol added together. Can you explain this result? Do you think that it might be possible to mix two different liquids and get a mixture volume that is larger than the sum of the volumes of the two components? Explain.
Liquid wter contains some hydrogen-bonded structures similar to ice, in which the water molecules are spaced fairly wide apart. These structures are disrupted when ethyl alcohol is added because the alcohol competes for hydrogen bonds with water molecules and the water structure collapses. Thus, mixtures of water and ethyl alcohol have less volume than the sum of the volumes of the components. Mixing two liquids could result in a volume greater than the sum of the volumes of the components if the structural ordering in the mixture is greater than in the separated components.
______________ in liquid water are packed closer together than they are in ice.
Molecules.
73. When the humidity is low and the temperature high, humans must take in large quantities of water or face serious dehydration. Why do you think water is so important for the proper functioning of your body?
Most of the important chemical reactions of life take place in aqueous solutions inside cells.
65. Are all liquids soluble in each other? Explain.
No, nonpolar molecules do not dissolve in polar molecules.
A compound that does not conduct an electric current in either aqueous solution or the molten state is called a ____________?
Nonelectrolyte. Some polar molecular compounds are nonelectrolytes in the pure state, but become electrolytes when the dissolve in water. This occurs because such compounds ionize in solution.
Both oil and gasoline are composed of ______________ molecules.
Nonpolar. The attractive forces that hold two oil molecules together are similar in magnitude to the forces that hold two gasoline molecules together. Oil molecules can easily separate and replace gasoline molecules to form a solution. As a rule, polar solvents such as water dissolve ionic compounds and polar compounds; nonpolar solvents such as gasoline dissolve nonpolar compounds. This relationship can be summed up in the expression "like dissolves like."
75. Why is the midday sky blue while the evening sky in the west is often an orange or red color? (Hint: There are colloidal-size particles in the atmosphere).
Particles in the atmosphere that are approximately the same size as the wavelengths of visible light cause light from the sun to scater and split into individual components. Oxygen and nitrogen in the atmosphere scatter violet and blue light due to their small size. Thus the sky appears blue at midday, when the sun is closest to Earth. At sunrise and sunset, the sun is at its greatest distance from Earth. It shines through a layer of atmospheric aerosols consisting of small particles of dust and other pollutant particles mixed with air. The sunlight is reflected and absorbed by these particles. The shorter wavelengths (blue and violet) are scattered away from the path of the sun, but the longer wavelengths (yellow and red) pass through an dare the most visible.
Key Equation FYI
Percent H₂O = mass of water/mass of hydrate x 100%
Water is a _________ __________ molecule.
Simple triatomic, H₂O. The oxygen atom forms a covalent bond with each of the hydrogen atoms. Because of its greater electronegativity, oxygen attracts the electron pair of the covalent O-H bond to a greater extent than hydrogen. As a result, the oxygen atom acquires a partial negative charge (looks like an 8-) The less electronegative hydrogen atoms acquire partial positive charges (looks like an 8+). Thus, the O-H bonds are highly polar.
31. Distinguish between a solution in general and an aqueous solution.
Solutions are homogeneous mixtures in which a solute is dissolved in a solvent. Aqueous solutions are solutions that have water as the solvent.
49. Arrange colloids, suspensions, and solutions in order of increasing particle size.
Solutions, colloids, suspensions.
What are homogeneous mixtures that are formed when compounds such as inorganic acids, bases, and ionic salts mix with water are classified as ______________.
Solutions.
The process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules is called _____________?
Solvation. Figure 15.7 shows a model of the solvation of an ionic solid such as soldium chloride.
35. Describe the process of solvation.
Solvent molecules surround positively charged and negatively charged ions.
In the dissolving process, individual solute ions break away from the crystal. ___________ molecules surround the negatively and positively charged ions in a process called solvation and the ionic crystal dissolves.
Solvent.
23. Why do the particles at the surface of a liquid behave differently from those int he bulk of the liquid?
Surface molecules are attracted to the liquid molecules below but not the air. Molecules inside the liquid are attracted in all directions.
33. Why is water an excellent solvent for most ionic and polar covalent compounds but not for nonpolar compounds?
The polar water molecules attract ions and polar covalent molecules. Nonpolar compounds are unaffected because they have no charges.
How do the polarities of the two O-H bonds affect the polarity of the water molecule?
The shape of the molecule is the determining factor. The bond angle of the water molecule is approximately 105°, which gives the molecule a bent shape. The two O-H bond polarities do not cancel, so the water molecule as a whole is polar.
A ________ dissolves the _______.
The solvent dissolves the solute. The solute becomes dispersed in the solvent. Solvents and solutes may be gases, liquids, or solids. Solutions are homogeneous mixtures. They are also stable mixtures. Solutes particles can be atoms, ions, or molecules, and their average diamaters are usually than a nm (10⁻⁹ m). Substances that dissolve most readily in water include ionic compounds and polar covalent molecules. Nonpolar covalent molecules, such as methane, and compounds found in oil, grease, and gasoline, do not dissolve in water. However, oil and grease will dissolve in gasoline.
27. Explain why bodies of water with large surface areas such as lakes and oceans do not evaporate rapidly.
The water has low vapor pressure.
True or false. All liquids have a surface tension.
True. Water's surface tension is higher than most. The surface tension of water tends to hold a drop of liquid in a spherical shape. The drop is not a perfect sphere because the force of gravity tends to pull it down, causing it to flatten. This is why, on some surfaces, water tends to bead up rather than spread out. It is possible to decrease the surface tension of water by adding a surfactant.
The scattering of visible light by colloidal particles is called the ________________ __________.
Tyndall effect. Suspensions also exhibit the Tyndall effect, but solutions DO NOT. The particles in solutions are too small to scatter light. Figure 15.15 shows how the Tyndall effect can differentiate solutions from colloids and suspensions.
79. After a winter of alternate periods of freezing and thawing, some roads have broken pavement and potholes. Using what you know of the properties of water, explain why potholes form.
Water enters cracks in pavement and expands when it freezes, creating larger cracks. Continuous freeze-thaw cycles cause pavement to break up and form potholes.
37. Explain why gasoline does not dissolve in water.
Water is polar and gasoline is nonpolar.
57. Water has its maximum density at 4⁰C. Discuss the consequences of this fact.
Water molecules at 4⁰C are tightly packed and have maximum density. Below 4⁰C the water molecules arrange in a regular network because of the attractions between them. As a result, ice has a lower density than water and floats on water.
83. How many significant figures are in each measurement? a. 56.003 g b. 0.0056 cm c. 750 mL d. 0.4005 dg
a. 5 b. 2 c. 2 d. 4
87. Balance these equations. a. CO₂ + H₂O → C₆H₁₂O₆ + O₂ b. Na + H₂O → Na⁺ + OH⁻ + H₂
a. 6CO₂ ₊ 6H₂O → C₆H₁₂O₆ ₊ 6O₂ b. 2Na ₊ 2H₂O → 2Na ₊ 2OH⁻ ₊ H₂
63. You have a solution containing either sugar or salt dissolved in water. a. Can you tell which it is by visual inspection? b. Give two ways by which you could easily tell which it is.
a. No, both form clear, colorless solutions. b. dry to examine the crystals, test for electrical conductivity, do a flame test.
59. Explain which properties of water are responsible for these occurances. a. Water in tiny cracks in rocks helps break up the rocks when it freezes. b. Water beads up on a newly waxed car. c. A longer time is needed for a teaspoon of water to evaporate than a teaspoon of alcohol.
a. Water expands when it freezes to ice. b. Water is polar and wax is nonpolar, and water has a higher surface tension. c. A network of hydren bonding between water molecules lowers the vapor pressure. Two ethanol molecules can hydrogen bond, but a network is not formed.
61. Water is a polar solvent; gasoline is a nonpolar solvent. Decide which compounds are more likely to dissolve in water and which are more likely to dissolve in gasoline. a. CCl₄ b. Na₂₂SO₄ c. methane (CH₄) d. CKl
a. gasoline. b. water. c. gasoline. d. water.
93. A mixture of 40 cm³ of oxygen gas and 60 cm³ of hydrogen gas at STP is ignited. a. Which gas is the limiting reagent? b. What is the mass of water produced? c. Which gas remains after reaction? d. What is the volume, at STP, of the remaining gas?
a. hydrogen b. 0.048 g H₂O c. oxygen d. 0.010 L
81. Cobal chloride test paper is blue. THis paper is made by soaking strips of paper in an aqueous solution of CoCl₂∙6H₂O. The paper strips are then dried in an oven. CoCl₂∙6H₂O (pink) + heat → CoCl₂ (blue) + 6H₂O a. When colbalt (III) chloride hexahydrate is dissolved in water, what is the color of the solution. b. What is the color of wet cobalt chloride paper? c. What is the color of dry cobalt chloride paper? d. What is the percent by mass of water in the hexahydrate? e. What does cobalt chloride test paper test for?
a. pink b. pink c. blue d. 45.4% e. water or water vapor
67. Name these hydrates and determine the percent by mass of water in each. a. Na₂CO₃∙H₂O b. MgSO₄∙7H₂O
a. sodium carbonate monohydrate, 14.5% H₂O b. magnesium sulfate heptahydrate, 51.2% H₂O
43. Name each hydrate. a. SnCl₄∙5H₂O b. FeSO₄∙7H₂O c. BaBr₂∙4H₂O d. FePO₄∙4H₂O
a. tin (IV) chloride pentahydrate b. iron (II) sulfate heptahydrate c. barium bromide tetrahydrate d. iron (III) phosphate tetrahydrate
Colloidal particles also tend to stay suspended because they become ______________ by adsorbing ions from the dispersing medium onto their surface.
charged. Some colloidal particles become positively charged by adsorbing positively charged ions. Other colloidal particles become negatively charged by adsorbing negatively charged ions. All the particles in a particular colloidal system will have the same charge. The repulsion betwee the like-charged particles prevents the particles from forming heavier aggregates that would have a greater tendency to settle out. Thus, a colloidal system can be destroyed or coagulated by the addition of ions having a charge opposite to that of the colloidal particles. The added ions neutralize the charged colloidal particles. The particles can clump together to form heavier aggregates and precipitate from the dispersion.
All ionic compounds and electrolytes because they ______________ into ions and thus can conduct electricity.
dissociate.
As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal _______________.
dissolves.
When the vapor pressure drops below 1.0 kPa, the hydrate ___________________.
efforesces. Washing soda, or sodium carbonate decahydrate (Na₂CO₃∙10H₂O), is efflorescent. As the crystals lose water of hydration, they effloresce and become coated with a white powder of anhydrous sodium carbonate (Na₂CO₃). The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained. Because the water molecules are held by weak forces, hydrates often have an appreciable vapor pressure. If a hydrate has a vapor pressure higher than the pressure of water vapor in the air, the hydrate will lose its water of hydration or effloresce.
A colloid is a __________ type of mixture.
gelatin. It is a heterogeous mixture containing particles that range in size from 1 nm to 1000 nm. The particles are spread throughout the dispersion medium, which can be a solid, liquid, or gas. The first substances to be identified as colloids were glues. Other colloids include such mixtures as gelatin, paint, aerosol sprays, and smoke. Table 15.3 lists some common colloidal systems and gives examples of familiar colloids.
Colloid comes from the Greek work killa meaning _____________.
glue. The word colloid is based on the glue-like appearance of many colloids and was coined by the English scientist Thomas Graham in 1861.
The formula of a __________ consists of the formula of the ionic salt followed by a dot and the number of water molecules associated with one formula unit of the salt.
hydrate.
The high surface tension and low vapor pressure of water are the results of ___________ ____________.
hydrogen bonding.
Water molecules are in continuous motion because of their ____________ energy.
kinetic. When a crystal of sodium chloride is placed in water, the water molecules collide with it. Remember that a water molecule is polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. The polar solvent molecules (H₂O) attract the solute ions (Na⁺, Cl⁻).
In a solution, a _________ dissolves the solute. The solute becomes dispersed in the solvent.
solution.
What makes it possible for some insects, such as the water strider, to walk on water?
surface tension. Water molecules at the surface of the water drop above cannot form hydrogen bonds with air molecules, so they are drawn into the body of the liquid, producing surface tension.
A ________ differs from a soliution because the component particles ofa suspension are much larger and do not stay suspended indefinitely.
suspension.
55. What makes a colloidal dispersion stable?
the addition of an emulsifier.
The Solute is the what?
the dissolved particles.
The solvent is the what?
the dissolving medium.
The structure of ice is a regular open framework of ___________ ___________ held together by hydrogen bonds and arranged like a honeycomb.
water molecules.
