Chapter 5: The Chemistry of Seawater

By measuring the pH of seawater, one can indirectly determine the water's ____ concentration.

carbon dioxide

Rank the major constituents of seawater according to their abundance by weight, with the highest weight at the top and the lowest weight at the bottom.

(top to bottom) chloride, sodium, sulfate, magnesium, calcium, potassium

Rank the following substances according to pH, with the most acidic on top.

(top to bottom) sulfuric acid, soda pop, milk, pure water, bleach

Describe how a solar still works to desalinate seawater by putting the steps in the right order, with the first step on top.

A pool of seawater is capped with a plastic dome. Solar radiation evaporates the seawater. Evaporated water condenses on the dome and trickles down the sides where it is collected.

How is the abundance of major seawater constituents related to solubility and residence time?

Abundant constituents are soluble and have a high residence time.

What is the Redfield ratio?

C:N:P = 106:16:1

Match the seawater constituent to its approximate residence time.

Chloride - 100,000,000 years Calcium - 1,000,000 years Iron - 200 years

The maximum amount of gas that can be held in solution is called the saturation concentration, and it is affected by pressure, temperature, and salinity. Which of the following correctly describe the relationship between these factors and the saturation concentration?

Colder water can hold more gas Water under high pressure can hold more gas

True or false: Trace elements are labeled as such because they are unimportant to ocean processes.

False

True or false: Water solutions contain only H2O molecules.

False - Water solutions also have H+ and OH- ions

True or false: Ratios of nutrients such as nitrogen and phosphorus do not vary in seawater.

False - they are nonconservative

Which of the following statements best describes the seasonal changes in ocean water salinities of polar areas?

Ice forming in the water increases salinity, and ice melting in the summer decreases salinity. - remember that the formation of ice includes only water molecules, leaving the salts behind

Which of the phrases does not correctly describe compensation depth?

This depth is permanently at 200 meters below the surface

Choose the three major sources of carbon dioxide in the oceans.

Transfer from the atmosphere Oxidation of organic matter Respiration of marine organisms

A salinometer measures the salinity of ocean water by measuring ____.

electrical conductivity

A charged atom, group of atoms, or molecules is called a(n) ____. Positively charged examples are called ____, and negatively charged examples are ____.

ion; cation, anion

____ bonds, those formed due to the attraction of oppositely charged particles, are easily broken in water due to the polar nature of the water molecule.

ionic

Ocean salinity changes with ____, as rates of precipitation, evaporation, and runoff are strong influences on how much salt is dissolved in ocean water.

latitude

Iron is not abundant in seawater and has a low solubility; correspondingly, it has a ____ residence time.

low

Organisms undergoing respiration remove ____ from seawater and produce and release carbon dioxide.

oxygen - bacteria are the major agent of oxygen removal in seawater at depth because they are the most abundant organisms in the deep-water column

The most abundant dissolved gases present in the ocean are ____, ____, and carbon dioxide.

oxygen, nitrogen

Trace elements in seawater are present at concentrations that are measured in ____.

parts per million or less

Nitrogen, ____, and silicate are important to different forms of marine life for growth, as they act like the fertilizers of the ocean. These ions are present in very low concentrations in seawater.

phosphate

Marine plants take up carbon dioxide from seawater in order to ____.

photosynthesize

Which of the following geographic locations are known for their high ocean salinities resulting from both high evaporation rates and low freshwater input?

Persian Gulf, Red Sea, Mediterranean Sea

What happens to the organic substances present in seawater?

They accumulate in sediments They are broken down into smaller particles They are used by organisms

How are the concentrations of H+ and OH- ions in a water solution related to each other?

They are inversely proportional

True or false: The formation of new crust at sea floor ridge systems both adds salts to and removes salts from seawater.

True

The relative ratios of major seawater constituents ____.

remain the same, regardless of depth or geographic location

The average time a substance remains in the ocean (or any specific part of the hydrologic cycle) is called its ____ time.

residence

Two atoms held together by ionic bonding do so through ____.

the attraction of opposite charges

Rank the gases present in the ocean according to their abundance (percent by volume), with the most abundant on top.

(top to bottom) carbon dioxide, nitrogen, oxygen

Rank the residence times of these constituents with the highest at the top of the list.

(top to bottom) chloride, potassium, carbonate, iron

How does the formation of new ocean crust at ridge systems affect the salt content of seawater?

It both adds and removes salts from seawater

What is the most important process for removing elements from seawater?

Adsorption

About ____ percent of the weight of seawater is dissolved salts.

3.5

What is the average pH of seawater?

7.8 - seawater has more OH- ions than H+

What type of creatures would be found to be living in anoxic conditions in the ocean?

Anaerobic

In which of the following settings would anoxic conditions be likely to occur?

Bottom of an ocean trench Isolated, deep basins

In warm climates, salt in seawater is concentrated using evaporation and then extracted for use. How is salt recovered in cold climates?

By allowing the water to freeze and then collecting the leftover brine

How are concentrations of oxygen in seawater measured directly?

By using a probe that sends an electronic signal back to a receiver on a ship or buoy

Match the type of constituents in seawater to their descriptions.

Conservative - ratios do not change with changes in total salt content Nonconservative - concentrations change due to biological and chemical processes

The cations found in seawater originally came from ____, whereas the anions originated from ____.

Earth's crust; Earth's mantle - large quantities of cations are present in rocks that are formed from volcanic processes

Which of the following are involved in using reverse osmosis to desalinate water?

Forced pressure, semipermeable membrane

Match the type of desalination that works well with low-salinity water to its description.

Ion exchange - resins are used in a column of water to extract unwanted ions Electrodialysis - an electrical field is used to transport ions out of solution and through a semipermeable membrane

Which of the following statements correctly describe the Keeling curve?

It is a graph of CO2 concentrations in the atmosphere over the past 50+ years. The data for the curve have been measured at Mauna Loa Observatory, Hawaii.

Approximately 300 million metric tons of excess oxygen are produced each year by photosynthetic marine organisms. Where does this excess oxygen go?

It is incorporated into sediments and then eventually formed into rocks. It is consumed by bacteria.

Which of the following correctly describe the pH scale?

It ranges from 0 to 14. A pH above 7 represents an alkaline solution. It is a logarithmic scale.

Which of the following statements correctly describe the principle of constant proportion of seawater?

It was based upon the analysis of data collected during the Challenger expedition of 1872 to 1876. It states that ratios between the amounts of major ions in the open ocean are constant.

The well-known graph of increasing CO2 concentrations in the atmosphere over the past 50+ years is known as the ____ curve.

Keeling

Why does ocean salinity change with latitude?

Latitude influences evaporation and precipitation within the hydrologic cycle. - high precipitation rates, such as occur along the equator, result in lower salinity, and high evaporation rates, such as occur along 30°N and 30°S, result in higher salinity

Choose the advantages (over the disadvantages) of using reverse osmosis to desalinate seawater.

No thermal pollution Removal of contaminants Heating of water is not required

Match the term to the correct description.

Osmosis - The movement of water across a semipermeable membrane Reverse osmosis - the forcing of water movement through a semipermeable membrane using pressure

Put the phrases in the correct order in which they occur in the oceans to help characterize compensation depth. The top sentence should represent conditions in shallow ocean water.

Photosynthetic organisms produce oxygen Photosynthesis and respiration are balanced at the compensation depth Carbon dioxide is produced through respiration at the expense of oxygen

Match the way in which salt ions are removed from seawater to its description.

Sea spray - seawater is blown onto land, taking along and depositing the salts Adsorption - ions bind to clay particles, sink in the water column, and are incorporated into sediments Chemical precipitation - ions react with each other to form insoluble products Biological uptake - animals take in seawater and incorporate the calcium and/or silica into their hard parts Isolation - seawater bodies are cut off, and evaporation leaves behind the salts

Match the seawater depth to the expected relative pH level due to CO2 concentrations.

Surface water - the pH is high (alkaline) due to low concentrations of CO2 Deep water - the pH is low (acidic) due to high concentrations of CO2

How does water pouring out from underwater hydrothermal vents affect the salinity of seawater?

The outgoing water has a much higher salinity than the surrounding ocean water

How can measuring the amount of chlorine in seawater lead to an estimate of salinity?

The principle of constant proportion can be used, as chlorine is present in seawater at a set ratio compared to any of the other major constituents

Although oxygen content generally decreases with increasing depth in the ocean, a noticeable reverse in the trend occurs around 800 meters. Why?

There is a significant drop in the number of animals that are using oxygen

How do water molecules keep dissolved ions from combining into molecules?

Water molecules are polar and are able to surround ions, preventing them from joining with other ions

Match the way in which dissolved constituent concentrations in water are expressed with its scientific unit.

Weight - mg/kg Volume - mg/l Molar terms - moles/kg

How do changes in CO2 concentration in seawater affect pH?

When CO2 increases, then pH decreases. - an increase in CO2 makes seawater more acidic

The relative ____ of major seawater constituents remain the same for different geographic locations and water depths.

ratios

What process is used to concentrate the salt content of seawater in order to extract halite for use as table salt?

evaporation in shallow ponds

Rates of evaporation are high ____, so salinity is also high.

along the equator

The increase in atmospheric CO2 concentrations has resulted in ____ of dissolved CO2 concentrations in seawater.

an increase - the increase of carbon dioxide concentrations in seawater is leading to the acidification of the oceans

Respiration by ____ is the most important process in the removal of oxygen from seawater in the deep ocean.

bacteria

Photosynthesis in the ocean results in the addition of CO2 into intermediate and deep-ocean water when organisms sink and decay through a process called the ____ pump.

biological

A substance that acts to prevent the sudden change of pH in a solution is called a(n) ____.

buffer

Oxidation of organic matter, respiration of marine organisms, and transfer from the atmosphere all add ____ ____ to seawater.

carbon dioxide

Using the law of constant proportion, you can calculate the salinity of seawater if you know the ____ content.

chlorine

The principle of ____ ____ of seawater states that regardless of variations in salinity, the ratios between the amounts of major ions in open-ocean water are constant.

constant proportion

Water becomes supersaturated if it ____.

contains more than 100% oxygen

Increased CO2 concentrations in seawater result in a(n) ____ in pH.

decrease

In general, oxygen content ____ with water depth in the ocean.

decreases

The ____ zone is shallow in coastal waters but may extend to 200 meters in the open ocean where there are fewer suspended particles and sunlight can reach greater depths.

euphotic

pH levels are ____ in surface ocean waters because carbon dioxide levels are relatively ____.

high; low

Water coming out of hydrothermal vents on the sea floor has a much ____ salinity than the water that enters the system. Scientists ____ how this affects ocean salinity overall.

higher; do not understand

A negatively charged chloride atom dissolved in water will be surrounded with the positively charged ____ atoms of the water molecule, which prevents the chloride atom from combining with other ions.

hydrogen - oxygen atoms are negatively charged in water molecule

Anthropogenic carbon dioxide concentrations in the atmosphere have ____ over the past 150 years.

increased - burning of fossil fuels and deforestation are two major sources

Methods of desalination that work particularly well with ____ water include ion exchange resins and electrodialysis.

low-salinity

The salinity of surface ocean water in the polar areas gets ____ in the summer because of the input of melted ice.

lower

The ____ present in seawater include proteins, carbohydrates, and fats.

organics

Marine plants undergoing photosynthesis produce and release ____ into seawater.

oxygen

What dissolved gas do heterotrophic organisms take up from seawater in order to respire?

oxygen

Which two of the following gases are the most important for biological activity in the oceans?

oxygen, carbon dioxide

The ____ of the oceans has remained relatively constant over time because the input of salts is equal to the output.

salinity

The ocean contains an average of 35 parts per thousand dissolved salt; this is the ocean's ____.

salinity

Sea spray that is blown ashore, precipitation in the formation of insoluble deposits on the sea floor, and biological processes are all ways in which ____ is removed from the ocean.

salt

Adsorption is the most significant process in removing ____ from seawater. Other processes include evaporation, sea spray, biological uptake, and chemical precipitation.

salt ions

Volcanism and land runoff provide the ____ now found dissolved in ocean water.

salts

What is the term used to describe the maximum amount of gas that can be held in solution?

saturation concentration - increases with decreasing water temperature and salinity, and increases with increasing pressure

Under the correct conditions, water can become ____ with oxygen and rise to a value of more than 150%. Such conditions include quiet, cold waters at high latitudes that have abundant sunlight, nutrients, and photosynthetic organisms.

supersaturated

The concentrations of nutrient ions in seawater are nonconservative, and they vary because ____.

they are closely related to the life cycles of marine organisms

Dissolved constituent concentrations in water are expressed in terms of ____, ____, and molarity.

weight, volume