Chemistry: The Central Science

Chemistry is sometimes referred to as “the central science” due to its interconnectedness with a vast array of other STEM disciplines (STEM stands for areas of study in the science, technology, engineering, and math fields). Chemistry and the language of chemists play vital roles in biology, medicine, materials science, forensics, environmental science, and many other fields.

A The basic principles of physics are essential for understanding many aspects of chemistry, and there is extensive overlap between many subdisciplines within the two fields, such as chemical physics and nuclear chemistry. B Mathematics, computer science,and information theory provide important tools that help us calculate, interpret, describe,and generally make sense of the chemical world. C Biology and chemistry converge in biochemistry, which is crucial to understanding the many complex factors and processes that keep living organisms (such as us) alive. D Chemical engineering, materials science, and nanotechnology combine chemical principles and empirical findings to produce useful substances, ranging from gasoline to fabrics to electronics. Agriculture, food science, veterinary science, and brewing and winemaking help provide sustenance in the form of food and drink to the world’s population. Medicine, pharmacology, biotechnology, and botany identify and produce substances that help keep us healthy. Environmental science, geology, oceanography, and atmospheric science incorporate many chemical ideas to help us better understand and protect our physical world. Chemistry’s usefulness also extends outside of our own world to help us better understand the universe and the composition of space in disciplines like astronomy and cosmology.

Q. In paragraph 2 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

Biochemistry is one of the five major branches of chemistry, which can be divided into many sub-branches.

Types of Mixtures

A mixture is composed of two or more types of matter that can be present in varying amounts and can be separated by physical changes, such as evaporation. A A mixture with a composition that varies from point to point is called a heterogeneous mixture. B Italian dressing is an example of a heterogeneous mixture. C Its composition can vary because we can make it from varying amounts of oil, vinegar, and herbs. D It is not the same from point to point throughout the mixture—one drop may be mostly vinegar, whereas a different drop may be mostly oil or herbs because the oil and vinegar separate and the herbs settle. Other examples of heterogeneous mixtures are chocolate chip cookies (we can see the separate bits of chocolate, nuts, and cookie dough) and granite (we can see the quartz, mica, and more).

A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout. An example of a solution is a sports drink, consisting of water, sugar, coloring, flavoring, and electrolytes mixed together uniformly. Each drop of a sports drink tastes the same because each drop contains the same amounts of water, sugar, and other components. Note that the composition of a sports drink can vary—it could be made with somewhat more or less sugar, flavoring, or other components, and still be a sports drink. Other examples of homogeneous mixtures include air, maple syrup, gasoline, and a solution of salt in water.

Q. In paragraph 1 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

Though there are specific types of mixtures within them, the two principal categories of mixture are heterogeneous and homogeneous.

The Process of Scientific Research

Scientific knowledge is advanced through a process known as the scientific method. Basically, ideas (in the form of theories and hypotheses) are tested against the real world (in the form of empirical observations), and those empirical observations lead to more ideas that are tested against the real world, and so on. In this sense, the scientific process is circular. The types of reasoning within the circle are called deductive and inductive. In deductive reasoning, ideas are tested against the empirical world; in inductive reasoning, empirical observations lead to new ideas. These processes are inseparable, like inhaling and exhaling, but different research approaches place different emphasis on the deductive and inductive aspects.

In the scientific context, deductive reasoning begins with a generalization—one hypothesis—that is then used to reach logical conclusions about the real world. If the hypothesis is correct, then the logical conclusions reached through deductive reasoning should also be correct. A deductive reasoning argument might go something like this: All living things require energy to survive (this would be your hypothesis). Ducks are living things. Therefore, ducks require energy to survive (this would be your logical conclusion). In this example, the hypothesis is correct; therefore, the conclusion is correct as well. Sometimes, however, an incorrect hypothesis may lead to a logical but incorrect conclusion. Consider this argument: all ducks are born with the ability to see. Quackers is a duck. A Therefore, Quackers was born with the ability to see. B Scientists use deductive reasoning to empirically test their hypotheses. C Returning to the example of the ducks, researchers might design a study to test the hypothesis that if all living things require energy to survive, then ducks will be found to require energy to survive. D

Q. In paragraph 2 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

This means that they create studies that are designed specifically to analyze their observations or experiences in order to prove or disprove their hypotheses.

Cultural Universals

Often, a comparison of one culture to another will reveal obvious differences. Still, it’s important to note that all cultures also share some common elements. Cultural universals are patterns or traits that are globally common to all societies. One example of a cultural universal is the family unit: every human society recognizes a family structure that regulates sexual reproduction and the care of children. Even so, how that family unit is defined and how it functions vary. A In many Asian cultures, for example, family members from all generations commonly live together in one household. B In these cultures, young adults will continue to live in the extended household family structure until they marry and join their spouse’s household, or they may remain and raise their nuclear family within the extended family’s homestead. C In the United States, by contrast, individuals are expected to leave home and live independently for a period before forming a family unit consisting of parents and their offspring. D

Q. In the paragraph there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

In conclusion, commonalities, known as cultural universals, – like that of the family structure – between cultures can be noticed all over the world whether one compares continents or countries.

The Terrestrial Planets

A The terrestrial planets are quite different from the giants. In addition to being much smaller, they are composed primarily of rocks and metals. B These, in turn, are made of elements that are less common in the universe as a whole. C The most abundant rocks, called silicates, are made of silicon and oxygen, and the most common metal is iron. D We can tell from their densities that Mercury has the greatest proportion of metals (which are denser) and the Moon has the lowest. Earth, Venus, and Mars all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be oxidized.

When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.

Differentiation is the process by which gravity helps separate a planet’s interior into layers of different compositions and densities. The heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust. Later, when the planet cools, this layered structure is preserved. In order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than 1,800 F.

Q. In paragraph 1 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

The terrestrial planets, as they are known, consist of the four innermost planets of our solar system – namely, Mercury, Venus, Earth, and Mars.

Metabolism and Body Weight

Our body weight is affected by a number of factors, including gene-environment interactions, and the number of calories we consume versus the number of calories we burn in daily activity. If our caloric intake exceeds our caloric use, our bodies store excess energy in the form of fat. If we consume fewer calories than we burn off, then stored fat will be converted to energy. A Our energy expenditure is obviously affected by our levels of activity, but our body’s metabolic rate also comes into play. B A person’s metabolic rate is the amount of energy that is expended in a given period of time, and there is tremendous individual variability in our metabolic rates. C People with high rates of metabolism are able to burn off calories more easily than those with lower rates of metabolism. D

We all experience fluctuations in our weight from time to time, but generally, most people’s weights fluctuate within a narrow margin, in the absence of extreme changes in diet and/or physical activity. This observation led some to propose a set-point theory of body weight regulation. The set-point theory asserts that each individual has an ideal body weight, or set point, which is resistant to change. This set-point is genetically predetermined and efforts to move our weight significantly from the set-point are resisted by compensatory changes in energy intake and/or expenditure.

Some of the predictions generated from this particular theory have not received empirical support. For example, there are no changes in metabolic rate between individuals who had recently lost significant amounts of weight and a control group. In addition, the set-point theory fails to account for the influence of social and environmental factors in the regulation of body weight. Despite these limitations, set-point theory is still often used as a simple, intuitive explanation of how body weight is regulated.

Q. In paragraph 3 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

This variability in metabolic rate is due to a variety of factors including age, gender, physical activity, hormone function, and muscle-to-fat ratio.

The Giant Impact Hypothesis

In an effort to resolve these apparent contradictions, scientists developed a fourth hypothesis for the origin of the Moon, one that involves a giant impact early in Earth’s history. There is increasing evidence that large chunks of material—objects of essentially planetary mass—were orbiting in the inner solar system at the time that the terrestrial planets formed. The giant impact hypothesis envisions Earth being struck obliquely by an object approximately one-tenth Earth’s mass—a “bullet” about the size of Mars. This is very nearly the largest impact Earth could experience without being shattered.

Such an impact would disrupt much of Earth and eject a vast amount of material into space, releasing almost enough energy to break the planet apart. Computer simulations indicate that material totaling several percent of Earth’s mass could be ejected in such an impact. Most of this material would be from the stony mantles of Earth and the impacting body, not from their metal cores. This ejected rock vapor then cooled and formed a ring of material orbiting Earth. It was this ring that ultimately condensed into the Moon.

While we do not have any current way of showing that the giant impact hypothesis is the correct model of the Moon’s origin, it does offer potential solutions to most of the major problems raised by the chemistry of the Moon. A First, since the Moon’s raw material is derived from the mantles of Earth and the projectile, the absence of metals is easily understood. B Second, most of the volatile elements would have been lost during the high- temperature phase following the impact, explaining the lack of these materials on the Moon. C Yet, by making the Moon primarily of terrestrial mantle material, it is also possible to understand similarities such as identical abundances of various oxygen isotopes. D

Q. In paragraph 3 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

Therefore, it is for these satisfactory explanations that the giant impact hypothesis is the favored scientific theory of the formation of the Moon.

Surface Temperature on Venus

The largest volcanic mountains of Mars are found in the Tharsis area, although smaller volcanoes dot much of the surface. The most dramatic volcano on Mars is Olympus Mons (Mount Olympus), with a diameter larger than 500 kilometers and a summit that towers more than 20 kilometers above the surrounding plains—three times higher than the tallest mountain on Earth. The volume of this immense volcano is nearly 100 times greater than that of Mauna Loa in Hawaii. Placed on Earth’s surface, Olympus would more than cover the entire state of Missouri.

A Images taken from orbit allow scientists to search for impact craters on the slopes of these volcanoes in order to estimate their age. B Many of the volcanoes show a fair number of such craters, suggesting that they ceased activity a billion years or more ago. C However, Olympus Mons has very, very few impact craters. D Its present surface cannot be more than about 100 million years old; it may even be much younger. Some of the fresh-looking lava flows might have been formed a hundred years ago, or a thousand, or a million, but geologically speaking, they are quite young. This leads geologists to the conclusion that Olympus Mons possibly remains intermittently active today—something future Mars land developers may want to keep in mind.

Q. In paragraph 2 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

Generally, the more craters that appear on a surface, the older that surface is.

Long-Term Memory

A Long-term memory (LTM) is the continuous storage of information. B Unlike short-term memory, the storage capacity of LTM has no limits. C It encompasses all the things you can remember that happened more than just a few minutes ago to all of the things that you can remember that happened days, weeks, and years ago. D In keeping with the computer analogy, the information in your LTM would be like the information you have saved on the hard drive. It isn’t there on your desktop (your short-term memory), but you can pull up this information when you want it, at least most of the time. Not all long-term memories are strong memories. Some memories can only be recalled through prompts. For example, you might easily recall a fact— “What is the capital of the United States?”—or a procedure—“How do you ride a bike?”—but you might struggle to recall the name of the restaurant you had dinner at when you were on vacation in France last summer. A prompt, such as that the restaurant was named after its owner, who spoke to you about your shared interest in soccer, may help you recall the name of the restaurant.

Long-term memory is divided into two types: explicit and implicit. Understanding the different types is important because a person’s age or particular types of brain trauma or disorders can leave certain types of LTM intact while having disastrous consequences for other types. Explicit memories are those we consciously try to remember and recall. For example, if you are studying for your chemistry exam, the material you are learning will be part of your explicit memory.

Implicit memories are memories that are not part of our consciousness. They are memories formed from behaviors. Implicit memory is also called non-declarative memory.

Q. In paragraph 1 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

It is defined in contrast to short-term memory.

Pioneer and Voyager

A The first spacecraft to investigate the regions past Mars were the NASA Pioneers 10 and 11, launched in 1972 and 1973 as pathfinders to Jupiter. B One of their main objectives was simply to determine whether a spacecraft could actually navigate through the belt of asteroids that lies beyond Mars without getting destroyed by collisions with asteroidal dust. C Another objective was to measure the radiation hazards in the magnetosphere (or zone of magnetic influence) of Jupiter. Both spacecraft passed through the asteroid belt without incident, but the energetic particles in Jupiter’s magnetic field nearly wiped out their electronics, providing information necessary for the safe design of subsequent missions. D

Pioneer 10 flew past Jupiter in 1973, after which it sped outward toward the limits of the solar system. Pioneer 11 undertook a more ambitious program, using the gravity of Jupiter to aim for Saturn, which it reached in 1979. The twin Voyager spacecraft launched the next wave of outer planet exploration in 1977. Voyagers 1 and 2 each carried 11 scientific instruments, including cameras and spectrometers, as well as devices to measure the characteristics of planetary magnetospheres. Since they kept going outward after their planetary encounters, these are now the most distant spacecraft ever launched by humanity.

Voyager 1 reached Jupiter in 1979 and used a gravity assist from that planet to take it on to Saturn in 1980. Voyager 2 arrived at Jupiter four months later, but then followed a different path to visit all the outer planets, reaching Saturn in 1981, Uranus in 1986, and Neptune in 1989. This trajectory was made possible by the approximate alignment of the four giant planets on the same side of the Sun. About once every 175 years, these planets are in such a position, and it allows a single spacecraft to visit them all by using gravity-assisted flybys to adjust course for each subsequent encounter; such a maneuver has been nicknamed a “Grand Tour” by astronomers.

Q. In paragraph 1 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

The former of these was the first spacecraft to make direct observations and take close- up pictures of Jupiter.