ACT Science Practice Test 57

PASSAGE VII

The molar heat of fusion is the amount of heat necessary to melt (or freeze) 1.00 mole of a substance at its melting point at a constant pressure. The molar heat of fusion for water is 6.02 kilojoules per mole (kJ/mol).

The equation for molar heat of fusion is:

q = ΔH_fus(mass/molar mass)

In this equation, q is the total amount of heat involved, ΔH_fus represents the molar heat of fusion (this value is a constant for a given substance), and (mass/molar mass) represents the number of moles of a given substance.

The following table lists molar heats of fusion, boiling points, and melting points for several elements.

PASSAGE I

In recent decades, astronomers deduced that there is approximately five times more material in clusters of galaxies than expected based on visible galaxies and hot gas. Most of the material in these galaxies is, in fact, invisible. Since galaxies are the largest structures in the universe held together by gravity, some scientists concluded that most of the matter in the entire universe is invisible. They called this invisible material dark matter. Two scientists offer theories on whether dark matter exists.

Scientist 1

Recent studies by researchers at Northeastern University and the University of Victoria may suggest that dark matter- a substance previously considered viable in light of Newton's theories of gravity-does not actually exist. Dark matter is not readily observable because it does not directly refract light or energy. Its existence could only be deduced because of the perceived gravitational effect that it has on surrounding matter, (i.e., warped galaxies).

This new research is based upon Einstein's theory of general relativity. Although Newtonian physics may provide for the cohesive nature of solar systems, when applied to galaxies the 'numbers' do not add up. Because there is not enough visible matter for the various gravitational equations to balance, dark matter was theorized to make up this deficit. Without a source for the rest of the missing matter, there was previously nothing in Newtonian physics to explain the movement or shape of galaxies.

In terms of general relativity, a galaxy, seen collectively, has its own gravity and essentially drives its own rotation at a constant rate. Aaron Romanowsky of Harvard University and several colleagues point to the existence of several elliptical-shaped galaxies surrounded by very little dark matter as evidence that dark matter is not, in fact, the cause of the warped galaxies. The results of their studies cast doubt on some of the conventional theories of galaxy formation and manipulation.

This theory does not explain everything, such as how large clusters of galaxies are able to bind to one another, but it does allow for already proven equations to explain the motion of galaxies without dark matter.

Scientist 2

Without dark matter, there are many cosmological phenomena that are difficult to explain. Some scientists believe that the interaction between dark matter and other smaller, nearby galaxies is causing the Milky Way galaxy to take on a warped, elliptical profile. This interaction involves two smaller galaxies (called Magellanic Clouds) near the Milky Way, moving through an enormous amount of dark matter, which in effect enhances the gravitational pull that the two Magellanic Clouds could exert on the Milky Way and other surrounding bodies. Computer models from the University of California at Berkeley seem to support this theory. Without the existence of the dark matter, the Magellanic Clouds would not have sufficient mass to have such a strong effect on the bend of the Milky Way galaxy.

The strongest evidence for the validity of this hypothesis rests in Newtonian physics and the hypothesis that anything with mass will exert a gravitational pull. However, there is nothing readily observable in the vicinity of the Milky Way with sufficient mass that could cause such a high level of distortion via gravitational pull.

In addition, theoretical arguments for the existence of dark matter can be made by looking at the cosmic microwave background in the universe. This "leftover" light radiation, emitted only a few hundred thousand years after the formation of the universe, provides information about conditions in the universe on a very large scale. Measurements of cosmic microwave radiation imply the existence of dark matter, although even dark matter cannot solve all of the mysteries of the universe.