MCAT-Section-3-Physical-Sciences Section Three : Physical Sciences - Practice Questions

Question 7

X-rays are produced by a device which beams electrons with an energy between 103 and 106 eV at a metal plate. The electrons interact with the metal plate and are stopped by it. Much of the energy of the incoming electrons is released in the form of X-rays, which are high-energy photons of electromagnetic radiation. An example of such a device is shown below. Electrons are accelerated from the cathode towards the anode by an electric field.

There are two mechanisms by which the X-rays are produced within the metal. The first mechanism is called bremsstrahlung, which is German for “breaking radiation.” X-rays are emitted by the electrons as they are brought to rest by interactions with the positive nuclei of the anode.

The second mechanism occurs when an incoming electron knocks an inner electron out of one of the metal atoms of the anode. This electron is replaced by an electron from a higher energy level of the atom, and a photon making up the energy difference is emitted.

X-rays are absorbed by a material when they pass through it. The amount of X-rays absorbed increases with the density of the material. In addition, lower energy X-rays are more likely to be absorbed than higher energy X-rays. (Note: 1 eV = 1.6 x

10-19 J; Planck’s constant h = 4.1 x 10–15 eV•s; speed of light c = 3 x 108 m/s.)

In an X-ray tube, electrons of charge e are accelerated through a potential difference of V. The anode is cooled by water of mass m with specific heat c. If n electrons per second strike the anode, what is the maximum possible rise in the temperature of the water after 100 s?

Select an option, then click Submit answer.

○
nVe/100mc
○
100Ve/mc
○
100n(Ve + mc)
○
100nVe/mc

Question 8

In 1965, Boris Deryagin reported the discovery of an unusual substance formed during the condensation of water vapor in quartz capillaries. The material, called poly-water, appeared to be a polymer of water monomers and differed from normal water in a number of ways. It had a freezing point of –40° C and solidified into a glass-like solid with substantially less volumetric expansion than that of ordinary water upon freezing. It had a density 40% greater than water and a refractive index of 1.48.

An intricate apparatus was used to produce the poly-water. Ordinary distilled water was placed in a chamber held at 160° C with pressure below atmospheric pressure. This chamber was connected to a second chamber by a tube held at 500° C in order to prevent the passage of liquid water. The second chamber was held at 0° C and contained a drawn quartz capillary in which the water vapor condensed, forming poly-water.

Hypothesis 1

Deryagin proposed that polywater was a polymer of water monomers arranged in a network of hexagonal units. The polymerization was catalyzed by the silicate surface of the quartz capillary.

Proposed Structure of Polywater Hypothesis 2

Another researcher was skeptical. Analysis indicated that polywater was merely a solution of water and dissolved particles including silicon, carbon dioxide, and substantial concentrations of ions These contaminants dissolved from the quartz capillary and from materials used in the apparatus.

(constants for normal water : density = 1 g/c , index of refraction = 1.33 , freezing point depression constant = 1.86°C )

Which of the following pieces of evidence would most support Hypothesis 1?

Select an option, then click Submit answer.

○
The mass of the quartz capillary did not change throughout the experiment.
○
Filtration of the polywater increased its freezing temperature.
○
The polywater was found to differ from normal water in its boiling point.
○
The second chamber could be kept at 50° C with similar results.

Question 9

What is the structure associate with BF3 molecule?

Select an option, then click Submit answer.

○
Linear
○
Triangle
○
Trigonal Pyramidal
○
Tetrahedron

MCAT-Section-3-Physical-Sciences – Section Three : Physical Sciences