# CST Science Pre-test Prep Test 8th Graders

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• 1.

### Two students are pushing a cart as shown. The right is pushing 200 N and the left is pushing 150 N. The cart will move as if it were acted on by a single force with a magnitude of...

• A.

50 N.

• B.

150 N.

• C.

200 N.

• D.

350 N.

A. 50 N.
Explanation
The cart will move as if it were acted on by a single force with a magnitude of 50 N because the forces applied by the two students are in opposite directions. The net force acting on the cart is equal to the difference between the two forces, which is 200 N - 150 N = 50 N.

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• 2.

### The  galaxies pictured above would BEST be classified as...

• A.

Barred galaxies.

• B.

Spiral galaxies.

• C.

Irregular galaxies.

• D.

Symmetrical galaxies.

B. Spiral galaxies.
Explanation
The galaxies pictured above would best be classified as spiral galaxies because they exhibit a distinct spiral structure with arms that extend outwards from a central bulge. This classification is based on their visual appearance, which resembles the characteristic spiral shape commonly associated with spiral galaxies. Barred galaxies typically have a bar-like structure running through their central region, while irregular galaxies lack a defined shape or structure. Symmetrical galaxies is a vague term that does not provide enough information to accurately classify the galaxies in the picture.

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• 3.

### An athelete can run 9 kilometers in 1 hour. If the athelete runs at that same average speed for 30 minutes, how far will the athelete travel?

• A.

18 kilometers

• B.

9 kilometers

• C.

4.5 kilometers

• D.

3.3 kilometers

B. 9 kilometers
Explanation
The athlete can run 9 kilometers in 1 hour, which means the athlete's average speed is 9 kilometers per hour. If the athlete runs at the same average speed for 30 minutes, which is half an hour, the athlete will cover half the distance. Therefore, the athlete will travel 9 kilometers.

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• 4.

### To express the distance between the MIlky Way galaxy and other galaxies, the MOST appropriate unit of measurement is the...

• A.

Meter.

• B.

Kilometer.

• C.

Light-year.

• D.

Astronomical unit.

C. Light-year.
Explanation
The most appropriate unit of measurement to express the distance between the Milky Way galaxy and other galaxies is the light-year. A light-year is the distance that light travels in one year, which is approximately 9.46 trillion kilometers. Since the distances between galaxies are extremely vast, using kilometers or meters would be impractical. The light-year provides a more convenient and meaningful measure of these enormous distances in the context of astronomical observations.

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• 5.

### Which characteristic of motion could change without changing the velocity of an object?

• A.

The speed

• B.

The position

• C.

The direction

• D.

The acceleration

B. The position
Explanation
The characteristic of motion that could change without changing the velocity of an object is the position. Velocity is a vector quantity that includes both speed and direction. However, if only the position of an object changes, while the speed and direction remain constant, the velocity will remain the same. This means that the object can move to a different location but still have the same velocity.

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• 6.

### Which of the following compounds is MOST likely to be part of living organisms?

• A.

C4 H12 O6

• B.

BF3

• C.

MoCl2

• D.

CsI

A. C4 H12 O6
Explanation
C4H12O6, also known as glucose, is the most likely compound to be part of living organisms. Glucose is a simple sugar and a key source of energy in biological systems. It is a primary fuel for cellular respiration and is used in various metabolic processes. Glucose is a fundamental component of carbohydrates and is essential for the functioning of living organisms, making it the most likely compound to be found in them.

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• 7.

### A ball is dropped from the top of a tall building. As the ball falls, the upward force of air restistance becomes equal to the downward pull of gravity. When these two forces become equal in magnitude, the ball will...

• A.

Flatten due to the forces.

• B.

Fall at a constant speed.

• C.

Continue to speed up.

• D.

Slow to a stop.

B. Fall at a constant speed.
Explanation
As the ball falls, the force of gravity pulls it downward, while the force of air resistance acts in the opposite direction. Initially, the force of gravity is greater than the force of air resistance, causing the ball to accelerate. However, as the ball gains speed, the force of air resistance also increases. Eventually, a point is reached where the force of air resistance becomes equal to the force of gravity. At this point, the ball stops accelerating and falls at a constant speed, known as terminal velocity. Therefore, the correct answer is that the ball will fall at a constant speed.

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• 8.

### A force of 5 N is required to increase the speed of a box from a rate of 1.0 m / s to m / s within 5 salong a level surface. What change would MOST likely require additional force to produce the same results?

• A.

Reduce the mass of the box

• B.

Increase the mass of the box

• C.

Make the surfaces of the box smooth

• D.

Make the surface of the floor smooth

B. Increase the mass of the box
Explanation
Increasing the mass of the box would require additional force to produce the same results because according to Newton's second law of motion, force is directly proportional to mass. Therefore, increasing the mass would increase the force required to accelerate the box.

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• 9.

### A piece of pine wood floats on the surface of a lake because the water exerts...

• A.

An upward force equal to the weight of the wood.

• B.

A downward force equal to the weight of the wood.

• C.

An upward force equal to the weight of the displacement water.

• D.

A downward force equal to the weight of the displacement water.

C. An upward force equal to the weight of the displacement water.
Explanation
When an object is placed in a fluid, it experiences an upward force known as buoyancy. This force is equal to the weight of the fluid that the object displaces. In this case, the pine wood is floating on the surface of the lake because the water exerts an upward force on it that is equal to the weight of the water that the wood displaces. This buoyant force counteracts the downward force of gravity, allowing the wood to float.

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• 10.

### Which of the following is found farthest from the center of an atom?

• A.

Nucleus

• B.

Proton

• C.

Neutron

• D.

Electron

D. Electron
Explanation
Electrons are found farthest from the center of an atom. They exist in energy levels or orbitals surrounding the nucleus. Electrons have a negative charge and are much smaller and lighter compared to protons and neutrons, which are located in the nucleus. The electrons' position in the atom is not fixed but rather exists in a cloud-like region, making them the farthest from the center.

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• 11.

### Which of the following elements is BEST able to combine with itself and Hydrogen(H) to form large molecules?

• A.

Sodium (Na)

• B.

Lithium (Li)

• C.

Sulfur (S)

• D.

Carbon (C)

D. Carbon (C)
Explanation
Carbon is the best element to combine with itself and hydrogen to form large molecules because it has a unique ability to form strong covalent bonds with other carbon atoms, resulting in the formation of long chains and complex structures. This property of carbon is known as catenation, and it allows for the formation of diverse organic compounds, including carbohydrates, lipids, proteins, and nucleic acids, which are essential for life. Sodium, lithium, and sulfur do not possess this ability to the same extent as carbon, making carbon the correct answer.

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• 12.

### Which of the following represents the velocity of a moving object?

• A.

40

• B.

40 m north

• C.

40 m / s

• D.

40 m / s north

D. 40 m / s north
Explanation
The correct answer is 40 m / s north because velocity is a vector quantity that includes both magnitude (40 m/s) and direction (north). The other options either lack direction (40) or provide only magnitude without specifying the direction (40 m north, 40 m/s).

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• 13.

### Which of the following forms of energy is released or absorbed in MOST chemical reaction?

• A.

Light energy

• B.

Electrical energy

• C.

Sound energy

• D.

Heat energy

D. Heat energy
Explanation
In most chemical reactions, heat energy is released or absorbed. This is because chemical reactions involve the breaking and forming of chemical bonds, which requires energy. When bonds are broken, energy is absorbed, and when new bonds are formed, energy is released. The release or absorption of heat energy is a common indicator of a chemical reaction taking place. Light energy, electrical energy, and sound energy can also be involved in certain chemical reactions, but heat energy is the most commonly released or absorbed form of energy.

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• 14.

### Which of the following BEST describes an atom?

• A.

Protons and electrons grouped together in a random pattern

• B.

Protons and electrons grouped in an alternating pattern

• C.

A core of protons and neutrons surrounded by electrons

• D.

A core of electrons and neutrons surrounded by protons

C. A core of protons and neutrons surrounded by electrons
Explanation
An atom is described as a core of protons and neutrons surrounded by electrons. This is because an atom consists of a nucleus at its core, which contains protons (positively charged particles) and neutrons (neutral particles). The protons and neutrons are tightly packed together in the nucleus. Surrounding the nucleus are electrons (negatively charged particles) that orbit in specific energy levels or shells. Therefore, the correct answer is a core of protons and neutrons surrounded by electrons.

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• 15.

### Red-clay bricks have a density of approximately 2000 kg / m3. Air has a density of 1 kg / m3. Which of the following has the lowest mass?

• A.

2m3 of bricks

• B.

4m3 of bricks

• C.

6000m3 of air

• D.

10,000m3 of air

A. 2m3 of bricks
Explanation
The answer is 2m3 of bricks. This is because the density of bricks is much higher than the density of air. Therefore, even though the volume of 2m3 of bricks is smaller than the volume of air, the mass of the bricks will be greater.

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• 16.

### What do the the elements sulfur (S), nitrogen (N), phosphorus (P), and bromine(Br) have in common?

• A.

They are noble (inert) gases.

• B.

They are nonmetals.

• C.

They have the same thermal conductivity.

• D.

They have the same number of protons.

B. They are nonmetals.
Explanation
The elements sulfur, nitrogen, phosphorus, and bromine have in common that they are nonmetals. Nonmetals are a group of elements that do not exhibit typical metallic characteristics such as being shiny, malleable, or good conductors of heat and electricity. Instead, nonmetals tend to be brittle, dull, and poor conductors. Sulfur, nitrogen, phosphorus, and bromine all fall into this category, making them nonmetals.

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• 17.

### Which class of elements BEST conducts electricity?

• A.

Metals

• B.

Nonmetals

• C.

Semimetals

• D.

Noble (inert) gases

A. Metals
Explanation
Metals are the best conductors of electricity among the given options. This is because metals have a high number of free electrons that are mobile and can easily carry electric charge. The delocalized electrons in metals are not strongly bound to specific atoms, allowing them to move freely and conduct electricity efficiently. In contrast, nonmetals, semimetals, and noble gases have fewer free electrons or lack them altogether, making them poor conductors of electricity.

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• 18.

### What is the density of a 64-g iron cube that displaces 8 mL of water?

• A.

512 g / mL

• B.

32 g / mL

• C.

8 g / mL

• D.

4 g / mL

C. 8 g / mL
Explanation
The density of a substance is defined as its mass divided by its volume. In this question, we are given the mass of the iron cube (64 g) and the volume of water it displaces (8 mL). To find the density, we divide the mass by the volume: 64 g / 8 mL = 8 g / mL. Therefore, the correct answer is 8 g / mL.

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• 19.

### How much time is required for a bicycle to travel a distance of 100 m at an average speed of 2 m / s?

• A.

0.02 s

• B.

50 s

• C.

100 s

• D.

200 s

B. 50 s
Explanation
The average speed of the bicycle is given as 2 m/s. To find the time required to travel a distance of 100 m, we can use the formula time = distance/speed. Plugging in the values, we get time = 100 m / 2 m/s = 50 s. Therefore, the correct answer is 50 s.

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• 20.

### A galaxy is BEST described as a cluster of...

• A.

Hundreds of stars.

• B.

Thousands of stars.

• C.

Millions of stars.

• D.

Billions of stars.

D. Billions of stars.
Explanation
A galaxy is best described as a cluster of billions of stars. Galaxies are massive systems that contain a vast number of stars, along with other celestial bodies such as planets, nebulae, and black holes. The number of stars in a galaxy can vary, but on average, there are billions of stars within a single galaxy. This makes the option "billions of stars" the most accurate description of a galaxy.

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• 21.

### When magnesium (Mg) metal is burned in the presence of oxygen (O2), magnesium oxide (MgO) is produced. The properties of magnesium oxide are different than the individual properties of magnesium and oxygen because magnesium oxide is...

• A.

A solution.

• B.

A mixture.

• C.

A compound.

• D.

An element.

C. A compound.
Explanation
When magnesium metal is burned in the presence of oxygen, a chemical reaction occurs and magnesium oxide is formed. In a compound, the elements are chemically bonded together in a fixed ratio. Magnesium oxide is composed of one magnesium atom bonded to one oxygen atom, forming a stable compound with distinct properties different from those of the individual elements. Therefore, the correct answer is that magnesium oxide is a compound.

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• 22.

### Which of the following sets contains only objects that shine as a result of reflected light?

• A.

Moons, planets, and comets

• B.

Moons, comets, and stars

• C.

Planets, stars, and comets

• D.

Planets, stars, an moons

A. Moons, planets, and comets
Explanation
Moons, planets, and comets are all celestial objects that shine as a result of reflected light. Moons reflect the light of their parent planet, planets reflect the light of the sun, and comets reflect the light of the sun as well. Stars, on the other hand, shine through a process called nuclear fusion, where they generate their own light and energy. Therefore, the set "moons, planets, and comets" is the only set that contains objects that shine as a result of reflected light.

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• 23.

### Within a substance, atoms that collide frequently and more independently of one another are MOST likely in a...

• A.

Liquid.

• B.

Solid.

• C.

Gas.

• D.

Crystal.

C. Gas.
Explanation
In a gas, atoms have high kinetic energy and are in constant motion, colliding frequently with each other. These collisions are relatively independent, meaning that they do not have a strong attraction or bond to each other. This is in contrast to a solid or crystal, where atoms are tightly packed and have strong interactions, or a liquid where atoms are still close together but have more freedom of movement compared to a solid. Therefore, atoms that collide frequently and independently are most likely in a gas.

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• 24.

### In a comparison of metals to nonmetals, metals tend to have...

• A.

Lower melting points and greater conductivity than nonmetals.

• B.

Lower conductivity and lower density than nonmetals.

• C.

Higher density and lower melting points than nonmetals.

• D.

Greater conductivity and higher melting points than nonmetals.

D. Greater conductivity and higher melting points than nonmetals.
Explanation
Metals tend to have greater conductivity and higher melting points than nonmetals. This is because metals have a sea of delocalized electrons that can easily move and carry electric charge, making them good conductors of electricity. Nonmetals, on the other hand, have fewer free electrons and therefore have lower conductivity. Additionally, metals have strong metallic bonds, which require high temperatures to break, resulting in higher melting points compared to nonmetals.

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• 25.

### What characteristic of carbon (C) makes it essential to living organisms?

• A.

Carbon forms crystal structures under certain conditions.

• B.

Carbon can exist as a solid, liquid, or gas.

• C.

Carbon bonds in many ways with itself to form chains.

• D.

C. Carbon bonds in many ways with itself to form chains.
Explanation
Carbon bonds in many ways with itself to form chains. This characteristic of carbon allows it to form the backbone of organic molecules, which are the building blocks of life. Carbon can form single, double, and triple bonds with other carbon atoms, as well as form bonds with other elements such as hydrogen, oxygen, nitrogen, and sulfur. This versatility in bonding allows carbon to create a wide variety of complex and diverse molecules, including carbohydrates, lipids, proteins, and nucleic acids, which are essential for the structure and function of living organisms.

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• 26.

### As a smaple of water turns to ice,...

• A.

New molecules are formed.

• B.

The mass of the sample is increased.

• C.

The arrangement of the molecules changes.

• D.

Energy is absorbed by the molecules.

C. The arrangement of the molecules changes.
Explanation
When water turns to ice, the arrangement of the molecules changes. In the liquid state, the water molecules are constantly moving and have enough energy to overcome the attractive forces between them. However, as the temperature decreases, the molecules slow down and the attractive forces become stronger. This causes the water molecules to arrange themselves in a regular pattern, forming a solid structure known as ice. Therefore, the correct answer is that the arrangement of the molecules changes.

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• 27.

### A student divides several cubes into two groups, based on whether or not each cube can float in water. What property is the student using to classify the cubes?

• A.

Weight

• B.

Density

• C.

Conductivity

• D.

Mass

B. Density
Explanation
The student is using density to classify the cubes. Density is a property that describes how much mass is contained in a given volume. If a cube has a lower density than water, it will float, and if it has a higher density, it will sink. Therefore, by observing whether the cubes float or sink in water, the student can determine their relative densities and classify them accordingly.

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• 28.

### An object composed mainly of ice is orbiting the Sun in an elliptical path. This object is MOST likely...

• A.

A planet.

• B.

An asteroid.

• C.

A meteor.

• D.

A comet.

D. A comet.
Explanation
Given that the object is composed mainly of ice and is orbiting the Sun in an elliptical path, it is most likely a comet. Comets are composed of ice, rock, and dust and they follow elliptical orbits around the Sun. They often have a visible coma (a glowing atmosphere) and a tail when they approach the Sun due to the heat causing the ice to vaporize and release gas and dust. Planets, asteroids, and meteors do not typically have a composition of ice or exhibit the characteristics of a comet.

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• 29.

### Iron oxides, such as rust, form when iron metal reacts with oxygen in the air. What are the chemical symbols for the two elements found in iron oxide?

• A.

I and O

• B.

Ir and O

• C.

Fe and O

• D.

Pb and O

C. Fe and O
Explanation
Iron oxide is a compound that forms when iron metal reacts with oxygen in the air. The chemical symbol for iron is Fe, while the chemical symbol for oxygen is O. Therefore, the correct answer is Fe and O.

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