CA #4 and STAAR review - TEKS SCI 8.5E and SCI 8.6A

Common Assessment and

STAAR Review

TEKS SCI 8.5E and SCI 8.6A

Evidence of Chemical Reactions

TEK 8.5E: Investigate how evidence of chemical reactions indicates that new
substances with different properties are formed.

In a chemical reaction, the only absolute evidence of the chemical change is the creation
of a new substance.

There are a number of other possible signs of a chemical reaction, including:

●change of temperature

●change in color;

●release of a gas

●formation of a precipitate (insoluble powder) in a solution

●creation of light

●change in taste or smell.

Evidence of Chemical Reactions

In each of the following chemical reactions, the evidence of the reaction is listed:

●Baking Soda / Vinegar: creation of gas, decrease in temperature

●Acid / Metal: creation of gas, increase in temperature

●Burning Wood: creation of light, increase in temperature, creation of gas

●Spoiling Food: change in taste and smell, creation of gas, change in color

●Cooking Food: change in taste and smell, change in color

●Firefly Light: creation of light, change in color

●Rusting Iron or Tarnishing Metal: change in color.

Unbalanced Forces

TEK 8.6A: Demonstrate and calculate how unbalanced forces change the
speed or direction of an object's motion.

On Earth, it is rare for only one force to act upon an object. All objects are pulled
towards the center of the Earth by gravity, a long-range attractive force between
any two objects with mass. If the object is falling (accelerating) towards the center
of Earth because of gravity, that motion will be slowed by air resistance, the force
of air molecules pushing back against the falling motion. The object falls because
the force of gravity is stronger than the opposing force of air resistance. That sum
of all forces acting on the object is called the net force.

Unbalanced Forces

Unbalanced forces exist on an object when the net
force acting on that object is greater than zero. In the
above example, gravity pulls down more strongly than
the air resistance pushes back. The two forces can be
represented by arrows, as shown to the right. The size
of the arrows represent the relative strength of the two
forces. The larger down arrow represents gravity, and
the smaller up arrow represents air resistance. By
subtracting the down arrow length from the up arrow
length, the net force arrow shows the smaller
downward acting force

Unbalanced Forces

Newton’s Second Law of Motion
states that an object acted on by
an unbalanced force will
accelerate in the direction of the
force. In the above example, the
object falls because the net force
is downward and is greater than
zero.

Force (N) = mass x acceleration

Unbalanced Forces

Balanced forces exist when all of the forces acting
on an object cancel each other out, resulting in a
net force of zero. If the object pulled down by
gravity is resting on the ground, which is strong
enough to push up with equal force, the two
forces cancel each other out, and no motion
results In the diagram to the right, the down arrow
(gravity) and the up arrow (ground support) are of
equal length but opposite direction. If the net force
is zero, the forces are balanced, and no
acceleration (change of motion) occurs.

Unbalanced Forces

A good example of balanced and unbalanced forces is a
tug-of-war between two teams of students. If the opposing
teams are of equal strength, neither team can move the
other, because their net force is zero and the forces are
balanced. This is shown in the diagram to the right.

The same result occurs when a force pushes on an
unmovable object, or when friction prevents the motion of an
object because not enough force was applied to overcome
that friction. In both of these cases, the two forces are equal
but in opposite directions—they cancel each other out.

Unbalanced Forces

The same result occurs when a force pushes on an
unmovable object, or when friction prevents the motion
of an object because not enough force was applied to
overcome that friction. In both of these cases, the two
forces are equal but in opposite directions—they cancel
each other out.

If Team B becomes tired, and pulls less hard than Team
A, Team B eventually lose because they were moved
(accelerated) to the left, the direction of the greater
force.

Unbalanced Forces

Calculations of net force, where the two forces act in opposite directions, simply
require that you subtract the weaker force from the stronger force. The result of
this subtraction is the net force, acting in the direction of the stronger force.
Study the examples below to see how this process works.

Unbalanced Forces

When there are more than two forces, first add all of the forces acting in the
same direction together, and then subtract the smaller net force from the larger
net force. Again the resulting net force will be acting in the direction of the larger
net force.

Multiple Choice

A student obtains two strips of magnesium, Mg, ribbon that are each 3 cm long. One strip of magnesium is places in a test tube with 5 mL of water, and the other strip is placed in a test tube of 5 mL of hydrochloric acid, HCl. Both liquids are at room temperature. The students observations are recorded in the table.

Which of the following statements is NOT supported by the student's observations?

• A chemical reaction takes place between magnesium and hydrochloric acid.

• A gas is released in Test Tube 2.

• The substances in both test tubes are reactive only at high temperatures.

• Energy is released in the reaction involving hydrochloric acid.

Multiple Choice

Which of these provides evidence that a chemical reaction occurs?

• A gas is present.

• The total number of atoms remains unchanged.

• The state of matter remains the same.

• A brown gas is produced.

Evidence for Chemical Reactions

• color change

• temperature change (energy)

• light produced (energy)

• gas produced

• burning

• solid (precipitate) forms

Multiple Choice

Coal contains carbon and other elements. Carbon dioxide forms when coal burns in the presence of oxygen. Which of these is the best evidence that a chemical reaction occurs when coal burns?

• The shape of the coal changes.

• Oxygen is present.

• A new substance is produced.

• Coal is made up of more than one element.

Multiple Choice

For an investigation, a student poured a blue solution of CuSO₄ into a beaker. The student then placed a shiny, silver-colored strip of zinc metal into the solution and observed the changes (seen in the diagram).

The student inferred that a chemical reaction occurred. What evidence supports this inference?

• A dark solid formed on the zinc metal.

• The zinc metal remained silver-colored and shiny.

• The CuSO₄ solution turned blue when the metal zinc was added.

• None of these make sense and I am wondering why my time is currently being wasted with this inane drivel.

Multiple Choice

Some students in a chemistry lab conducted an investigation in which they added four different solid substances to a separate beaker of water. They stirred the mixtures for one minute and then recorded their observations in the table shown.

Which substance most likely caused a new substance to be formed when mixed with the water?

• Substance 1

• Substance 2

• Substance 3

• Substance 4

Evidence for Chemical Reactions

• color change

• temperature change (energy)

• light produced (energy)

• gas produced

• burning

• solid (precipitate) forms

Multiple Choice

A scientist performed four investigations using eight different liquids. In each investigation, the scientist combined two of the liquids together under a fume hood and recorded observations in the following table.

In which investigation is it LEAST likely that the liquids reacted chemically?

• 1

• 2

• 3

• 4

Chemical vs Physical Changes

In a physical change, no new substance is formed.

A chemical change is always accompanied by one or more new substance(s).

Physical change is easily reversible i.e original substance can be recovered. Chemical changes are irreversible i.e. original substance cannot be recovered.

Physical Changes (examples)

• melting

• freezing

• condensing

• baking

• crushing

• cutting

• bending

Multiple Choice

A scientist in a lab spilled a few drops of dilute hydrochloric acid (HCl) on a lab table. For safe purposes, the scientist sprinkled some baking soda (NAHCO₃) onto the spill. Which observation would provide the best evidence that a chemical reaction occurred?

• The baking soda and hydrochloric acid combined, and bubbles formed.

• The baking soda absorbed the hydrochloric acid.

• Some of the baking soda dissolved in the hydrochloric acid.

• The hydrochloric acid evaporated, leaving only the baking soda behind.

Multiple Choice

During an investigation, a student combined the two clear, colorless solutions shown in the diagram.

The student observed that the temperature changed from 25 degrees C to 23 degrees C when the solutions were combined and that a white substance rapidly formed and settled at the bottom of the container.

What most likely happened to produce these results?

• One of the original compounds came out of the solution.

• The solutions reacted chemically.

• Some of the water froze into ice crystals.

• Rapid evaporation of water occurred, leaving a solid behind.

Multiple Choice

A student observes some sugar as it is heated and burns. The student concludes that a chemical reaction has occurred. Which of the following observations about the burning sugar provides evidence of a chemical reaction?

• Heat is added to the sugar crystals.

• The sugar melts and becomes a liquid.

• The temperature of the sugar increases.

• Gas is produced as the sugar turns black.

Multiple Choice

Two students are working together to build a birdhouse. Student 1 applies a force of 10 N to a wooden board in order to slide it across the table to Student 2.

If the force of friction reducing the student's push is 4 N, what is the net force acting on the board?

• 4 N

• 6 N

• 10 N

• 14 N

Formulas to Remember

• Force (N) = mass (kg) x acceleration (m/s²)

• F = m x a

• m = F/a

• a = F/m

• Force is measured in newtons

• mass is measured in kilograms

• acceleration is measured in meters per second squared

Multiple Choice

Deep Space 1 was a spacecraft powered by an engine that gave off xenon particles to change velocity. It had a mass of 550 kg. Which expression can be used to find the spacecraft's acceleration if the engine created a net force of 0.10 N?

• $$\frac{0.10\ N}{500\ kg}$$

• $$500\ kg\ \cdot\ 0.10\ N$$

• $$\frac{500\ kg}{0.10\ N}$$

• 500 kg + 0.10 N

Math Response

A golfer hits a golf ball with a club. The mass of the ball is 0.05 kg. The ball accelerates at 2000 m/s².

What is the net force, to the nearest newton, that accelerates the ball?

Record your answer and be sure to use the correct place value.

Type answer here

Correct Answer: 100

Deg^°

Rad

Multiple Choice

Four students pushed carts filled with sports equipment across the gym. Each student pushes with the same amount of force. Which cart has the greatest change in speed?

Math Response

A basketball with a mass of 0.60 kg is accelerated with a force of 10.8 N. If resisting forces (e.g. air resistance and friction) are ignored, what is the acceleration of the basketball to the nearest m/s²?

Record your answer and be sure to use the correct place value.

Type answer here

Correct Answer: 18

Deg^°

Rad

Multiple Choice

A student uses a video camera to record another student dropping a marble through water in a graduated cylinder. The student watched the video in slow motion and made the observations seen in the diagram.

During which part or parts of the marble's fall did the marble experience unbalanced forces?

• Part 1 only

• Parts 1 and 2 only

• Part 3 only

• Parts 2 and 3 only

Math Response

When a space shuttle was launched, the astronauts on board experienced an acceleration of 29 m/s². If one of the astronauts had a mass of 60.0 kg, what net force in newtons did the astronaut experience?

Record your answer and be sure to use the correct place value.

Type answer here

Correct Answer: 1740

Deg^°

Rad

Multiple Choice

A student uses a magnet to move a 0.025 kg metal ball. The magnet exerts a force of 5 N, which causes the ball to begin moving. What is the acceleration of the ball when it begins to move?

• 200 m/s²

• 0.125 m/s²

• 5 m/s²

• 5.025 m/s²

Multiple Choice

The diagram shows a sled moving along a smooth and frictionless track.

In which sections of the track will the sled experience an unbalanced force?

• Sections 1 and 3

• Sections 2 and 3

• Sections 2 and 4

• Sections 3 and 4

Unbalanced Forces

To have unbalanced forces means that the force applied in one direction is greater than the force applied in the opposite direction.

When unbalanced forces are acting on an object, there is a change in speed and/or direction.

If an object is accelerating, it is changing speed or direction, and is therefore being acted on by an unbalanced force.

Multiple Choice

The diagram shows a homemade car being pushed with the force of 25 N.

The force causes the car to move at a constant speed of 3 m/s. What will happen if the force is changed to 35 N?

• The car will move at a constant speed of 13 m/s.

• The speed of the car will not change.

• The speed of the car will increase.

• The speed of the car will decrease to 1 m/s.

Multiple Choice

For an investigation, a student measures the speed of a cart as it rolls down a ramp. The student then records data in the table seen in the diagram.

Which of these best explains the student's data?

• The speed of the cart decreases as the cart rolls down the ramp because of friction between the cart and the ramp.

• The speed of the cart increases as the cart rolls down the ramp because the force acting on the cart is greater than the force of gravity.

• The speed of the cart increases as the cart rolls down the ramp because the forces acting on the cart are unbalanced.

• The speed of the cart decreases as the cart rolls down the ramp because the forces acting of the cart are balanced.

Multiple Choice

Some students measure the acceleration of a wheeled cart being acted on by varying forces. The students record their data in the table shown here. According to the data table, how much force is acting on the cart when the students measure an acceleration of 2.6 m/s².

• 13 N

• 7 N

• 8 N

• 12 N

Multiple Choice

The diagram shows a boat moving north in a river at 3 m/s while the current in the river moves south at 1 m/s.

How will the boat be affected if it enters a part of the river where the current is moving south at 2 m/s?

• The boat will move slower.

• The boat will move faster

• The boat will stop

• The boat will move to the west.