Unit Test: Chemical Reactions (OpenSciEd 7.1)

Question 1

A student mixes two clear liquids in a cup. Which observation is the BEST evidence that a chemical reaction occurred?

Accepted Answer

A new solid forms and settles to the bottom

Question 2

Which statement best describes a molecule?

Accepted Answer

Two or more atoms bonded together

Question 3

Carbon dioxide has the chemical formula CO₂. How many atoms in total are in ONE molecule of carbon dioxide?

Accepted Answer

3 atoms

Question 4

A student's investigation used a sealed flask. Use the data table below. The substances react inside the sealed flask and bubbles form. What will the total mass of the sealed flask and its contents be after the reaction?

Accepted Answer

Exactly 195.0 g, because no matter left

Question 5

A class heated a white powder and recorded its properties before and after. What conclusion is best supported by the data?

Accepted Answer

A new substance formed, because several properties changed

Question 6

John Dalton proposed an atomic theory in the early 1800s. Which statement is a key claim of Dalton's atomic theory?

Accepted Answer

Atoms of the same element are alike, and atoms combine in whole-number ratios.

Question 7

A student opens a jar of vinegar across the room, and a few seconds later another student smells it. Which explanation best describes how the second student detects the smell?

Accepted Answer

Vinegar molecules travel through the air and fit receptors in the nose that signal the brain.

Question 8

A student models the reaction of methane burning with this balanced equation: CH4 + 2O2 → CO2 + 2H2O The student counted the atoms on each side. What does the table show about this reaction?

Accepted Answer

The number of each type of atom is the same before and after the reaction.

Question 9

Which example describes a chemical change rather than a physical change?

Accepted Answer

Baking soda and vinegar mix and produce a gas that was not there before.

Question 10

A class placed a beaker of vinegar and a spoonful of baking soda on a scale in an open container, then mixed them. Sample data from their investigation: Which explanation best accounts for the change in mass?

Accepted Answer

A gas formed and escaped into the air, carrying atoms out of the container.

Question 11

During a chemical reaction, some of the atoms in the starting substances are destroyed, which is why new substances form.

Accepted Answer

false

Question 12

Each pure substance has its own set of characteristic properties, such as density, melting point, and odor, that can be used to identify it.

Accepted Answer

true

Question 13

In the chemical formula H2O, the subscript 2 shows that each molecule contains two atoms of hydrogen.

Accepted Answer

true

Question 14

Two substances made of the same kinds of atoms can smell different if their atoms are arranged in different molecular structures.

Accepted Answer

true

Question 15

A student measures a sample's density before and after heating it and finds the density changed slightly while its color, odor, and melting point stayed the same. This density change by itself is enough to prove a chemical reaction occurred.

Accepted Answer

false

Question 16

A molecule of water has the formula H2O. Draw a particle model of ONE water molecule. Include a key that shows which symbol or shape stands for each type of atom. Then explain in 1-2 sentences what your model shows about how the atoms are connected.

Accepted Answer

Drawing: one large circle labeled O with two smaller circles labeled H attached to it, one on each side, forming a bent shape. Key: large circle = oxygen atom, small circle = hydrogen atom. Explanation: A water molecule is made of 3 atoms, 2 hydrogen and 1 oxygen, bonded together. Both hydrogen atoms are connected to the oxygen atom, not to each other.

Question 17

A student's model shows this reaction in a sealed container: Use the table to explain WHY the total mass stays the same even though new substances form. Refer to what happens to the atoms.

Accepted Answer

The mass stays at 36 g because the atoms are only rearranged, not created or destroyed. Before the reaction there are 4 hydrogen atoms and 2 oxygen atoms, and after the reaction there are still 4 hydrogen and 2 oxygen atoms. The bonds break and the atoms join in new ways to make water molecules, but since every atom has mass and the same atoms are still there, the total mass does not change.

Question 18

When scientists want to know if a new substance has formed, they test characteristic properties. Name TWO properties you could measure or observe to identify a substance, and briefly describe how you would test each one.

Accepted Answer

1. Melting point: heat a sample and use a thermometer to record the temperature when it melts. 2. Density: measure the mass on a balance and the volume with a graduated cylinder, then divide mass by volume. (Other acceptable: boiling point, solubility, odor, color, flammability.)

Question 19

At the start of the unit, a student drew a substance as a single solid blob. Later, the student drew it as tiny separate particles. By the end of the unit, the student drew it as molecules made of different atoms bonded together. Explain why scientists (and students) revise their models, and describe ONE piece of evidence from a reaction investigation that would make the student change their model.

Accepted Answer

Models are revised when new evidence shows the old model cannot explain what we observe. For example, when two substances reacted and made a new substance with different properties like a new smell and different melting point, a blob model could not explain where the new substance came from. A model with atoms that can break apart and rearrange into new molecules explains how new substances form while the same atoms remain.

Question 20

Nylon fabric is a synthetic material made in a factory. Its starting chemicals come from petroleum (crude oil) taken from the ground. Explain how a synthetic material like nylon can come from a natural resource. In your answer, describe what happens to the atoms, and give ONE way making this material affects society.

Accepted Answer

Petroleum is a natural resource made of molecules containing carbon and hydrogen atoms. In a factory, chemicals from petroleum go through chemical reactions where their atoms are rearranged into new, very long molecules that form nylon. Nylon has different properties than petroleum, so it is a new substance, but its atoms came from the natural resource. One effect on society is that nylon makes strong, cheap clothing and rope, but using petroleum can cause pollution and uses up a nonrenewable resource.

Question 21

The marble on some old monuments is slowly crumbling. Marble is made mostly of calcium carbonate (CaCO3). Rain in polluted areas contains sulfuric acid (H2SO4). When they react, the products are calcium sulfate (CaSO4), water (H2O), and carbon dioxide (CO2):

CaCO3 + H2SO4 → CaSO4 + H2O + CO2

Use an atomic model to explain what happens to the atoms in the marble during this reaction and why the marble surface wears away over time. In your answer, describe how atoms are rearranged and what happens to the carbon dioxide.

Accepted Answer

In the reaction, the calcium carbonate in the marble and the sulfuric acid in rain break apart and their atoms rearrange to form new molecules. The calcium atom ends up bonded with sulfur and oxygen as calcium sulfate. Hydrogen and oxygen form water, and carbon and oxygen form carbon dioxide. Counting atoms, both sides have 1 Ca, 1 C, 7 O, 2 H, and 1 S, so no atoms are created or destroyed. The marble wears away because the original substance, calcium carbonate, is no longer there. Carbon dioxide is a gas, so it escapes into the air, and calcium sulfate is a different substance with different properties that crumbles and washes away in rain. Over many years, the surface is lost.

Question 22

A student mixed two liquids in a closed container and recorded the following sample data. Make a claim about whether a chemical reaction occurred. Support your claim with at least TWO pieces of evidence from the table and explain your reasoning.

Accepted Answer

A chemical reaction occurred. First, the solid formed after mixing has a density of 2.71 g/cm³, which is very different from both starting liquids (1.00 and 1.12 g/cm³). Second, both starting liquids dissolve in water, but the new solid does not. Third, the solid does not boil below 500°C while the liquids boil near 100°C. Because characteristic properties like density, solubility, and boiling point are unique to each substance, and these properties are different after mixing, a new substance must have formed, which means the atoms rearranged in a chemical reaction.

Question 23

Design an investigation that would show mass is conserved when baking soda reacts with vinegar (a reaction that produces a gas). Describe: (1) the materials and setup you would use, (2) the measurements you would take and when, (3) how you would control for possible sources of error, and (4) what results would support conservation of mass and why, using the idea of atoms.

Accepted Answer

I would put vinegar in a plastic bottle and baking soda inside a balloon stretched over the bottle opening, so nothing mixes yet. I would place the whole setup on a digital balance and record the total mass. Then I would lift the balloon so the baking soda falls in, without removing anything. After the bubbling stops, I would record the total mass again. To reduce error, I would tape the balloon tightly to prevent leaks, keep everything on the balance, use the same balance, and do three trials and average them. If the mass before and after is the same, it supports conservation of mass. The atoms in the baking soda and vinegar rearrange into new molecules, including carbon dioxide gas, but the gas is trapped in the balloon, so all the atoms are still in the system and the total mass does not change.

Question 24

A city has a marble monument that is being damaged by acid rain. A planning team is considering three solutions, shown in the sample estimates below. Recommend the BEST solution (or a combination). Justify your choice using the chemical reaction between acid and marble, the data, and at least one impact on people or the environment. Then describe one way the city could monitor whether the solution is working.

Accepted Answer

I recommend combining reducing factory sulfur emissions with a sealant in the short term. The marble crumbles because sulfuric acid reacts with calcium carbonate, rearranging its atoms into new substances that wash away. Reducing sulfur emissions removes the reactant at its source, cutting damage by about 80%, and it also gives cleaner air for people and protects other buildings, plants, and lakes. Since it takes years and is expensive, the sealant (60% reduction, medium cost) can block acid from touching the marble in the meantime. The glass cover is most effective at 95%, but it is very costly, blocks the view, and does not help anything else. To monitor, the city could measure the pH of rain each month and place small marble test blocks nearby, weighing them each year to see if they lose less mass.

Question 25

In the early 1800s, John Dalton proposed that matter is made of tiny, indivisible atoms; that atoms of the same element are identical; and that in chemical reactions atoms are rearranged, not created or destroyed. Evaluate Dalton's model. Describe TWO strengths of the model for explaining chemical reactions you studied in this unit, and ONE limitation of the model. Explain why scientists still find the model useful today even though it has limits.

Accepted Answer

One strength of Dalton's model is that it explains conservation of mass: if atoms are only rearranged and never created or destroyed, the total mass in a closed system stays the same, which matches our sealed-bottle data. A second strength is that it explains how new substances form: when atoms regroup into different molecules, the new substance has different properties, like when marble reacts with acid. One limitation is that Dalton said atoms cannot be divided, but scientists later found atoms contain smaller particles like protons, neutrons, and electrons. His model also does not explain why atoms bond the way they do. Scientists still use the model because it is simple and accurately predicts what happens in everyday chemical reactions. A model does not have to explain everything to be useful for explaining and predicting phenomena.

Unit Test: Chemical Reactions (OpenSciEd 7.1)

Part A: Multiple Choice (20 pts)

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