7th grade - Unit 1 Test

What is the chemical symbol for water?

• H2O

• CO2

• O2

• H2

Answer explanation

The chemical symbol for water is H2O, which indicates that each molecule consists of two hydrogen atoms and one oxygen atom. The other options represent different compounds: CO2 is carbon dioxide, O2 is oxygen, and H2 is hydrogen gas.

Tags

NGSS.MS-PS1-1

Which of the following is a simple molecule?

• Water

• DNA

• Protein

• Starch

Answer explanation

Water is a simple molecule composed of two hydrogen atoms and one oxygen atom (H2O). In contrast, DNA, proteins, and starch are complex macromolecules made up of many atoms and smaller units.

Tags

NGSS.MS-PS1-1

Which of the following best describes the law of conservation of mass?

• Mass is conserved in a chemical reaction.

• Mass can be created and destroyed.

• Mass is always increasing.

• Mass is always decreasing.

Answer explanation

The law of conservation of mass states that mass is conserved in a chemical reaction, meaning it cannot be created or destroyed. Therefore, the correct choice is that mass is conserved in a chemical reaction.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-5

What happens to the motion of particles when thermal energy is added to a substance?

• Particles move faster.

• Particles stop moving.

• Particles move slower.

• Particles remain the same.

Answer explanation

When thermal energy is added to a substance, it increases the kinetic energy of the particles, causing them to move faster. This is why the correct answer is that particles move faster.

Tags

NGSS.MS-PS1-4

Which of the following is an indicator of a chemical reaction?

• Change in color

• Change in shape

• Change in size

• Change in texture

Answer explanation

A change in color is a clear indicator of a chemical reaction, as it often signifies the formation of new substances. In contrast, changes in shape, size, or texture may occur without a chemical reaction.

Tags

NGSS.MS-PS1-2

Which model best represents the atomic composition of a water molecule?

• Two hydrogen atoms and one oxygen atom

• One hydrogen atom and two oxygen atoms

• Two oxygen atoms and one hydrogen atom

• Three hydrogen atoms

Answer explanation

A water molecule consists of two hydrogen atoms bonded to one oxygen atom, making the correct representation 'Two hydrogen atoms and one oxygen atom'. This is the fundamental structure of water.

Tags

NGSS.MS-PS1-1

How can you determine if a chemical reaction has occurred when two substances are mixed?

• By observing a change in color

• By measuring the mass before and after

• By checking if the substances dissolve

• By observing a change in shape

Answer explanation

A change in color is a strong indicator of a chemical reaction, as it often signifies the formation of new substances. Other options may not necessarily indicate a reaction, making color change the most reliable choice.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-5

What happens to the state of a substance when thermal energy is removed?

• It changes from gas to liquid to solid.

• It changes from solid to liquid to gas.

• It remains the same.

• It changes from liquid to gas to solid.

Answer explanation

When thermal energy is removed, a substance loses heat, causing it to transition from gas to liquid and then to solid. This process is known as condensation and freezing, respectively, confirming the correct choice.

Tags

NGSS.MS-PS1-4

Which of the following is a property that can be used to determine if a chemical reaction has occurred?

• Formation of a precipitate

• Melting point

• Boiling point

• Density

Answer explanation

The formation of a precipitate is a clear indicator of a chemical reaction, as it involves the creation of a solid from a solution. In contrast, melting point, boiling point, and density are physical properties and do not indicate a chemical change.

Tags

NGSS.MS-PS1-2

Explain how a model can be used to describe the atomic composition of a simple molecule like CO2.

Answer explanation

A model can represent CO2 by showing one carbon atom (C) bonded to two oxygen atoms (O). This visualizes the molecular structure and indicates the atomic composition, highlighting the ratio of elements in the molecule.

Tags

NGSS.MS-PS1-1