Question 1

The first image shows a sample of Substance X. Examine the three changes shown in the following images. For each change, determine if it is possible, and if so, whether it is a physical or chemical change. (Assume the sample is in a sealed box and you can see all atoms.)

Change 1: The molecules are arranged in a regular, repeating pattern compared to the original.
Change 2: Some molecules appear to have split and recombined, forming new types of molecules.
Change 3: Some molecules are missing one of their original atoms, and there are free atoms present.

Which of the following best describes Change 2?

Accepted Answer

A chemical change, because new molecules are formed from the original ones.

Question 2

Which of the following best describes Change 1 (second image) compared to the original sample (first image)?

Accepted Answer

A physical change, because the molecules are the same but arranged in a regular pattern (solid formation).

Question 3

Which of the following best describes Change 3 (fourth image) compared to the original sample (first image)?

Accepted Answer

A chemical change, because some molecules have broken apart and there are free atoms present.

Question 4

If the original sample (first image) is carbon dioxide ($$\mathrm{CO_2$}$$), which change would most likely represent the freezing of carbon dioxide (dry ice formation)?

Accepted Answer

Change 1 (second image)

Question 5

If the original sample is a gas, which change would most likely represent a chemical reaction with another substance present in the box?

Accepted Answer

Change 2 (third image)

Question 6

Which change(s) could be reversed by simply changing the temperature, without altering the chemical identity of the molecules?

Accepted Answer

Change 1 (second image) only

Question 7

If the original sample is $$\mathrm{CO_2$}$$ and after a change you observe both $$\mathrm{CO_2$}$$ molecules and individual O atoms, what type of change has occurred?

Accepted Answer

A chemical change, because new substances (O atoms) have formed.

Question 8

Which of the following statements is true about physical and chemical changes as represented in molecular diagrams?

Accepted Answer

Chemical changes involve breaking and forming of chemical bonds, resulting in new substances.

Question 9

If you observe that the number of molecules remains the same but their arrangement changes from random to ordered, what type of change has occurred?

Accepted Answer

A physical change

Question 10

Which of the following best describes the difference between a physical and a chemical change at the molecular level?

Accepted Answer

A chemical change results in the formation of new substances, while a physical change does not.

Analyzing Physical and Chemical Changes Using Molecular Sketches

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