Question

prepwork part 1: physical & chemical changes practice

1. in the diagram below, circle any elements, box any compounds, and star* any molecules.
2. **a sample of h₂ is cooled so that it transitions from a liquid to a solid. represent this transition in the boxes below. note - h is the symbol for hydrogen.
3. **in class, students are asked to classify h₂ as an element, compound, molecule, or atom. three student responses are below.

student 1: h₂ is a compound because it has more than one atom bonded together.
student 2: h₂ is a molecule because it has more than one atom bonded together.
student 3: h₂ is an element because both of the atoms have the same identity (h).
with whom do you agree, and why? you may agree with more than one student!

Explanation:

Step1: Define elements, compounds and molecules

An element is a pure - substance made of only one type of atom. A compound is a substance made of two or more different types of atoms chemically bonded together. A molecule is a group of two or more atoms held together by chemical bonds.

Step2: Analyze the diagram for question 1

Elements are single - type atoms. Compounds have different types of atoms bonded. Molecules are groups of atoms bonded. Without seeing the actual atom - types in the diagram, in general, single non - bonded atoms are elements (circle them), groups of different atoms bonded are compounds (box them), and any group of atoms bonded are molecules (star them).

Step3: Represent phase transition for question 2

In a liquid, $H_2$ molecules are more spread out and can move around each other. In a solid, $H_2$ molecules are more closely packed and in a more ordered arrangement. You can draw the molecules in the first box more spread out and in the second box more closely packed and arranged in a regular pattern.

Step4: Evaluate student responses for question 3

$H_2$ is an element because it is made up of only one type of atom (hydrogen). It is also a molecule because it consists of two atoms bonded together. Student 2 and Student 3 are correct. Student 2 is correct because $H_2$ is a molecule as it has two atoms bonded. Student 3 is correct because since both atoms are of the same type (hydrogen), $H_2$ is an element. Student 1 is incorrect because a compound must have different types of atoms.

Answer:

1. Without seeing the specific atom - types in the diagram, follow the general rules for circling elements, boxing compounds and starring molecules.
2. Draw $H_2$ molecules more spread out in the liquid box and more closely - packed and ordered in the solid box.
3. I agree with Student 2 and Student 3. Student 2 is correct as $H_2$ is a molecule with two bonded atoms. Student 3 is correct as $H_2$ is an element with only hydrogen atoms.