Unit 6 - WS # 6 - Why Structure Is Important? Video Answer Key - Free Printable

Let’s go step by step to understand and solve this problem.

We are given a table with three substances: PbI₂ (lead(II) iodide), CO₂ (carbon dioxide), and Ar (argon). For each, we’re told:

- The crystal structure (shown as diagrams)
- The type of bonding/structure: Ionic, Molecular, or Atomic
- Melting point and boiling point in °C
- We need to figure out the phase at room temperature for each — solid (S), liquid (L), or gas (G)

Room temperature is usually around 20–25°C. So we compare each substance’s melting and boiling points to that range.

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Step 1: Look at PbI₂ (Ionic)

- Melting point = 402°C → This means it turns from solid to liquid at 402°C.
- Boiling point = 954°C → Turns from liquid to gas at 954°C.
- Room temp (~25°C) is WAY below 402°C → So it’s still solid.
→ Phase at room temperature: Solid (S)

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Step 2: Look at CO₂ (Molecular)

- Melting point = -78°C → Turns from solid to liquid at -78°C? Wait — actually, CO₂ doesn’t have a normal liquid phase at 1 atm! But according to the table, they list melting pt = -78°C and boiling pt = -57°C. That seems off — but let’s use what’s given.

Actually, real-world fact: At standard pressure, CO₂ sublimes — goes straight from solid to gas at -78.5°C. But since the table gives both melting and boiling points, maybe they’re using hypothetical or different conditions? Let’s stick to the numbers given.

Given:
- Melting point = -78°C
- Boiling point = -57°C

So between -78°C and -57°C, it would be liquid. Below -78°C → solid. Above -57°C → gas.

Room temp = ~25°C → which is above -57°C → so it should be gas.

But wait — in reality, CO₂ is a gas at room temperature. Even if the table has slightly wrong values, the conclusion matches.

→ Phase at room temperature: Gas (G)

*(Note: In some tables, CO₂’s “melting point” isn’t listed because it sublimes — but here we’ll go with the data provided.)*

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Step 3: Look at Ar (Atomic)

- Melting point = -189°C
- Boiling point = -186°C

That’s a very small range — only 3 degrees between melting and boiling!

So:
- Below -189°C → solid
- Between -189°C and -186°C → liquid
- Above -186°C → gas

Room temp = ~25°C → way above -186°C → so it’s gas

→ Phase at room temperature: Gas (G)

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Now, fill in the last column:

For PbI₂ → S
For CO₂ → G
For Ar → G

Wait — looking back at the table, under “Phase at room temperature on...”, there are letters E, M, P — probably meaning:

E = Element? No — that doesn’t fit.

Looking again — perhaps E, M, P stand for:

Actually, looking at the layout — it might be a typo or mislabeling. But in many such worksheets, they ask you to write S, L, or G.

Alternatively, maybe E = Solid (from Latin “solidus”? Unlikely.)

Wait — look at the first row: PbI₂ — already filled with E, M, P vertically? Actually, no — in the image, for PbI₂, under “Phase at room temperature on...”, it says:

E
M
P

That doesn’t make sense unless it’s asking for something else.

Wait — re-examining the original image description:

In the user’s image, for PbI₂, under “Phase at room temperature on...”, it shows:

E
M
P

And same for others — but those are probably placeholders or labels for rows? Or maybe it’s meant to be filled with S, L, G?

Actually, looking closely — in the handwritten part, for PbI₂, someone wrote “IONIC” in the Type column, and for CO₂ “MOLECULAR”, for Ar “ATOMIC”.

Then under “Phase at room temperature on...”, for PbI₂, it has E, M, P written vertically — but that might just be column headers or something else.

Wait — perhaps “E”, “M”, “P” refer to states? Like:

E = Earth? No.

Another idea: Maybe it’s a mistake, and it’s supposed to be “State at room temperature” and you write S, L, or G.

Given that, and based on our reasoning:

PbI₂ → Solid → S
CO₂ → Gas → G
Ar → Gas → G

But let’s double-check CO₂.

Real science check: Carbon dioxide at room temperature and pressure is a gas. Its sublimation point is -78.5°C, so yes — definitely gas at 25°C.

Argon is a noble gas — always gas at room temp.

Lead iodide is a yellow solid — used in photography — melts at high temp — so solid.

So phases:

PbI₂: Solid
CO₂: Gas
Ar: Gas

Now, how to write it in the table? Since the column says “Phase at room temperature on...” and has E, M, P — maybe those are options? Or maybe it’s a formatting error.

Perhaps “E”, “M”, “P” stand for:

E = Element? But PbI₂ is compound.

Wait — another thought: In some contexts, especially older texts, they might use:

S = Solid
L = Liquid
G = Gas

But here it’s labeled E, M, P — which might be typos or misprints.

Looking at the student’s hand — they’re holding a pen, ready to write — so likely they need to fill in S, L, or G.

Alternatively, maybe E = Solid (from “Earth”?), M = Liquid (“Water”?), P = Gas (“Air”?) — too speculative.

Best approach: Use standard notation — S, L, G.

So final answers per row:

Row 1 (PbI₂): Phase = Solid → write S

Row 2 (CO₂): Phase = Gas → write G

Row 3 (Ar): Phase = Gas → write G

But wait — in the table, for CO₂, the melting point is listed as -78°C and boiling as -57°C — which suggests that between -78 and -57 it’s liquid. But at room temp (25°C), it’s above boiling point → gas. Correct.

Similarly, Ar boils at -186°C → so at 25°C, definitely gas.

PbI₂ melts at 402°C → so at 25°C, solid.

All good.

Now, the question also says: “Account for differences in the melting and boiling points of the three types of structures.”

This is an explanation part — but since the user didn’t select “ExplainAnswer”, we don’t need to include it unless required. But the main task is to fill the phase column.

Since the instruction is to solve the problem accurately, and the visible task is to complete the table, we focus on filling the phase column.

Final decision:

For each substance, write the phase at room temperature as:

PbI₂ → S
CO₂ → G
Ar → G

But how to present it? Since the table has “E, M, P” under that column, maybe those are meant to be replaced? Or perhaps it’s a multi-choice where you circle one?

Looking again at the image description — for PbI₂, under “Phase at room temperature on...”, it has:

E
M
P

Vertically aligned — suggesting that for each substance, you choose one of E, M, P.

What could E, M, P mean?

Possibility:

E = Solid (from “Earth” or “Elemental solid”?)

M = Liquid (“Molten”?)

P = Gas (“Plasma”? No — too hot. Or “Particles far apart”?)

Another common abbreviation:

In some systems:

s = solid
l = liquid
g = gas

But here it’s uppercase E, M, P.

Wait — perhaps it’s a typo, and it’s supposed to be S, L, G — and E is miswritten for S? Unlikely.

Maybe:

E = Exists as solid?
M = Exists as liquid?
P = Exists as gas?

That makes sense! So for each substance, you mark which phase it exists in at room temperature by selecting E, M, or P — where:

E = Solid
M = Liquid
P = Gas

That fits!

Because:

- PbI₂ is solid → choose E
- CO₂ is gas → choose P
- Ar is gas → choose P

Yes! That must be it.

In the table, for each row, under “Phase at room temperature on...”, there are three lines: E, M, P — and you’re supposed to indicate which one applies.

Probably by circling or writing next to it.

So for PbI₂: it’s solid → so E
For CO₂: gas → P
For Ar: gas → P

Perfect.

So now we can finalize.

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Final Answer:
For PbI₂: E
For CO₂: P
For Ar: P