Soap Scum Scaffolded Assessment
Soap Scum Task
Chemistry Assessment (ELL)
NAME:
DATE:
Have you ever washed your hands and they did not feel clean? This might be soap scum!
Soap scum forms when you use soap with hard water. Hard water has minerals like calcium bicarbonate: \(Ca(HCO_3)_2\).
Process:
\(Ca(HCO_3)_2 \rightarrow Ca^{2+} + 2HCO_3^-\)
(Breaks into ions when dissolved in water)
1
Initial Model
Task: Draw a model showing the system before soap scum forms.
Include:
Drain surface
Water (\(H_2O\))
Calcium (\(Ca^{2+}\))
Level 1-2 Stems:
The calcium ions are ______________. The water molecules have charges that are ______________ and ______________.
Level 3 Stems:
Oxygen pulls the ions because it has a ______________ charge, while the calcium ion has a ______________ charge.
Page 1
Molecular Structures
Soluble (Dissolves)
Sodium Stearate (Soap) mixes with water.
Insoluble (Solid)
Calcium Stearate (Scum) is a solid.
| Soap (Sodium Stearate) | Soluble |
|---|
 | |
| Scum (Calcium Stearate) | Insoluble |
|---|
 | |
Page 2
2
Balance the Equation
\(Ca(HCO_3)_2(aq)\) + \(NaC_{18}H_{35}O_2(aq)\)
\(NaHCO_3(aq)\) + \(Ca(C_{18}H_{35}O_2)_2(s)\)
| Element / Ion | Reactants (Left) | Products (Right) |
|---|
| \(Ca\) (Calcium) | | |
| \(HCO_3\) (Bicarbonate) | | |
| \(Na\) (Sodium) | | |
| \(C_{18}H_{35}O_2\) (Stearate) | | |
Atomic Properties (Electronegativity)
\(Na\): 0.93 \(H\): 2.22 \(C\): 2.55 \(O\): 3.44
Page 3
3a
The Pull of Oxygen
Calculate the electronegativity difference (O - Bond Partner):
\(O - H\)
\(3.44 - 2.22\)
\(O - C\)
\(3.44 - 2.55\)
\(O - Na\)
\(3.44 - 0.93\)
Oxygen pulls sodium because oxygen has a much _______________ electronegativity. This makes the bond _______________ and easier for water to pull apart.
3b
Charge Balance
Why does one calcium (\(Ca^{2+}\)) need two stearate ions (\(C_{18}H_{35}O_2^-\))?
The charge of calcium is ______________. Each stearate has a charge of ______________. To make the compound neutral (zero), we must have ______________ stearate ions.
Page 4
3c
Where do the ions go?
Explain why the calcium ion stays close to the oxygen atoms and not the carbon tail.
| Name | Structure |
|---|
| Stearate Ion |  |
| Soap |  |
The calcium ion is ______________ (positive/negative). It stays near oxygen because oxygen is more ______________ (positive/negative).
Oxygen has a ______________ electronegativity. This creates a ______________ charge on that end which attracts the calcium ion.
Page 5
4
Prediction Challenge
Which mineral will form scum?
Mineral A
\(KCO_3\)
Potassium is in Group 1
Mineral B
\(Mg(HCO_3)_2\)
Magnesium is in Group 2
Choose your answer:
Mineral A
Mineral B
Why? Use the Periodic Table in your answer:
Mineral ____________ is most likely to form scum because it has a charge of ____________, which is the same as ____________.
End of Assessment
Page 6
