Why can a filter remove sand but not dissolved salt?
You will be able to: Choose filtration or distillation from the physical form and volatility of components.
Why can a filter remove sand but not dissolved salt?
A mixture of sand, salt and water looks like a useful separation challenge. A filter catches sand grains, but dissolved sodium and chloride ions pass through with water.
A useful starting point: Which end of water faces a dissolved ion? →
Words and symbols before equations
- Filtration
- Separating suspended particles from a fluid using a porous barrier.
- Distillation
- Vaporizing and condensing a component to separate by volatility.
- Volatility
- Tendency to enter the vapor phase.
- Residue
- Material left behind after a separation step.
What this picture assumes
Schematic material-flow diagram, not apparatus construction instructions. Filtration leaves dissolved salt in the filtrate. Distillation collects condensed water; dissolved NaCl is treated as nonvolatile.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Feed: suspended sand + dissolved NaCl + water. Filter → sand retained on filter. Filtrate: water AND dissolved Na⁺ and Cl⁻. Clear filtrate can still contain dissolved solute.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
A normal filter separates sufficiently large undissolved particles. It does not sort individual dissolved ions or molecules from the solvent in an ordinary solution.
Distillation uses differences in vapor composition and volatility. Water can vaporize from a salt solution and condense elsewhere while nonvolatile salt remains. Covalent bonds in water do not need to break.
For two volatile liquids, vapor is often enriched in the more volatile component. A single distillation does not guarantee perfect separation, especially for close boiling points or nonideal mixtures.
Choose the method from the separation goal. Evaporating away solvent may recover a solid but does not collect the solvent unless its vapor is condensed.
A worked example, step by step
Plan a sequence to recover sand and water from sand mixed with aqueous NaCl.
- Filter first: sand is an undissolved solid, so it is retained.
- Recognize that the filtrate still contains dissolved Na⁺ and Cl⁻.
- Distill the filtrate: vaporize water and condense it into a separate receiver.
- Water is collected as distillate; nonvolatile salt remains. The explanation uses size/form first, then volatility.
Clear filtrate is not necessarily pure water. Dissolved particles may pass through a filter.
Would filtering an ordinary glucose solution remove the dissolved glucose?
Compare with an explanation
No. Its dissolved molecules pass through normal filter pores with the water.
Predict. Change one thing. Explain.
Switch between filtering sand plus saltwater and distilling saltwater. Trace which labeled species reaches the collected liquid and explain why.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Feed: suspended sand + dissolved NaCl + water. Filter → sand retained on filter. Filtrate: water AND dissolved Na⁺ and Cl⁻. Clear filtrate can still contain dissolved solute.
Schematic material-flow diagram, not apparatus construction instructions. Filtration leaves dissolved salt in the filtrate. Distillation collects condensed water; dissolved NaCl is treated as nonvolatile.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using particle interactions, concentration, gas behavior or energy transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA student filters muddy saltwater and claims the clear liquid contains no solute. Explain the error and propose a way to recover water separately from dissolved salt.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Filtration can remove suspended mud particles.
- 1 point: Dissolved ions pass through and can remain in clear filtrate.
- 1 point: Distillation vaporizes water and condenses it into a receiver.
- 1 point: The separation exploits nonvolatile salt versus volatile water; clarity alone does not establish purity.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What does an ordinary filter target?
Sufficiently large undissolved particles.
RECALL 2What property supports distillation?
Differences in volatility.
RECALL 3Does a phase change necessarily break covalent bonds?
No; water can vaporize and condense while remaining H₂O.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why can a filter remove sand but not dissolved salt?
- Filtration: suspended particle size/form.
- Distillation: different tendencies to enter the vapor.
Remember: Clear filtrate is not necessarily pure water. Dissolved particles may pass through a filter.
Conditions: Schematic material-flow diagram, not apparatus construction instructions. Filtration leaves dissolved salt in the filtrate. Distillation collects condensed water; dissolved NaCl is treated as nonvolatile.
Refresh Kid · AP Chemistry Unit 3 · Objectives 3.9.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 3.9, objective 3.9.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 3: Properties of Substances and Mixtures, Topics 3.1–3.13. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Colligative-property calculations and solution molality/mass-percent/volume-percent calculations are not required here. The optional speed-density model illustrates distributions; it does not require memorizing its mathematical derivation.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
Want to work through this with a tutor?
Bring your question about Why can a filter remove sand but not dissolved salt? Your explanation and answers remain free to access.