A Secondary 1 student wipes a blue sentence off a school whiteboard and then discovers that a line drawn accidentally with a permanent marker is much harder to remove. I would not begin by telling the child one marker is ‘stronger’. Why does one coloured line release from the board while another stays attached? The answer is about ink chemistry, the material of the writing surface and how a dried film behaves.
For families looking for a Secondary 1 Chemistry Tutor Hougang, this small classroom incident gives us an excellent route into mixtures, evaporation, particles, surface adhesion, polymers, experimental comparisons and accurate scientific language. Dry-erase inks are deliberately formulated so that the pigment-bearing residue, often assisted by release agents, can be removed relatively easily from a suitable smooth nonporous board. Many permanent-marker formulations leave a more strongly adhering deposit. Neither outcome is simply a contest between ‘weak blue’ and ‘strong black’.
In Singapore, Secondary 1 students ordinarily learn these foundations in integrated lower-secondary Science, not a separate national Pure Chemistry examination. G1, G2 and G3 lesson content differs by school and level. The advanced materials chemistry is enrichment when not taught in class. This article follows a question-first, evidence-based approach: start with what happened, explain the mechanism, correct a misconception, practise a changed question and check independent learning. No child needs to use chemical cleaning solvents to investigate marker ink.
1. Describe the event before interpreting it
The dry-erase line can become faint after rubbing; the permanent line remains under ordinary wiping. Those are observations. The conclusion that the ink formulations differ needs further information about pigments, resins, release agents and surface properties.
I ask children to write ‘What did I see?’ and ‘Why might it happen?’ separately. This distinction becomes especially useful when a photograph suggests a chemical reaction but does not directly show one.
2. A whiteboard is a particular material
A dry-erase board generally has a smooth, relatively nonporous finish, although enamel, painted steel and polymer coatings differ. The surface determines how readily an ink film can attach and be released.
A glossy plastic, an ordinary sheet of white paper and a classroom whiteboard are not interchangeable just because they share a colour. The chemical and physical properties of the substrate matter.
3. The marker is a mixture of materials
Marker ink is not a pure element or compound. Its formulation can include solvent, colourant, binder or resin, and additives chosen for storage, flow, drying and adhesion.
A child should identify those roles rather than memorise one imaginary universal ingredient list. Different products and manufacturers use different formulations, so a general explanation should remain conditional.
4. A volatile carrier makes drawing possible
Liquid carrier moves pigment or dye through the marker tip onto the surface. Once deposited, some solvent evaporates into the surrounding air, leaving colour-bearing material behind.
Evaporation is not proof that all the ink has disappeared. The visible line remains because other parts of the formulation are still present. This is a valuable distinction from the familiar change of state of water.
5. Why a dry line can still be erased
Dry erase means the mark can generally be removed by a suitable dry eraser from the intended nonporous board. It does not mean that the marker was dry when applied.
Pigment particles or film fragments are transferred away from the board during wiping. The colour does not have to undergo chemical decomposition or turn into invisible gas.
6. What a release agent accomplishes
Many dry-erase inks include release agents that prevent pigments from adhering too tenaciously to suitable board surfaces. The manufacturer EXPO describes pigments and release agents as central to this behaviour.
Do not insist on a specific silicone compound unless the actual product formulation provides it. The relevant scientific role is deliberate low adhesion, not a universally fixed chemical name.
7. Why permanent marker behaves differently
A permanent marker can include binders and solvents selected to create a film that adheres well to many surfaces after drying. The deposited colourant may resist ordinary dry wiping.
The word permanent is a product-description shorthand for ordinary conditions, not an absolute law that no method can ever remove any mark. Surface, temperature, formulation and handling matter.
8. Not all permanent markers are identical
Different permanent inks use different dyes or pigments, resins and solvents. Two markers can make visually similar lines while leaving materials with different adhesion and durability.
A student cannot infer exact chemical composition from line colour. Scientific identification needs reliable ingredient information or appropriate analysis, not an attractive photograph.
9. The importance of paper porosity
Paper is a network of cellulose fibres with pores and tiny gaps. Ink liquid and colourant can enter that network. Once pigment sits within or between fibres, an ordinary whiteboard eraser may not reach it.
This explains why dry-erase ink can stain clothes or paper despite being designed for easy removal from nonporous boards. The material’s response is not one intrinsic property of the ink alone.
10. A surface’s roughness can matter
Fine scratches or roughened spots on a board can trap pigment-bearing residue. A worn board may retain faint marks even when the marker and eraser are otherwise appropriate.
This means poorer erasure may reflect the condition of the surface, not simply a defective pen. A good Science answer considers more than one variable before making a causal claim.
11. Adhesion versus cohesion
Adhesion concerns the interaction of the mark with the board. Cohesion concerns how its residual film holds together. Strong adhesion can make removal difficult, while film cohesion can affect whether a layer breaks or peels.
The child should not replace these ideas with a vague description of the ink as sticky. Ask which boundary is resisting separation and where the material goes during erasure.
12. Polymer binders and film strength
Polymers or resins can help the dried ink retain colourant and adhere to a surface. A permanent formulation may create a stronger or more compatible film, whereas a dry-erase formulation is deliberately suited for removal.
This is a materials-design decision. The strongest bond is not always the desired outcome. The entire function of a dry-erase marker is to make a temporary visible mark.
13. A dry-erase mark has a lifetime
Ink can dry over time, and the resulting film’s removal behaviour can change with conditions, contamination and product compatibility. The intended use and recommended cleaning interval vary.
A learner should avoid the claim that every dry mark will wipe off perfectly after any number of years. Real materials age and surfaces can accumulate residues.
14. What ghosting means
Ghosting is a faint remnant visible after erasing. It may arise from residues, contamination or changes to a board’s finish. The exact cause should be investigated from the circumstances.
A shadow does not automatically mean that the ink became a new chemical compound inside the board. A simple physical residue can produce a visible effect.
15. Rubbing does not destroy pigments
An eraser can mechanically detach ink-bearing particles and hold them on its pad or distribute them into debris. The mark on the board becomes faint, but matter is not destroyed.
Ask the pupil to label where the colour was before and after. This is a small but useful application of conservation of mass in an open system.
16. Friction is only part of the mechanism
Rubbing supplies mechanical action, but its success depends on relative adhesion of the material, eraser condition and surface properties. Friction alone is not a guarantee of clean removal.
A student who says ‘rub harder forever’ may overlook surface damage and smearing. Appropriate material compatibility matters more than maximum applied force.
17. Compare graphite and marker ink
Pencil marks involve graphite particles transferred to paper and subsequently lifted by an eraser. Dry-erase marks involve a formulated colour-bearing residue deposited on a smooth board.
Both illustrate physical removal, but their compositions and surfaces differ. The existing pencil-erasing guide supplies the comparison.
18. Dye and pigment are related but different
Many dyes are dissolved molecules while many pigments are solid dispersed colour particles. Ink formulations can use either or both depending on design. Neither word is synonymous with solvent.
Students should not describe every blue marker as one universal pigment compound. Similar colour is an optical observation, not proof of matching molecular identity.
19. Solvent versus solute
A liquid carrier can dissolve or disperse other ink components, allowing them to travel through the tip. After application, solvent evaporation alters the mixture’s composition.
This supports the foundational language of mixtures and solutions. The colourant typically remains behind while volatile components leave, so drying is different from erasing.
20. Permanent does not mean chemically fused forever
A strong mark is not necessarily joined to every atom in the board through a single chemical bond. Surface interactions, penetration into pores, film strength and polymer compatibility can all contribute.
Students should avoid absolutes: permanent under ordinary dry wiping is a practical description, not a statement about every possible environment or cleaning method.
21. Why some stains become trapped in clothes
Textile fibres form a porous, irregular network that can hold coloured particles or molecules. That is why a dry-erase mark on a uniform may not respond to the same erasing action as one on a board.
The manufacturer EXPO FAQ specifically notes that its pigments can become trapped by porous clothing and surfaces. This is a source-grounded reminder that ‘erasable’ has conditions.
22. Why a white-painted wall is not a whiteboard
A white wall may have a porous or textured paint finish. Marker components may penetrate or attach differently from those on a purpose-made dry-erase board.
Students should not experiment on school walls. The correct Science lesson can be taught by comparing labelled material diagrams rather than making marks on property.
23. Dry-erase and wet-erase products
Wet-erase formulations are designed to be removed using a damp cloth rather than normal dry wiping. Their intended surfaces and ink properties can differ from dry-erase markers.
A common naming similarity does not imply a universal mechanism. The class should read the described marker type and not interchange cleaning requirements.
24. What happens when a marker is uncapped
Volatile ink ingredients may leave through an uncapped tip, altering the remaining ink’s flow or concentration. A marker may stop writing well as its carrier evaporates.
This is another physical process separate from the later erasure of a dried mark. Students should not try to revive markers by adding unknown chemicals.
25. Does a strong smell reveal the composition?
Some volatile ink carriers have recognisable odours, but smell alone cannot identify every ingredient or establish safe exposure levels. A marker designed for school use still has handling instructions.
The useful safety message is to avoid deliberately sniffing markers and maintain normal classroom ventilation. Chemical curiosity does not require inhaling fumes.
26. Why the blue line does not prove a reaction
The appearance changes after wiping because colour-bearing material was detached. That does not by itself establish that new chemical substances were formed.
For the general criterion of a chemical change, see Physical vs Chemical Changes. A change in what we see can result from moving existing material.
27. Design a fair marker comparison
A worksheet can compare the same length and darkness of lines from different markers on identical clean boards, with the same drying interval and number of eraser strokes.
The independent variable might be ink type. The dependent measure could be residual darkness. Board finish, starting mark, eraser condition, wiping pressure and timing are controls.
28. A fictional data table
Suppose dry-erase marker A leaves 15 units of darkness and permanent marker B leaves 72 units after the same defined wiping procedure, from identical starting readings of 100. The observed residual is much lower for A.
Those values are fictional practice data. They do not prove that 85% of the pigment molecules were destroyed; optical darkness need not be directly proportional to pigment mass.
29. Calculate a percentage correctly
If an index falls from 100 to 25 units after wiping, the measured decrease is 75 units, or 75% relative to the starting index. This calculation is about the index used in the task.
Do not substitute grams or molecules for units that were never measured. Correct arithmetic must stay tied to the measurement’s meaning.
30. Worked explain question
Question: Why can suitable dry-erase ink be removed easily from a smooth whiteboard? Answer: The ink is formulated to leave colour-bearing material that adheres only weakly to the intended nonporous surface, allowing a dry eraser to lift it.
This is better than ‘it disappears because it dries’. Drying leaves the mark; wiping removes it. Name both the formulation and the substrate.
31. Worked compare question
Question: Compare a dry-erase marker and a permanent marker on the same suitable board. Answer: Both deposit coloured formulations, but the first is designed for easy dry removal whereas the second can leave a more strongly adhering mark.
The comparison is conditional on the described materials. It does not say the permanent line is chemically indestructible.
32. Worked predict question
Question: What might happen when a dry-erase marker is used on absorbent paper? Answer: The ink may enter the fibre structure and become difficult to remove using an ordinary whiteboard eraser.
The explanation should identify porosity, rather than assume that a marker’s name guarantees it will erase from all white surfaces.
33. Worked evidence question
Question: The coloured line vanishes after wiping. Does that prove the pigment became a colourless gas? Answer: No. The pigment-bearing residue can be moved onto the eraser.
This question repairs confusion between evaporation of the carrier and mechanical removal of the dried colour.
34. Worked uncertainty question
Question: Two blue pens produce identical-looking lines. Do they necessarily use the same dye? Answer: No. Different colourants and formulations can produce similar visible colours.
The result does not permit a unique identification of chemical composition without more information.
35. Worked board-wear question
Question: Why might an old classroom board show faint ghosts after wiping? Answer: Ink residue, surface contamination or changes to the finish may make release less complete.
The student should present possible causes if the task supplies no more details, not claim that a specific molecule caused all marks.
36. The G1 learning pathway
For G1, use a drawing of a coloured film on a smooth surface and ask where the material goes when an eraser passes over it. One short accurate sentence about physical removal is a useful answer.
Follow the MOE G1 syllabus and school topic order. Detailed ink chemistry may be enrichment.
37. The G2 learning pathway
For G2, distinguish surface porosity from ink formulation and explain why the same marker can behave differently on two substrates. Ask students to identify variables in a hypothetical erasure test.
Use the MOE G2/G3 syllabus as the curriculum reference and match the child’s present understanding.
38. The G3 learning pathway
A secure G3 learner can explore solvent volatility, polymer binders, pigment dispersion and release agents. They can distinguish adhesion and cohesion in a model of ink-film removal.
A strong answer recognises that ink performance depends on both formulation and the board surface, and that an optical darkness graph does not measure removed mass directly.
39. A 90-minute small-group lesson
First 15 minutes: Diagnose each learner’s first explanation and retrieve mixtures and evaporation. Next 25: Compare marker film, board coating and paper fibres with diagrams. Next 20: Work through explain and predict questions.
Next 20: Interpret a supplied residual-darkness table, identify controlled variables and correct a misleading ‘ink evaporates away’ statement. Final 10: Ask each student to answer a changed example independently and choose a targeted retrieval task.
40. Teach ahead without confusing the student
A curious learner can hear about resins and surface release, but an early G1 or G2 response may need only ‘different ink formulations and smooth versus porous surfaces’. Vocabulary has value when the pupil can use it.
More advanced chemistry is not a substitute for reading the school question accurately. Correct first principles now make later polymer and materials lessons easier.
41. A seven-day retrieval plan
Day 1: Explain the basic marker contrast. Day 3: Distinguish solvent evaporation from pigment removal without notes. Day 5: Predict what happens on porous paper. Day 7: Read a simple experimental table and state one unsupported conclusion.
The 20-minute Science revision plan provides a simple framework. A new correct explanation is better evidence than repeating the same paragraph ten times.
42. Safety: never mix cleaners
Do not combine bleach, alcohol, aerosol cleaners or unknown chemicals to try to remove marks. Such mixtures can be dangerous and can damage property. Let the teacher or facilities staff use board-manufacturer-approved methods.
The purpose of the article is to explain why inks behave differently, not to supply a chemical cleanup recipe for children.
43. What parents should ask a tutor
Ask which concept the child missed: solvents, material transfer, surface compatibility, question words or evidence. The tutor should show a corrected reasoning step and then test an unfamiliar question.
For Hougang and Kovan families, confirm lesson venue, class size, G1/G2/G3 level, timetable and availability. A local educational article is not proof of a particular physical class or guaranteed marks.
44. Common family questions
Does dry erase mean no liquid ink? No. The carrier can evaporate after application. Is permanent ink indestructible? No, but normal dry wiping may not remove it. Is the release agent the same in every marker? Not necessarily.
Does marker colour prove its chemical composition? No. Why does dry erase stain clothes? Pigment can enter the porous fibres. Is wiping a chemical reaction? Not usually; it commonly moves material.
45. More misconceptions and corrections
‘The marker is erasable because the ink evaporates.’ Incorrect: usually the volatile carrier evaporates and colourant remains to be removed. ‘A whiteboard is just white paper.’ Incorrect: its surface is designed differently.
‘The darkest line must contain more of one specific chemical.’ Not established without formulation and measurement information. Scientific reasoning improves when children replace unsupported absolutes with explanations tied to a defined system.
46. Original sources and continuing study
The EXPO dry-erase FAQ gives direct manufacturer evidence about pigment release agents and porous-surface staining. Related articles include Graphite and Pencil Erasers, Why Glue Sticks to Paper and Physical vs Chemical Change.
For the wider series see the Hougang Chemistry Tuition and Tutor Library. The next standalone concept, Why Paper Turns Yellow with Age, deals with changes within paper rather than ink removal.
Applied Practice Bank: 28 Questions with Worked Answers
A two-mark reason for easy removal
Question: ‘Explain why a suitable whiteboard marker can be wiped away after it dries.’ Answer: Its ink contains colourant and release components designed to remain weakly attached to the smooth nonporous board, allowing rubbing to lift the residue.
Tutor check: The student must not say that all ink evaporated or disappeared. A strong answer distinguishes the liquid carrier leaving first from pigment-bearing material being transferred during wiping. Ask for those two stages in the correct order.
A two-mark reason for persistent ink
Question: ‘Why might permanent-marker ink remain on the same board after ordinary dry wiping?’ Answer: Its formulation can leave a more strongly adhering colour-bearing film or residue, so the standard eraser cannot easily detach it.
Tutor check: Saying permanent means ‘impossible to remove under any conditions’ overstates the product label. The explanation should refer to the specified surface, formulation and dry-erasure method rather than an absolute physical law.
Predict a porous-surface result
Question: ‘A dry-erase marker leaves a mark on uncoated paper. What difficulty may arise when erasing?’ Answer: Colour-bearing liquid or particles can enter gaps among the fibres and become difficult to remove with a conventional whiteboard eraser.
Tutor check: The student should mention porosity, not the everyday colour of the material. Ask for one other porous example, such as fabric, and have the child predict why it may hold a stain.
What happens to the marker’s solvent?
Question: ‘The mark dries on a board. Where did the volatile solvent go?’ Answer: Much of it evaporated into the surrounding air, while pigments and less volatile components remained on the board.
Tutor check: If the child claims the whole ink is gone, ask why the line remains visible. The response should connect a liquid carrier to a gas phase without inventing chemical decomposition.
What happens to the pigment?
Question: ‘The line becomes faint after wiping. Must the pigment have turned into gas?’ Answer: No. Pigment-bearing material can be detached and moved to the eraser or nearby residue.
Tutor check: This is a conservation-of-matter question. Ask for a before-and-after drawing showing the pigment on the board initially and elsewhere after erasure. The mechanism should account for material transfer.
Compare a whiteboard with a uniform
Question: ‘Why can an erasable board marker stain a school uniform?’ Answer: The dye or pigment-bearing material may penetrate and lodge among porous textile fibres even when it releases readily from a smooth board.
Tutor check: The child should explain the surface difference in one sentence. A product being suitable for one surface does not establish that it can be safely removed from every other material.
Compare a board with a painted wall
Question: ‘Can the pupil assume that dry-erase marker will wipe from a white-painted wall as easily as from a classroom whiteboard?’ Answer: No. The wall’s coating may be more porous, rough or chemically different.
Tutor check: Ask which evidence would be required for a responsible conclusion. Actual surface specification, product instructions and known compatibility matter more than simply observing that both surfaces look white.
Explain ghosting
Question: ‘A faint shadow remains on an older whiteboard. Give one possible cause.’ Answer: Residual ink, surface contamination or coating wear may reduce how completely the mark can be lifted.
Tutor check: The word ‘possible’ is important. Without additional evidence, the student should not declare that every ghost mark proves a particular chemical reaction or single component has failed.
Explain why wiping too hard is not ideal
Question: ‘Why is greater rubbing force not always a better removal method?’ Answer: Removal depends on adhesion and surface properties, and aggressive rubbing can spread residue or damage a surface.
Tutor check: Ask the child to distinguish the goal—selectively removing the mark—from the uncontrolled action of increasing force. Good materials engineering balances effectiveness and preservation of the substrate.
Distinguish release agent from pigment
Question: ‘What are the different roles of pigment and release agent in a typical dry-erase formulation?’ Answer: Pigment supplies the visible colour; the release agent helps prevent it from adhering too strongly to the intended board.
Tutor check: The words should not be used interchangeably. The same colour can occur in products with very different removal behaviour because colour production and attachment are different functions.
Distinguish binder from solvent
Question: ‘Which component is more likely to evaporate rapidly after writing: a volatile solvent or a polymeric film binder?’ Answer: The volatile solvent, under the intended conditions.
Tutor check: Avoid the rule that all liquid components evaporate at exactly the same rate. The question requires recognising that parts of a formulation play different physical roles during and after application.
Determine whether a chemical change occurred
Question: ‘A dry-erase mark disappears after an eraser moves over it. Is a new compound necessarily formed?’ Answer: No. The observation can be explained through physical removal of existing coloured material.
Tutor check: Students should use a criterion about chemical identity rather than treat every dramatic visual change as proof of a reaction. Ask for a genuine chemical-change counterexample from the school syllabus.
Judge two blue marker lines
Question: ‘Two blue lines look identical. Must the ink formulations have the same chemicals?’ Answer: No. Similar visible colours can be produced by different colourants, solvent mixtures and binders.
Tutor check: Separate what the eye tells us from what chemical analysis would be needed to establish. This is the same principle used for judging unknown white powders or colour-changing indicators.
Read a fictional data table
Question: ‘Two marks start at a darkness index of 100. After the same dry wiping, A measures 10 and B measures 70. Which has less residual darkness?’ Answer: A, by 60 index units.
Tutor check: The numbers are hypothetical optical readings, not a measurement of chemical concentration. Do not allow the student to claim a specific number of pigment molecules was destroyed.
Calculate a decrease
Question: ‘The darkness index changes from 90 to 18. What is the decrease?’ Answer: 72 units, or 80% of the starting index if a percentage comparison is appropriate.
Tutor check: Ask how the arithmetic was obtained and what the units represent. A percentage drop in measured darkness is not necessarily the same as a percentage reduction in total pigment mass.
Identify a fair-test weakness
Question: ‘Two marker types are wiped on two boards with different finishes. Can the result isolate the effect of marker formulation?’ Answer: Not reliably, because board finish differed too.
Tutor check: Propose a better comparison using the same surface condition, initial mark size, drying interval and eraser action. The student must name the deliberately changed variable.
Choose an independent variable
Question: ‘The aim is to compare erasure performance among marker formulations. What is the independent variable?’ Answer: The marker formulation or type.
Tutor check: Time can still appear on a graph, but that does not automatically make it the main independent treatment factor. The research question determines which variable is deliberately changed.
Choose a dependent variable
Question: ‘How can the outcome of a wiping comparison be measured?’ Answer: One possibility is the residual darkness after a defined number of strokes under the same conditions.
Tutor check: The response should identify a measurable outcome rather than the vague label ‘cleaner’. Discuss the need for fixed lighting and initial marks when interpreting an optical measurement.
Make a prediction about an old board
Question: ‘A board has many fine scratches. Might dry-erase marks become harder to remove?’ Answer: Yes, they may, because scratches or surface damage can retain residue; the extent depends on the board and ink.
Tutor check: Avoid claiming that every scratch traps every pigment particle. A conditional prediction tied to the surface mechanism is stronger than an absolute statement.
Explain why no cleaning experiment is needed
Question: ‘Would mixing several household cleaners prove the marker’s chemistry more accurately?’ Answer: No. It introduces hazards and confounding reactions, and the school mechanism can be learned from known product information and controlled diagrams.
Tutor check: Safety is part of scientific method. A hazardous intervention is not justified when a lower-risk source or model already answers the learning question.
Compare board erasing with graphite erasing
Question: ‘What do pencil erasers and dry-erase board erasers have in common?’ Answer: Both can remove existing coloured material from a substrate through physical transfer, although the marks and surfaces differ.
Tutor check: Ask for one difference. Pencil marks are associated with graphite on paper; dry-erase marks are formulated to release from particular smooth board finishes. The shared outcome does not imply one identical chemical recipe.
Explain why marking is not a measure of safety
Question: ‘Does a strong smell identify a marker as harmful or harmless?’ Answer: Not reliably. Odour reflects volatile components but does not establish full composition or exposure risk.
Tutor check: The practical response is to use classroom-approved products appropriately, avoid deliberate sniffing and follow the label. Sensory impressions are not a chemical safety test.
The G1 parent checkpoint
Ask the G1 student to draw three stages: liquid marker touches board, solvent leaves, then dried colour is lifted. A correct short explanation in the pupil’s own words is the main target.
If they confuse the disappearance of liquid carrier with removal of pigment, return to the diagram instead of introducing extra polymer chemistry. Progress means explaining the sequence without the tutor pointing at the answer.
The G2 parent checkpoint
Ask the G2 pupil why the same marker may behave differently on smooth whiteboard and porous textile. Require a prediction and one reason tied to surface structure.
If they simply repeat that one surface is ‘good’ and the other ‘bad’, ask for the scientific property. Porosity, interaction and material transfer are more informative than everyday judgments.
The G3 parent checkpoint
Ask the G3 learner to distinguish volatile carrier, binder, pigment and release agent and then describe why two films can differ in adhesion while producing similar colours.
Give a fictitious experimental table and ask for its limitations. This checks whether the pupil can use the chemical model and evaluate evidence, not just recite the formula of a random solvent.
Quick FAQ on removal and permanence
Does dry erase mean the mark is always removable? No, it depends on the intended surface and conditions. Does permanent mean chemically indestructible? No. What evaporates from many marker marks? The volatile carrier.
Where does the pigment go after wiping? It may move to the eraser or debris. Why can clothing stain? Porous fibres can trap colourant. Why should children avoid experimenting with chemical cleaners? Safety, surface damage and unreliable conclusions.
A final changed-case question
Imagine an unknown coloured mark on a glossy tile. Can the student tell from colour alone whether it will dry-erase? No. They need information about both the ink formulation and the tile surface.
This is the lesson’s core transfer. The correct answer is not a memorised classification of blue or black lines. It is a habit of asking which materials interact, what changes on drying, and how the coloured residue can leave the surface.