'A' Level Chemistry Problem Analysis: Determining if Ink Smudges Are Ionic or Covalent In Nature

QN: You have in your possession a 50 dollars Singapore note (bill). There are several ink or dirt smudges on it (of more than 1 colour or appearance). Describe and explain, using easily available (ie. non-controlled) chemical substances and/or tools, to determine if the smudges are covalent or ionic in nature.

Thought process:

Using a tissue paper moistened with water, wipe the smudges. If some of the smudges come off, the nature of the composite pigment could either be soluble ionic, or polar covalent (possibly with the capacity to form hydrogen bonds with water). If the smudges do not come off, the pigment could either be insoluble ionic, or non-polar covalent.

To determine which, wipe the remaining smudges with an alcohol (eg. isopropyl alcohol a.k.a. propan-2-ol) swab that can be purchased from any pharmacy (eg. NTUC, Guardian, Watsons, etc). If the smudges come off, the pigment must have been non-polar covalent. If the smudges remain, the pigment must be insoluble ionic.

An ionic compound is soluble in water, if the Gibbs Free Energy for the solution process (ie. lattice dissociation + hydration enthalpies) is feasible. Otherwise, it is insoluble ionic.

A covalent compound that is soluble in water, is either strongly polar, and/or has the capacity to form hydrogen bonds with water. Water would rather hydrogen bond within itself (ie. water molecules with other water molecules), than interact favourable with non-polar covalent compounds.

(Indeed, this is the basis for the so-called hydrophobic interactions of non-polar 'R' groups of amino acids in proteins. They get pushed away by water molecules (which prefer to bond within themselves, macham elitist), until they (the non-polar groups) end up in close proximity, afterwich induced dipole-dipole van der Waals interactions take over to hold them together (hence, "hydrophobic interactions").)

Alcohol is relatively non-polar (to be precise, the longer the non-polar or "fatty" hydrocarbon chain, the more non-polar the alcohol is; this explains why fats are "fatty" - they have long non-polar hydrocarbon chains). Hence, relatively non-polar and/or hydrogen bonding incapable covalent compounds, that are insoluble to water, may be soluble in alcohols (as well as oil/fats), due to favourable induced dipole-dipole interactions between solute and solvent. Which explains why the alcohol swab is able to wipe away non-polar and/or hydrogen bonding incapable covalent pigments that water cannot.

Tip : for you students who write messily and end up with ink-stained hands after taking an exam paper, wipe your hands with alcohol swabs (which can be bought cheaply from any pharmacy, eg. NTUC, Guardian, Watsons, etc). Often, the ink pigments are relatively non-polar and/or hydrogen bonding incapable covalent compounds, that can be wiped away easily with alcohol swabs.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


'A' Level Chemistry Problem Analysis: Organic Reactions (Electrophiles)

'A' Level Chemistry Problem Analysis: Reaction of 2,3-Dimethyl-1,3-butadiene with Bromine

'A' Level Chemistry Problem Analysis: Chemical Energetics