How can you dissolve a sediment using chemical reagents?

Let's discuss some chemical exercise that is very often given to freshmen at the departments of chemistry. Aqueous solution of which substance shall you take to dissolve magnesium hydroxide sitting at the bottom of your test-tube?

Options:

1. Calcium chloride.
2. Sodium phosphate.
3. Hydrogen chloride.
4. Potassium hydroxide.

Choose a correct option and write down the corresponding chemical equation for certainty.

SEARCH FOR SOLUTION

Magnesium hydroxide Mg(OH)2 is a poorly soluble substance. In water, it exists as a precipitate at the bottom of the reaction vessel.

Poorly soluble substances interact with water molecules insufficiently, being thereby unable to get involved into electrolytic dissociation. Thus, a fraction of cations and anions in the test-tube containing Mg(OH)2 is pretty modest.

You have been given four electrolytes, one of which must react with Mg(OH)2 and produce exclusively water soluble products. All electrolytes are soluble in solvents, e.g. in water. This is just a definition of an electrolyte. Aqueous solutions of electrolytes contain a large number of hydrated ions. These substances are able to interact chemically with Mg(OH)2, that is known as a weak electrolyte, i.e. it produces a way less ions than true electrolytes. Reactions between electrolytes are referred to as ion-exchange reactions. You should not expect redox reactions, substitution reactions, decomposition, and combination reactions to take place in such problems.

Which conditions are required for ion-exchange reactions to happen? What should occur for equilibrium to shift towards products? We consistently teach our students that equilibrium is always shifted towards the least dissociated substance, irrespective of whether it is a reagent or a product. As long as ionic substances are considered, solubility and strength are very well correlated.

Poorly dissociated substances are gases, molecular compounds (such as water), insoluble ionic substances and crystals. Pay attention that a common property of poorly dissociated substances is their weak physical interaction with water leading, thus, to insufficient hydration. Consequently, a reaction that would dissolve Mg(OH)2 must give rise to one or more poorly dissociated substances. Those substances would leave a solution and, therefore, hydrolysis of the product would be unable to reverse the reaction.

Precipitate dissolution implies a strong shift of chemical equilibrium towards reaction products. To rephrase, the right part of the reaction should contain even weaker electrolyte(s) than magnesium hydroxide.

A student shall clearly understand what and how is summarized in the Solubility Table for ionic compounds in water. It may be useful to invest certain time to remember solubilities of most common bases, acids and salts for saving time during an essential exam. As we teach our students, it is possible to remember qualitative solubilities of inorganic ionic compounds by analyzing polarities and sizes of the ions, as well as their polarizabilities.

Read the Problem formulation again. It has been mentioned that precipitate Mg(OH)2 must dissolve. Let’s repeat this in a different manner. Initially, we had a precipitate in a test-tube, then something happened, and the precipitate vanished. If the reaction produced another precipitate, its formation would have been difficult to notice visually. We would have been, most likely, unable to speak about any reaction that took place. The Problem description saying that ‘the precipitate of Mg(OH)2 vanishes’ is a convincing hint that no new ionic precipitate is formed. Instead, a molecular product (either water or gas) should be looked for.

Let’s discuss each proposed options and choose a correct answer. Find all eight hypothetical products of ion‑exchange reactions in the Solubility Table: (1) magnesium chloride, (2) calcium hydroxide, (3) magnesium phosphate, (4) sodium hydroxide, (5) magnesium chloride, (6) water, (7) magnesium hydroxide (again), (8) potassium hydroxide. It is straightforward to notice that the only molecular compound out of all possible products is water. Water does not dissociate significantly (see the pH paragraph of the text-book) while producing a very limited quantity of hydrogen cations (often referred to as protons) and hydroxide anions. Formation of water is enough to shift a reaction equilibrium rightwards, i.e. to completely dissolve Mg(OH)2.

Let’s write down the reaction hereby taking place:

Mg(OH)2 + 2HCl = MgCl2 + H2O.

Magnesium chloride is a more soluble compound, as compared to magnesium hydroxide, therefore, all the initial precipitate in the test-tube dissolves.

Finally, we note that the fourth reactant, potassium hydroxide, does not foster any chemical reaction at all. The composition of products is identical to the composition of reactants. Magnesium phosphate, as most other phosphates, is insoluble in water. Therefore, it represents another poorly dissociated substance. To predict regularities and yield of this reaction, one must have an access to full information regarding aqueous solubilities of Mg(OH)2 and Mg3(PO4)2. Even if such reaction exhibits a substantial yield, its progress does not lead to dissolution of a precipitate. Instead, the older precipitate, Mg(OH)2, gets substituted by the newer precipitate, Mg3(PO4)2. Herewith, it is not straightforward to determine chemical identity of the new precipitate without involving an advanced analytical chemistry analysis.

Do you know that criteria of ion-exchange reaction progress / high yield – formation of a gas, a molecular product, or a precipitate – can also be formulated in thermodynamic terms? According to chemical thermodynamics, reactants are less stable substances and products are more stable substances at given conditions. Chemical reaction is a process of transforming less thermodynamically stable particles into more thermodynamically stable particles. A weak electrolyte does not dissociate into ions (to a substantial extent), because it exhibits inherent stability. Formal cation and anion prefer to stay together in space, rather than getting hydrated. We hereby use the term formal, because H2O or CO2 or NH3 cannot directly issue ions. They are non-electrolytes (or, in some cases, very weak electrolytes), thanks to their particular thermodynamic stabilities. It is, therefore, correct to rephrase that ion-exchange reactions (including hydrolysis reactions) are driven by thermodynamic factors and can be rationalized in those terms. Weak electrolytes represent a nickname of more stable compounds, while strong electrolytes are to be interpreted as less stable compounds.

Correct choice is 3.