Question

A student wants to determine whether an unknown solid is an ionic compound. He puts some in water and finds that it does not dissolve. He says that this proves the solid is ionic. Which best evaluates the student's reasoning?

Answer 1

Answer:

The student's reasoning is incorrect.  

Many times ionic compounds can be recognized by their properties. Ionic compounds are solids that normally melt at high temperatures and boil at even higher temperatures.

In solid form, an ionic compound is not electrically conductive because its ions cannot flow ("electricity" is the flow of charged particles). However, when it is melted, it can conduct electricity because its ions are able to move freely through the liquid.

Explanation:

Ionic compounds are compounds that have cations and anions.

A polar molecule is one that is neutral, that is, it does not have a net charge. But it has an internal distribution of charges that form a partially positive region and a partially negative region.

Given that "the like dissolves the like" and that water is a polar molecule, it is possible to say that water is good for dissolving ions and polar molecules, but bad for dissolving non-polar molecules.

As mentioned, water molecules are polar, and partial positive charges on hydrogens, a negative partial charge on oxygen and a general angular structure. Its distribution of charges is due to the greater electronegativity of oxygen with respect to hydrogen, which makes the shared electrons of the O-H bonds more attracted by the O atom than by the H atom.

There are water insoluble ionic compounds. For example, silver chloride, AgCl is a very insoluble ionic salt. When mixing a solution of silver nitrate, AgNO3, which contains the Ag + ion, with another solution of sodium chloride containing the Cl– ion, precipitation of AgCl occurs.

But mixing any cation with any anion will not always lead to the formation of an insoluble ionic compound. For example, salt, which is an ionic compound of the NaCl formula, should be considered.

As the salt mixes in the water, the charged water molecules break the charged salt molecules, which are joined by a weaker bond. Then the salt dissociates in water in Na + and Cl-. The positively charged Na + ions are surrounded by the negative partial oxygen charges of the water molecules, while the negatively charged ions are surrounded by the hydrogen ends with a positive partial charge.  Then it can be said that the intermolecular forces that are acting in this case are the ion-dipole forces. This is the force that exists between an ion and a neutral polar molecule that has a permanent dipole moment, where there is electrostatic attraction between a positive ion and the negative end of a solvent molecule and between a negative ion and the positive part of the solvent molecule. In this way the solubility of the ionic compound is produced.

So the student's reasoning is incorrect.  

Many times ionic compounds can be recognized by their properties. Ionic compounds are solids that normally melt at high temperatures and boil at even higher temperatures.

In solid form, an ionic compound is not electrically conductive because its ions cannot flow ("electricity" is the flow of charged particles). However, when it is melted, it can conduct electricity because its ions are able to move freely through the liquid.

Answer 2

The student's reasoning is incorrect, because there are many ionic substances that are readily dissolved in water. An example of such a substance is table salt, sodium chloride. Another way of determining the type of bonding is by measuring the melting and boiling point of the solid. Ionic compounds tend to have high melting and boiling points.