These are properties of covalent compounds, also known as molecular compounds. Covalent compounds are a diverse group of molecules, so there are several exceptions to each 'rule'. When looking at a compound and trying to determine whether it is an ionic compound or a covalent compound, it's best to examine several properties of the sample.
Most covalent compounds have relatively low melting points and boiling points.
While the ions in an ionic compound are strongly attracted to each other, covalent bonds create molecules that can separate from each other when a lower amount of energy is added to them. Therefore, molecular compounds usually have low melting and boiling points.
Covalent compounds usually have lower enthalpies of fusion and vaporization than ionic compounds.
The enthalpy of fusion is the amount of energy needed, at constant pressure, to melt one mole of a solid substance. The enthalpy of vaporization is the amount of energy, at constant pressure, required to vaporize one mole of a liquid. On average, it takes only 1% to 10% as much heat to change the phase of a molecular compound as it does for an ionic compound.
Covalent compounds tend to be soft and relatively flexible.
This is largely because covalent bonds are relatively flexible and easy to break. The covalent bonds in molecular compounds cause these compounds to take form as gases, liquids and soft solids. As with many properties, there are exceptions, primarily when molecular compounds assume crystalline forms.
Covalent compounds tend to be more flammable than ionic compounds.
Many flammable substances contain hydrogen and carbon atoms which can undergo combustion, a reaction that releases energy when the compound reacts with oxygen to produce carbon dioxide and water. Carbon and hydrogen have comparable electronegativies so they are found together in many molecular compounds.
When dissolved in water, covalent compounds don't conduct electricity.
Ions are needed to conduct electricity in an aqueous solution. Molecular compounds dissolve into molecules rather than dissociate into ions, so they typically do not conduct electricity very well when dissolved in water.
Many covalent compounds don't dissolve well in water.
There are many exceptions to this rule, just as there are many salts (ionic compounds) that don't dissolve well in water. However, many covalent compounds are polar molecules that do dissolve well in a polar solvent, such as water. Examples of molecular compounds that dissolve well in water are sugar and ethanol. Examples of molecular compounds that don't dissolve well in water are oil and polymerized plastic.
Learn MoreDifference Between an Ionic and Covalent Bond
Examples of Covalent Compounds
How To Predict Formulas of Compounds Containing Polyatomic Ions