Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have sweet smell.

Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. As we have already learnt, molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass. The attractions get stronger as the molecules get bigger in size and
have more electrons.

Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms.

Solubility
The haloalkanes are very slightly soluble in water. In order to dissolve haloalkane in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. However, haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.

Which of the following has the highest density?

CCl₄

The correct answer is option 4. \(CCl_4\).

To understand why carbon tetrachloride \((CCl_4)\) has the highest density among the given compounds, let us delve deeper into the concepts of molecular mass and density:

Density is defined as mass per unit volume (d = m/v). It depends on both the molecular mass of the substance and how closely the molecules are packed together.

Molecular Mass and Structure

Higher molecular mass generally contributes to higher density, assuming the molecular volume does not change drastically. The compounds listed are all chlorinated methanes, and their densities are influenced by the number of chlorine atoms, which are relatively heavy.

Given Compounds:

1. \(CH_3Cl\) (methyl chloride):

Molecular mass: 50.5 g/mol

Density: ~0.91 g/cm³

 It has the smallest molecular mass among the listed compounds. It has only one chlorine atom, making it less dense compared to others.

2. \(CH_2Cl_2\) (methylene chloride):

Molecular mass: 84.5 g/mol

Density: ~1.33 g/cm³

It has a higher molecular mass than \(CH_3Cl\) due to the presence of two chlorine atoms. Consequently, it is denser than \(CH_3Cl\).

3. \(CHCl_3\) (chloroform):

Molecular mass: 119.5 g/mol

Density: ~1.48 g/cm³

It has an even higher molecular mass because of three chlorine atoms. Therefore, it is denser than both \(CH_3Cl\) and \(CH_2Cl_2\).

4. \(CCl_4\) (carbon tetrachloride):

Molecular mass: 153.5 g/mol

Density: ~1.59 g/cm³

It has the highest molecular mass, with four chlorine atoms. Chlorine atoms are much heavier than hydrogen or carbon, contributing significantly to the overall mass. This makes \(CCl_4\) the densest among the compounds listed.

Conclusion:

\(CCl_4\) (carbon tetrachloride) has the highest density because it has the greatest number of chlorine atoms, leading to the highest molecular mass. The presence of more chlorine atoms increases the overall mass significantly more than it increases the volume, resulting in the highest density among the given compounds.

This relationship between molecular mass and density explains why \(CCl_4\), with its four heavy chlorine atoms, is the densest compound in the list.