Final answer:
Ethanol is miscible with water due to the ability of its hydroxyl group to form hydrogen bonds with water, while ethanethiol has lower water solubility due to its sulfhydryl group's weaker hydrogen bonding.
Step-by-step explanation:
The reason ethanol is completely miscible with water is because both water and ethanol molecules are capable of forming hydrogen bonds with each other. This is due to the presence of a hydroxyl group (-OH) in ethanol, which engages in hydrogen bonding with water molecules.
On the other hand, ethanethiol (C2H5SH) contains a sulfhydryl group (-SH) instead of a hydroxyl group, leading to weaker hydrogen bonding with water and lower solubility. The completely miscible nature of ethanol with water can be attributed to its balanced hydrophilic (water-attracting) and hydrophobic (water-repelling) properties.
The hydroxyl group is hydrophilic and enables the formation of hydrogen bonds, which accounts for ethanol's high solubility in water. However, the carbon chain in both molecules contributes to the hydrophobic character; since ethanol has a shorter carbon chain, it is less hydrophobic and more soluble in water compared to ethanethiol, which has the same carbon chain length but a weaker hydrogen-bond-forming sulfhydryl group.