The molecule which dissolves in water is called a hydrophilic molecule. Hydrophilic molecules are those that have an affinity or attraction towards water. This property allows them to readily dissolve in water.

Water is a polar molecule, meaning it has a partial positive charge at one end (hydrogen) and a partial negative charge at the other end (oxygen). This polarity gives water its ability to form hydrogen bonds with other polar molecules or ions. Hydrophilic molecules also have polar regions or functional groups, such as hydroxyl (-OH) or amino (-NH2) groups, which can form hydrogen bonds with water molecules.

Key Points:
- Hydrophilic molecules have an affinity or attraction towards water.
- Water is a polar molecule with a partial positive charge at one end and a partial negative charge at the other end.
- Hydrophilic molecules have polar regions or functional groups that can form hydrogen bonds with water molecules.

Why do hydrophilic molecules dissolve in water?
Hydrophilic molecules dissolve in water due to the formation of hydrogen bonds between the polar regions of the molecule and water molecules. When a hydrophilic molecule is added to water, the partially positive hydrogen atoms in water molecules are attracted to the partially negative regions of the hydrophilic molecule, while the partially negative oxygen atom in water is attracted to the partially positive regions of the hydrophilic molecule. This interaction disrupts the attractive forces between water molecules, allowing the hydrophilic molecule to become surrounded by water molecules and eventually dissolve.

Key Points:
- Hydrophilic molecules form hydrogen bonds with water molecules.
- Hydrogen bonds form between the partially positive hydrogen atoms in water and the partially negative regions of the hydrophilic molecule.
- The partially negative oxygen atom in water is attracted to the partially positive regions of the hydrophilic molecule.
- The formation of hydrogen bonds between the hydrophilic molecule and water disrupts the attractive forces between water molecules, allowing the hydrophilic molecule to dissolve.

Examples of hydrophilic molecules:
1. Sugars: Sugars such as glucose and sucrose are hydrophilic molecules due to the presence of multiple hydroxyl groups (-OH) in their structure, which can form hydrogen bonds with water.
2. Amino Acids: Amino acids, the building blocks of proteins, contain both amino (-NH2) and carboxyl (-COOH) groups, which are hydrophilic and can interact with water molecules.
3. Ionic Compounds: Ionic compounds, such as sodium chloride (NaCl), dissolve readily in water due to the attraction between the positive and negative ions and the polar water molecules.

Key Points:
- Sugars, amino acids, and ionic compounds are examples of hydrophilic molecules.
- Sugars have hydroxyl groups that can form hydrogen bonds with water.
- Amino acids contain both amino and carboxyl groups, which are hydrophilic.
- Ionic compounds dissolve in water due to the attraction between positive and negative ions and water molecules.

In conclusion, a hydrophilic molecule is one that dissolves in water due to its affinity for water and the formation of hydrogen bonds between its polar regions or functional groups and water molecules. Examples of hydrophilic molecules include sugars, amino acids, and ionic compounds.