Question

Urea dissolves very readily in water, but the solution becomes very cold as the urea dissolves. How is this possible? It appears that the solution is absorbing energy.

Short answer

The solution gets cold because the dissolution of urea in water is an endothermic process that absorbs energy from the surroundings.

Step-by-step solution

- Understanding Dissolution and Energy

When a substance dissolves in water, it can either absorb or release energy. This process depends on the nature of the interactions between the solute (urea) and the solvent (water).

- Endothermic Process

The solution becoming very cold indicates an endothermic process. In an endothermic process, the system absorbs energy from its surroundings in the form of heat.

- Energy Absorption

As urea dissolves in water, it absorbs energy from its surroundings, causing the temperature of the solution to drop. This absorbed energy is used to break the bonds in the urea and to separate water molecules to make space for the urea molecules.

- Final Analysis

Although it seems like the solution is getting colder, it is actually absorbing energy from the environment. This energy absorption results in the observed temperature drop.

Key concepts

energy absorption

When urea dissolves in water, the process involves significant energy changes. Specifically, this event is termed as an endothermic process. What this means is that urea needs to absorb energy from the surroundings to dissolve properly. This energy is required to break the internal bonds of the urea molecules. Additionally, it also separates the water molecules to make space for the urea.

This absorbed energy primarily comes from the heat present in the surrounding environment, including the solution itself. As a result, the temperature of the solution drops. Hence, the energy absorption during the dissolution of urea is the key reason behind the solution becoming cold.

urea solubility

Urea, a simple organic compound, is highly soluble in water. The solubility of urea in water is facilitated by the polarity of urea molecules.

Water, being a polar solvent, interacts favorably with the polar urea molecules. This interaction causes urea to dissolve easily in water. When urea is introduced to water, it disrupts the hydrogen bonding between the water molecules, allowing the urea to occupy spaces in the water network.

It’s also essential to understand that the solubility of urea heavily depends on temperature. As temperature increases, so does the solubility of urea. Normally, this is due to the increased kinetic energy that helps to overcome the lattice energy of the urea crystal, promoting dissolution.

temperature change

During the dissolution of urea in water, a noticeable temperature change occurs. This effect is directly tied to the endothermic nature of urea dissolution.

The temperature change occurs because the solution absorbs energy to dissolve the urea. This consumption of energy from the surroundings lowers the overall temperature of the solution, making it feel cold to the touch.

It's important to note that not all dissolutions cause a temperature drop. Some dissolutions can actually release heat and increase the solution’s temperature, known as exothermic processes. In the case of urea, though, the dissolution process absorbs heat, confirming its endothermic characteristic.

solution interactions

The interactions between urea and water are essential to understand the dissolution process. As urea dissolves, it breaks its crystalline structure, requiring energy, and this energy is taken from the surrounding water.

The water molecules, in turn, reorient themselves to surround the urea molecules, forming what’s called a hydration shell. This process involves breaking and forming of hydrogen bonds. Even though making this hydration shell releases some energy, the total energy absorbed during the dissolution still exceeds the energy released, marking the overall process as endothermic.

These interactions ensure that the solute particles are integrated into the solvent, creating a homogeneous mixture, which is the essence of a solution.