Question

Which of the following has the maximum water potential? (a) \(1 \mathrm{M}\) of \(\mathrm{NaCl}\) (b) \(0.5 \mathrm{M}\) of glucose (c) Pure water (d) \(0.001 \mathrm{M}\) of \(\mathrm{HCl}\)

Short answer

Pure water has the maximum water potential.

Step-by-step solution

Understand the Water Potential Principle

Water potential is a measure of the potential energy in water. Pure water at standard conditions has the maximum water potential, designated as zero. If any solute is added to the water, it lowers the water potential giving it a more negative value.

Evaluate the Water Potential for Different Solutions

In the given problem, the water potential of the provided options can be evaluated based on the solute concentration in each. Pure water has zero water potential. Any other solution containing a solute, whether its NaCl, glucose, or HCl, automatically has a water potential which is less than zero (i.e more negative value compared to the water potential of pure water). Thus, the more concentrated the solution, the greater the solute potential and the lower the water potential.

Identify the Solution with Maximum Water Potential

From the analysis, the solution with maximum water potential will be the one with the least amount of solute or, in short, pure water.

Key concepts

Water Potential Principle

Imagine a scenario where plants absorb water from the soil. The driving force behind this natural phenomenon is a critical concept known as water potential. In scientific terms, water potential is the measure of the potential energy of water in a system compared to pure water. It determines the direction in which water will move. The principle behind water potential is quite straightforward: water always moves from an area of higher water potential to an area of lower water potential.

In a practical sense, when a solute is dissolved in water, it binds some of the water molecules, making them unavailable to move freely. This reduces the system’s water potential. Therefore, the more solute particles present, the more negative the water potential becomes, which influences water's movement across cell membranes in living organisms or between different environments in nature.

Solute Concentration

The concept of solute concentration is at the core of understanding water potential. A solute is any substance that is dissolved in a solvent, and the solute concentration refers to the quantity of solutes in a given volume of solution. For instance, in biology, a sugar solution's concentration might be measured in moles per liter (M).

The relationship between solute concentration and water potential is inversely proportional. This means as the solute concentration increases, the water potential decreases, becoming more negative. Think of solute concentration as the contributing factor that 'pulls' water towards it, commonly referred to as osmotic potential. This principle is crucial for processes such as plant nutrient absorption and cellular osmoregulation.

Pure Water

In the discourse of water potential, pure water stands as the baseline reference point. By definition, pure water is a solvent with no solutes dissolved in it. An important characteristic of pure water is its water potential value, which is designated as zero. This is because there are no solute particles to affect the availability of water molecules.

Pure water's pivotal role is evident when comparing it with solutions that contain dissolved substances. It symbolizes the state of highest water potential, indicating the maximum potential energy and hence the capacity to do work or move spontaneously towards solutions with lower water potential. In a practical understanding, the water potential of pure water sets the stage for grasping how and why water moves through various environments or systems.

Water Potential Measurement

Quantifying water potential is essential in various scientific and agricultural applications to predict and manipulate water movement. Water potential is measured in units of pressure, typically in megapascals (MPa). One of the widely used methods for measuring water potential is through the use of a device called a 'psychrometer'.

Another method involves a 'pressure chamber' or 'pressure bomb', which finds its utility in plant physiology. These instruments provide a direct measurement of the water potential as they subject the sample (like a leaf) to known pressures until water movement is halted, indicating equilibrium. Understanding how to measure water potential accurately is crucial for experiments and practical applications where water movement and retention are involved, such as in soil science and irrigation management.