You can change the molar concentration of pure water by altering its temperature. Specifically, lowering the temperature can increase its molar concentration.
The molar concentration of a substance, including pure water, is defined as the number of moles of the substance per unit volume (moles/L). For pure water, this value is determined by its density, as density relates mass to volume, and the molar mass of water relates mass to moles.
Understanding Water Concentration
While we often talk about the concentration of a solute in water, pure water itself has a molar concentration. This value represents how many moles of H₂O molecules are packed into one liter of pure water at a given temperature and pressure.
- Key Concept: Molar Concentration (Molarity) = Moles of Substance / Volume (Liters)
For pure water, the number of moles is derived from the mass of water (determined by density) and its molar mass (approximately 18.015 g/mol).
How Temperature Affects Water Concentration
The provided reference highlights a key method to change the molar concentration of pure water: changing its temperature.
According to the reference:
- At 25 °C, the concentration of water is approximately 55.2 mole/L (calculated as 1 mole/0.0181 L).
- By lowering the temperature from 25.0 °C to 4.0 °C, you can increase the molar concentration of water.
- At 4.0 °C, the concentration is approximately 55.6 mole/L (calculated as 1 mole/0.0180 L).
Why Lowering Temperature Increases Concentration
This phenomenon is directly related to the density of water. Water is unique in that its maximum density occurs at approximately 4°C, not at its freezing point (0°C).
- As temperature decreases from 25°C down to 4°C, the density of water increases.
- Increased density means a given mass (and thus a given number of moles) occupies a smaller volume.
- Since molar concentration is moles per volume, a smaller volume for the same number of moles results in a higher molar concentration.
Below 4°C, the density of water starts to decrease again (leading to ice floating), and thus the molar concentration would decrease slightly until freezing occurs.
Summary of Temperature Effects
Here's a quick comparison based on the provided reference:
| Temperature (°C) | Approximate Molar Concentration (mol/L) |
|---|---|
| 25.0 | 55.2 |
| 4.0 | 55.6 |
Therefore, the most effective way to slightly increase the molar concentration of pure liquid water is to cool it to its point of maximum density, around 4°C.
