Understanding the Basics of a Solution
To understand why water decreases concentration, we must first grasp the core components of a solution. A solution is a homogeneous mixture composed of a solute and a solvent. The solute is the substance being dissolved, and the solvent is the substance in which the solute dissolves. In an aqueous solution, water is the solvent.
When a solute is added to a solvent, the solute particles disperse evenly throughout the solvent, forming a uniform mixture. The concentration is a measure of the amount of solute present in a given amount of solvent or total solution. A higher ratio of solute to solvent means a more concentrated solution, while a lower ratio means a more dilute solution.
The Fundamental Principle of Dilution
The process of dilution involves adding more solvent (like water) to a solution without adding more solute. The key is that the total amount of solute, measured in moles, stays constant throughout the process. However, because the total volume of the solution increases, the solute particles are spread out over a larger space. This leads to a lower concentration of solute per unit of volume.
Think of it like a crowd of people in a room. If you start with 10 people in a small room, the density is high. If you then move those same 10 people into a much larger hall, the density of people decreases because they are spread out more. In this analogy, the people are the solute, and the room is the solvent. Adding more solvent (making the room larger) decreases the concentration (crowd density).
The Dilution Formula Chemists quantify this process using a simple formula: $M_1V_1 = M_2V_2$.
- $M_1$ and $V_1$ represent the molarity (concentration) and volume of the initial, more concentrated solution.
- $M_2$ and $V_2$ represent the molarity and volume of the final, diluted solution. This equation works because the product of molarity and volume ($M imes V$) equals the number of moles of solute, which remains constant.
Practical Examples of Dilution
Dilution is not just a theoretical concept; it's part of many everyday processes. For example, when you add water to a cleaning concentrate, you are performing a dilution to make it safe and effective for its intended use. In medicine, drugs are often stored in concentrated 'stock solutions' and must be diluted to the proper dosage before being administered to a patient. Even preparing a beverage from a frozen concentrate, like orange juice, is a common example of dilution.
The Inverse Relationship Between Volume and Concentration
The dilution formula, $M_1V_1 = M_2V_2$, perfectly illustrates the inverse relationship between volume and concentration. As the volume ($V$) increases, the concentration ($M$) must decrease proportionally to keep the product constant. This is because the moles of solute (product of $M$ and $V$) do not change. If you double the volume by adding more water, you will halve the concentration.
Comparison of Concentrated vs. Diluted Solutions
| Feature | Concentrated Solution | Diluted Solution |
|---|---|---|
| Ratio of Solute to Solvent | High | Low |
| Volume | Relatively low | Relatively high |
| Molarity (Concentration) | High | Low |
| Appearance (e.g., Colored Drink) | Dark or intense | Light or faint |
| Physical Effect | Strong (e.g., highly acidic) | Weaker (e.g., less acidic) |
| Taste | Intense or strong | Milder or weaker |
Conclusion
The simple act of adding water to a solution is a powerful display of a fundamental chemical principle. Why water decrease concentration is explained by the law of conservation of mass. By increasing the total volume of the solvent, the fixed amount of solute particles is spread out, inevitably lowering the solute-to-volume ratio. This phenomenon, known as dilution, is a cornerstone of chemistry and finds practical application in a myriad of fields, from preparing laboratory reagents to making a glass of juice from concentrate.
An excellent video tutorial explaining the process of dilution can be found at Khan Academy: https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:intermolecular-forces-and-properties/x2eef969c74e0d802:solutions-and-mixtures/v/dilution.
