Yes, a true solution can indeed pass through filter paper.
Understanding Solutions and Filtration
When we talk about a "solution" in the context of chemistry and filtration, we typically refer to a true solution. A true solution is a homogeneous mixture where one substance (the solute) is completely dissolved in another (the solvent), forming a single phase. This means the solute particles are broken down to their smallest possible units – individual molecules or ions – which are incredibly small.
Why True Solutions Pass Through Filter Paper
The key reason true solutions cannot be separated by filter paper lies in the relative sizes of the solute/solvent particles and the pores of the filter paper.
As the provided reference states, "if the solution is a true solution, where the solute is completely dissolved in the solvent, filter paper cannot separate them because both the solute and solvent can pass through the filter paper."
Standard filter paper works by having microscopic pores that allow smaller liquid molecules (the solvent) to pass through while trapping larger solid particles. However, in a true solution:
- Particle Size: The dissolved solute particles (molecules or ions) are typically on the order of 0.1 to 1 nanometer (nm) in size.
- Pore Size: The pores in most common laboratory filter papers are significantly larger, typically ranging from 1 to 100 micrometers (µm), which is thousands of times larger than the dissolved solute particles.
Because both the solute and solvent particles are much smaller than the filter paper's pores, they both flow through the paper unimpeded. This is why you cannot filter salt out of saltwater using filter paper; the salt ions are fully dissolved and pass right through along with the water molecules.
Distinguishing Solutions from Other Mixtures
It's important to differentiate true solutions from other types of mixtures that can be separated by filtration:
- Suspensions: These are heterogeneous mixtures where solid particles are large enough to be seen with the naked eye, eventually settle out, and are easily trapped by filter paper (e.g., muddy water, sand in water).
- Colloids: These are heterogeneous mixtures with intermediate-sized particles (between 1 nm and 1 µm). They do not settle and often appear cloudy. While they generally cannot be separated by standard filter paper, specialized ultrafilters with extremely small pores might be able to retain some colloidal particles.
The table below summarizes the filterability of different mixture types:
| Mixture Type | Particle Size | Settles Over Time? | Filterable by Standard Paper? | Example |
|---|---|---|---|---|
| True Solution | Very Small (<1 nm; molecular/ionic) | No | No (passes through) | Saltwater, Sugar Water |
| Colloid | Intermediate (1 nm - 1 µm) | No | No (usually) | Milk, Fog |
| Suspension | Large (>1 µm; visible particles) | Yes | Yes | Muddy Water, Chalk in Water |
Practical Implications and Alternative Separation Methods
Since filter paper is ineffective for separating the components of a true solution, other methods are required:
- Evaporation: Heating the solution to boil off the solvent, leaving the solid solute behind (e.g., obtaining salt from saltwater).
- Distillation: A process involving evaporation followed by condensation, used to separate a solvent from a solute, or to separate two liquids with different boiling points. This allows for the recovery of both the solvent and the solute.
- Crystallization: Allowing the solvent to slowly evaporate, leading to the formation of purified solid crystals of the solute.
In conclusion, filtration using standard filter paper is a technique for separating insoluble solids from liquids. It is not suitable for separating the components of a true solution because both the solute and solvent particles are too small to be retained by the filter's pores.
