Question

Each of the following mixtures is placed in a dialyzing bag and immersed in distilled water. Which substances will be found outside the bag in the distilled water? a. $\mathrm{NaCl}$ solution b. starch solution (colloid) and alanine, an amino acid, solution c. $\mathrm{NaCl}$ solution and starch solution (colloid) d. urea solution

Short answer

a. ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions; b. Alanine; c. ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions; d. Urea.

Step-by-step solution

– Understanding Dialysis

Dialysis is a process where solutes pass through a semi-permeable membrane from a region of higher concentration to a region of lower concentration. Generally, smaller molecules and ions move through the membrane, while larger molecules (like proteins and starch) do not.

– Analyze Each Mixture

Examine each mixture to determine which substances are small enough to pass through the dialysis membrane into the distilled water.

– $\text{NaCl}$ Solution

Sodium chloride, $\text{NaCl}$, dissociates into sodium ions ${\text{Na}}^{+}$ and chloride ions ${\text{Cl}}^{-}$, both of which are small enough to pass through the dialysis membrane.

– Starch Solution and Alanine Solution

Starch is a large molecule (colloid) and will not pass through the membrane. Alanine, an amino acid, is relatively small and will pass through the membrane.

– $\text{NaCl}$ and Starch Solution

In this mixture, ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions from $\text{NaCl}$ will pass through the membrane, but starch will not.

– Urea Solution

Urea is a small molecule and will pass through the dialysis membrane into the distilled water.

Key concepts

Semi-Permeable Membrane

In dialysis, a semi-permeable membrane plays a crucial role. It allows the movement of certain molecules while blocking others. Think of it like a sieve or filter that separates substances based on their size.

A semi-permeable membrane only lets smaller molecules and ions pass through. This is due to the tiny pores within the membrane. Larger molecules, like proteins and starch, are blocked because they are too big to fit through these pores.

This characteristic is essential in determining which substances can be found outside the dialyzing bag after the process. Only those small enough to pass through the pores will be located in the surrounding distilled water.

Solute Movement

Solute movement during dialysis involves molecules transitioning from areas of higher concentration to areas of lower concentration. This process is driven by diffusion, a natural phenomenon where particles spread out evenly in a solution.

In the context of dialysis, solutes move from the mixture inside the dialyzing bag (where they are in higher concentration) to the surrounding distilled water (where they are in lower concentration). This movement continues until equilibrium is reached.

For example:

• In NaCl solution, sodium and chloride ions move through the membrane because they are small.
• In a mixture of starch solution and alanine, only alanine, being small, will diffuse through the membrane, not starch.

The key to understanding solute movement in dialysis is recognizing that only solutes small enough to pass through the membrane can migrate to areas of lower concentration.

Molecule Size

Molecule size is a fundamental aspect when it comes to dialysis. It determines whether a substance can pass through a semi-permeable membrane.

Here’s how molecule size affects movement in dialysis:

• Small molecules, like sodium ions, chloride ions, alanine, and urea, easily pass through the pores of the membrane. They are small enough to fit through, allowing them to move from inside the dialyzing bag to the distilled water outside.
• Large molecules, such as starch and proteins, cannot pass through. Their size makes it impossible for them to fit through the membrane's tiny pores, so they remain inside the bag.

Understanding the size of molecules in the substances you are working with helps predict which will move through the membrane. For instance, when you have a mixture of NaCl and starch, you know that NaCl will dissociate into ions small enough to move through the membrane, while starch will stay inside the bag.