Diffusion: Definition, Examples, Properties, Osmosis Vs Diffusion

Diffusion is defined in scientific terms as the movement of individual molecules of any substance via a semipermeable barrier from an area of higher concentration to an area of lower concentration until the concentration becomes equal throughout. In other words, diffusion helps in the movement of substances in and out of the cells. The diffusion of small molecules and macromolecules (e.g., proteins) in aqueous solutions plays an important role in microorganisms, plants, and animals. Most kinetic phenomena in solids involve diffusion, or a unit step very similar to that operating in diffusion. It is an important process occurring in all living beings. In other words, diffusion is an important process that plays an important role in several fields of science.

This article deals with the topic of Diffusion in detail and elaborates on the types of diffusion, factors affecting diffusion, its causes and applications. Read on to get a complete idea about this important concept which is important for various branches of science.

Definition of Diffusion

The phenomenon of spreading a substance within another substance by the motion of the particles present in the substance is called diffusion. The phenomenon of diffusion of one substance into another substance goes on until a uniform mixture of the substance is formed.

Examples of Diffusion in Daily Life

• A tea bag immersed in a cup of hot water will diffuse into the water and change its colour.
• A spray of perfume or room freshener will get diffused into the air by which we can sense the odour.
• Sugar gets dissolved evenly and sweetens the water without having to stir it.
• As we light the incense stick, its smoke gets diffused into the air and spreads throughout the room.
• By adding boiling water into the dried noodles, the water diffuses causing rehydration and making dried noodles plumper and saturated.

Types of Diffusion

Diffusion is widely used in various fields such as biology, physics, chemistry, etc. Diffusion can be classified into two main types: Simple diffusion and facilitated diffusion.

Simple diffusion

A process in which the substance moves through a semipermeable membrane or in a solution without any help from transport proteins.  For example, bacteria deliver small nutrients, water and oxygen into the cytoplasm through simple diffusion.

Facilitated diffusion

Facilitated diffusion is a passive movement of molecules across the cell membrane from the region of higher concentration to the region of lower concentration by means of a carrier molecule.

Dialysis: It is the diffusion of solutes across a selectively permeable membrane. A selectively permeable membrane is one that allows only specific ions and molecules to pass through, while it obstructs the movement of others.

Osmosis: It is the movement of solvent molecules from the region of lower concentration to the region of higher concentration through a semipermeable membrane. Since water is solvent in every living being, biologists define osmosis as the diffusion of water across a selectively permeable membrane. For example, plants take water and minerals from roots with the help of osmosis.

Factors Affecting Diffusion

There are a couple of factors that affect the diffusion process, which individually and collectively changes the rate and extent of diffusion. These factors include:

• Temperature.
• Area of Interaction.
• Size of the Particle.
• The steepness of the concentration gradient.

Density: The rate of diffusion is inversely proportional to the density of a gas or liquid. The higher the density, the lesser will be the rate of diffusion.

Temperature: The rate of diffusion is directly proportional to the temperature. As the temperature increases, the kinetic energy of the particles increases, and they move with more incredible‌ speed resulting in an increased diffusion rate.

Diffusion in Chemistry

Diffusion in Solids: When particles of one substance occupy the vacant spaces present in the particles of the other substance, this is called diffusion. Many people think that diffusion does not take place in solids. But, the process of diffusion takes place very slowly in solids because the particles in solids do not move from their fixed positions. Instead, however, the constituent particles in solids vibrate about their fixed positions.

When two metal blocks were bound together tightly and kept undisturbed for a few years, then the particles of one metal have been found to diffuse into the other metal.

If we try to rub something written on the blackboard with chalk, after a gap of about a week or so, it becomes rather difficult to rub the same. Probably some particles of chalk in the form of dust have diffused in the pores of the blackboard which is normally made from some wooden material.

Diffusion in Liquids: The process of diffusion in liquids takes place slower than that in gases. It is because the constituent particles in liquids move slowly compared to the particles in gases.  When diffusion takes place between a liquid and a gas, it is known as intimate mixing. The kinetic theory can be used to explain this process. It states that collisions are taking place randomly between particles in a liquid or a gas. There is sufficient space between the particles of one substance for the other substances to move into. Let’s discuss this with some examples.

Example 1: The spreading of the purple colour of potassium permanganate into the water on its own.

In the above-said example, the particles of potassium permanganate and the water mix on their own without any external stirring. Since the particles of potassium permanganate and particles of water spread into each other and mix up on their own, they are moving or in motion. If the particles were not moving, the colour could not spread throughout the beaker on its own. This movement of different particles (on their own), so that they become mixed uniformly, is called Diffusion.

Example 2: When sugar is added to the water and stirred, it dissolves quickly. The sugar seems to disappear. What happens to the sugar? How does it disappear?  Where does the sugar go? We can explain how the sugar dissolves and disappears in water by using the idea of particles.

Both sugar and water are made up of tiny particles. These particles are too small to see even under a microscope. When sugar dissolves, tiny sugar particles break off from each solid sugar granule. These sugar particles go into the spaces between the particles of water and mix with them. So, the sugar solution will taste sweet even though we cannot see the sugar in it.

Please note that in this experiment, we have dissolved sugar in water by ‘stirring’. It has been done just to dissolve the sugar quickly. Even if we do not stir, sugar will dissolve in water, but quite slowly.

Diffusion in Gases: Since the interparticle spaces are vast and forces are quite weak, the particles of one gas can readily move into the empty spaces of another gas. That is why the diffusion is maximum among the gases.

For example, the smell of food, particularly of fish being cooked in the kitchen, spreads in the lobby, and even in different rooms because of diffusion. The food particles with specific smells diffuse into the air and as a result, their smell can be felt even from a distance.

In the air, all the constituting gases remain uniformly mixed. Carbon dioxide, which is \({{\rm{N}}_{\rm{2}}}\) and \({{\rm{O}}_{\rm{2}}}\) does not form the lower layer in the atmosphere.

If you observe, you can notice that the lighter gases move downwards and heavier gases move upwards, which means gravity’s diffusion is not influenced by gravity. For example, invert a cylinder containing hydrogen gas (colourless) over a cylinder containing nitrogen dioxide (reddish-brown) and separate the two cylinders by a lid.

Now remove the lid. What will you observe? Both the cylinders will acquire the same colour, i.e., light brown. It means that hydrogen gas has moved downwards and nitrogen dioxide upwards. Therefore, both the cylinders contain a uniform mixture of these two gases and are light brown.

Examples of Diffusion

Some common examples of diffusion that happen in our daily life are given below:

1. The spreading of ink or dye in water.
2. Dipping tea bags in hot water will cause diffusion of the tea in hot water.
3. When mixing salt with water, the salt dissolves and the ions move until they get evenly distributed.
4. Water diffuses into noodles while cooking them, this makes them bigger and softer.
5. A helium balloon deflates a little each day as helium diffuses through the balloon surface into the air.
6. We are able to leak of cooking gas in our homes detected due to the diffusion of a strong-smelling substance (Ethyl mercaptan) present in the cooking gas into the air.
7. The spreading of the blue colour of copper sulphate in water.

Difference Between Osmosis and Diffusion

Significance of Diffusion

Diffusion is a significant process, involving different life processes. As mentioned above, it is the net motion of particles, ions, solution, molecules, etc. In every living species, diffusion plays an extremely relevant role in the movement of the molecules during the metabolic process in the cells.

Diffusion is necessary for the following reasons:

• During the respiration process, difussion helps in diffusing the carbon dioxide gas escape the cell membrane into the blood.
• Diffusion also happens in plant cells. Water present in the soil diffuses into plants through their root hair cells.
• The movement of ions through the neurons that generates electrical charge is due to diffusion.

Importance of Diffusion in Plants

1. Diffusion helps in uniform diffusion of substances in the cytoplasm.
2. Flowers secrete aromatic chemicals. These chemicals diffuse into the air and attract insects for pollination.
3. The absorption of water by the roots occurs by diffusion.
4. The process of transpiration in plants occurs by diffusion.
5. The passive absorption of mineral salts occurs by diffusion.
6. The exchange of oxygen (O₂) and carbon-di-oxide (CO₂) gases from the atmosphere through the stomata of leaves takes place only by diffusion.

Diffusion Pressure

Diffusion pressure is a hypothetical term. It is related to the concentration of the diffusing substances. The pressure produced by the tendency of molecules or ions of a substance to diffuse is called diffusion pressure. This pressure is directly proportional to the concentration of the diffusing molecule. But the difference in diffusion pressure at two points is called the diffusion pressure gradient.

Diffusion always occurs from a region of high diffusion pressure (concentration) to an area of ​​low diffusion pressure, that is, the diffusion pressure is along a gradient. Thus the rate of diffusion in a region depends on the diffusion pressure difference between two points in that region.

Summary

Now we understand what diffusion is, how the molecules travel, how perfume smell moves around every corner of the room, how the naphthalene dissolves into petrol and the differences between Osmosis and Diffusion.

FAQs

The most commonly asked about diffusion are answered here:

1: What are the 3 types of diffusion?
Ans: The three types of diffusion are simple diffusion, osmosis, and facilitated diffusion.

2: What is diffusion? Explain with an example.
Ans: The phenomenon of the spreading of a substance within another substance by the motion of the particles present in the substance is called diffusion. The smell of hot food reaches us even when we are at a considerable distance. But to get the smell of cold food, we need to go close to it. It is because the rate of diffusion of hot gases released by the hot food is much faster than the rate of diffusion of cold gases released by the cold food.

3: What are examples of diffusion in daily life?
Ans: The examples of diffusion are:

1. The spreading of the blue colour of copper sulphate into the water on its own.
2. When shaking salt into water, the salt dissolves and the ions move until they evenly distributed.
3. The leakage of cooking gas in our homes detected due to the diffusion of a strong-smelling substance (Ethyl mercaptan) present in the cooking gas into the air.
4. Dipping the tea bags in hot water will diffuse the tea in hot water.

4: What are diffusion and osmosis?
Ans: Osmosis is the movement of solvent particles across a semipermeable membrane from a less concentrated solution to a more concentrated solution.
Diffusion is the movement of particles from an area of higher concentration to a lower concentration. The overall effect is to equalize concentration throughout the medium.

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