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Brownian Motion and Diffusion

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  • Last Updated : 24 Nov, 2021
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Everything in our environment is made up of microscopic pieces or particles. Atoms or molecules are the particles that make up matter. Particles make up our bodies, chairs, tables, books, and other objects. The number of particles in everything, on the other hand, is enormous. Even with a high-powered microscope, the particles that makeup matter cannot be seen because they’re so small. We have certain evidence that all things are made up of small particles even though they can’t be seen. Let’s take a look at some of the evidence that proves everything is made up of small particles.

Evidence for Particles in Matter

Experiments on diffusion, or the mixing of dissimilar substances on their own, and Brownian motion provide evidence for the presence of particles in matter. If we assume that matter is made up of microscopic particles in motion, it is simple to explain what happens in various tests. However, things become extremely difficult to describe without the concept of particles. We’ll now go over some experiments, such as ‘solid dissolving in liquid, ‘mixing of two gases,’ and ‘movement of pollen grains in water’ that can only be explained by thinking that everything is made up of microscopic particles that are continuously moving. These are discussed further below.

  • Dissolving a Solid in a Liquid

Since potassium permanganate is a purple-coloured solid and water is a colourless liquid, we’ll start by looking at how it dissolves in water. Even without stirring, a crystal of potassium permanganate in a beaker of water causes the water to become purple on its own. The purple colour of potassium permanganate spreading across the beaker’s water can be explained as follows.

Small particles make up both potassium permanganate crystals and water. Potassium permanganate particles are purple in colour, whereas water particles are colourless. When a crystal of potassium permanganate is submerged in water, the particles separate from one another. These purple-coloured potassium permanganate particles dispersed throughout the water, turning it purple. Actually, as potassium permanganate dissolves, the particles of potassium permanganate go into the gaps between the water particles.

Without any external stirring, the particles of potassium permanganate and those of water combine on their own in this experiment. The particles of potassium permanganate and water spread into each other and mix up on their own, therefore they are moving or they are in motion is the conclusion. The colour could not spread across the beaker on its own if the particles were not moving. Diffusion is the movement of various particles among themselves on their own, resulting in uniform mixing.

  • Mixing of Two Gases

Air is a colourless gas or a gas mixture. A gas jar is actually filled with air when it is empty. This is because we can’t see air in the gas jar because it’s colourless. Bromine is a liquid that is reddish-brown in colour. It produces a lot of vapour. Bromine vapour, often known as bromine gas, is a red-brown gas that is heavier than air. Let’s put the experiment to the test now.

An air-filled gas jar is placed upside down over a bromine-vapour-filled gas jar. The red-brown bromine vapours from the lower gas jar will spread up into the air in the higher gas jar. The gas jar containing air turns entirely red-brown over a period of time. Bromine vapour diffusing into the air can be described as follows. Air and vapour are both made up of microscopic particles that move around. As the moving particles of bromine vapour and air collide and bounce about in all directions, they are uniformly mixed. Despite the fact that bromine vapour is heavier than air, it overcomes gravity and mixes with air in the upper jar because its particles are travelling at fast speeds, giving them enough kinetic energy to overcome gravity and enter the gas jar containing air.

Diffusion- Diffusion is the spreading out and mixing of a substance with another substance as a result of the movement of its particles. It is a crucial process that occurs in all living things. Diffusion is a process that aids in the movement of molecules into and out of cells. The molecules travel from a high-concentration region to a low-concentration region until the concentration is uniform throughout. Diffusion occurs in liquids and gases because molecules can flow at random.

Diffusion is a process in which matter moves from a location of high concentration to a region of low concentration due to the random mobility of molecules. We can draw two conclusions about the nature of matter from the diffusion process. The following are the conclusions:

  • The substance is made up of thousands of tiny particles.
  • Matter particles are in constant motion.
  • Movement of Pollen Grains in Water

Robert Brown presented the best evidence supporting the existence and mobility of particles in liquids in 1827. Robert Brown used water to suspend exceedingly small pollen grains. The pollen grains were moving swiftly across the water in an extremely irregular or zig-zag pattern, as observed under the microscope. It was also discovered that the faster pollen grains move on the water’s surface, the warmer the water is.

The movement of pollen grains on the water’s surface can be explained in the following way. Water is composed of very small particles that move very quickly. Since water particles or water molecules are so minute, they are not visible under a microscope. As pollen grains are frequently impacted by fast-moving water particles, they float on the surface of the water. So, even though the water particles or water molecules are too small to see, their effect on the pollen grains is clearly visible. Brownian motion, named after the physicist Robert Brown who first noticed the phenomena, is the random motion of visible particles or pollen grains generated by much smaller unseen water particles.

Brownian Movement- Brownian movement can be seen in gases as well. When a beam of sunlight enters a room, we may sometimes observe microscopic dust particles suspended in the air moving quickly and randomly. This is an example where Brownian motion in gases (as air is a gas) is demonstrated. Since they are constantly impacted by fast-moving air particles, the small dust particles move around. Though we cannot see the incredibly minute particles or molecules of air, we can witness the effect created by their continuous and quick motions. The quick and random movement of small dust particles hanging in the air demonstrates that air is made up of particles that are continuously moving. Brownian motion is the zig-zag movement of tiny particles floating in a liquid or gas. The temperature rises as a result of Brownian motion. Brownian motion’s existence leads to two conclusions regarding the nature of matter. The following are the conclusions:

  • Small particles make up matter.
  • Matter particles are in continuous motion.

Brownian motion is the uncontrolled or irregular movement of particles in a fluid caused by collisions with other fast-moving molecules. Random particle movement is usually reported to be stronger in smaller particles, less viscous liquids, and at higher temperatures. These are some of the elements that influence a particle’s motion in a fluid.

Sample Questions

Question 1: In a beam of sunlight, little dust particles move at random. Why are these dust particles moving around?


Small dust particles hanging in the air move fast and randomly when a beam of sunlight enters a room due to Brownian movement, which is the zig-zag movement of tiny particles floating in a liquid or gas.

Question 2: Name the mechanism that causes an ink drop to spread across a beaker of water.


Diffusion is the process through which an ink drop spreads all over a beaker of water.

Question 3: How does the rate of diffusion alter when the temperature drops?


Since particles move slower at lower temperatures, so diffusion decreases when the temperature drops.

Question 4: Which key term best explains particle movement from a high to a low concentration?


The movement of particles from a high concentration to a low concentration is termed as diffusion.

Question 5: Why does gas diffuse so quickly?


Since the particles have a larger intermolecular space and kinetic energy in the vapor state, the gas diffuses quickly. As a result, these particles readily combine with air particles and move quicker.

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