How to Demonstrate Diffusion with Hot and Cold Water

What is Diffusion?

Have you ever smelled your neighbor’s lunch on your way home? Or smelled someone’s perfume minutes after that person was gone? You experienced diffusion!

Diffusion is a movement of particles from the area of high concentration to an area of low concentration. It usually occurs in liquids and gases.

Let’s get some complex-sounding terminology out of the way. When talking about diffusion, we often hear something about the concentration gradient (or electrical gradient if looking at electrons). Gradient just means a change in the quantity of a variable over some distance. In the case of concentration gradient, a variable that changes is the concentration of a substance. So we can define the concentration gradient as the space over which the concentration of our substance changes.

For example, think of the situation when we spray the air freshener in the room. There is one spot where the concentration of our substance is very high (where we sprayed it initially) and in the rest of the room it is very low (nothing initially). Slowly, the freshener starts to diffuse - its particles move through the air, from where the concentration is high toward where it is low. Once they are spread out evenly and the gradient disappears, we reach equilibrium - the state at which a substance is equally distributed throughout a space.

It’s important to note that particles never stop moving, even after the equilibrium is reached. Imagine two parts of the room divided by a line. It may seem like nothing is happening, but particles from both sides are moving back and forth. It's just that it is an equal probability of them moving from left to right as it’s from right to left. So we can’t notice any net change.

Diffusion is a type of passive transport. That means it doesn’t require energy to start. It happens naturally, without any shaking or stirring.

There is also a facilitated diffusion which happens in the cell membranes when molecules are transported with the help of the proteins.

You may remember hearing about Osmosis and wonder how it differs from diffusion. It is actually a very similar concept. Osmosis is just a diffusion through the partially permeable membrane. We talked about it more in our Gummy Bear Osmosis Experiment so definitely check it out.

What causes Diffusion?

Do particles really want to move somewhere less crowded? Well, no, not in the way we would think of it. There is no planning around, just the probability.

All fluids are bound to the same physical laws - studied by Fluid mechanics, part of the physics. We usually think of fluids as liquids, but in fact, air and other types of gas are also fluids! By definition, fluid is a substance that has no fixed shape and yields easily to external pressure.

Another property of the fluids is that they flow or move around. Molecules in fluids move around randomly and that causes collisions between them and makes them bounce off in different directions.

This random motion of particles in a fluid is called Brownian motion, named after the biologist Robert Brown, who observed and described it in 1827. While looking at pollen grains in water under his microscope, he noticed that they jiggled about and never settled. His first thought was the obvious one: perhaps the pollen was alive and swimming.

But then Brown did the thing that makes him a scientist rather than a storyteller. He tested his own idea and tried to break it. He looked at pollen from plants that had been dead for a century. It jiggled. He ground up rock, soot and glass, things that had never been alive at all, and looked at those. They jiggled too. So the motion could not be about life, and Brown said so.

He never worked out what was really causing it, but his careful observation turned out to be revolutionary. Decades later it helped Albert Einstein, who in 1905 explained the jiggling as the pollen being shoved around by countless invisible water molecules. A few years after that, in 1908 and 1909, Jean Baptiste Perrin measured it and proved Einstein right, giving us some of the first hard evidence that atoms and molecules physically exist. Perrin won a Nobel Prize for it.

Factors that influence Diffusion

There are several factors that influence the speed of diffusion. The first is the extent of the concentration gradient. The bigger the difference in concentration over the gradient, the faster diffusion occurs. This relationship is known as Fick's law of diffusion.

Another important factor is the distance over which our particles are moving. We can look at it as the size of a container. As you may imagine, with the bigger distance, diffusion is slower, since particles need to move further.

Then we have characteristics of the solvent and substance. The most notable is the mass of the substance and density of the solvent. Heavier molecules move more slowly; therefore, they diffuse more slowly. And it's a similar case with the density of the solvent. As density increases, the rate of diffusion decreases. It’s harder to move through the denser solvent, therefore our molecules slow down.

And the last factor we will discuss is the temperature. Both heating and cooling change the kinetic energy of the particles in our substance. In the case of heating, we are increasing the kinetic energy of our particles and that makes them move a lot quicker. So the higher the temperature, the higher the diffusion rate.

We will demonstrate the diffusion of food coloring in water and observe how it’s affected by the difference in temperature. Onwards to the experiment!

Materials needed for demonstrating Diffusion

• 2 transparent glasses - Common clear glasses will do the trick. You probably have more than needed around the house. We need one for warm water and one for cold water so we can observe the difference in diffusion.
• Hot and cold water - The bigger the difference in temperature in two glasses, the bigger difference in diffusion will be observed. You can heat the water to near boiling or boiling state and use it as hot water. Use regular water from the pipe as “cold water”. That is enough difference to observe the effects of temperature on diffusion.
• Food coloring - Regular food coloring or some other colors like tempera (poster paint) will do the trick. Color is required to observe the diffusion in our solvent (water). To make it more fun, you can use 2 different colors. Like red for hot and blue for cold.

Instructions for demonstrating diffusion

We have a video on how to demonstrate diffusion at the start of the article so you can check it out if you prefer a video guide more. Or continue reading instructions below if you prefer step by step text guide.

1. Take 2 transparent glasses and fill them with the water. In one glass, pour the cold water and in the other hot water. As we mentioned, near-boiling water for hot and regular temperature water from the pipe will be good to demonstrate the diffusion.
2. Drop a few drops of food coloring in each glass. 3–4 drops are enough - don't add too much. If you do, the concentration of food color will be too high and it will diffuse too fast in both glasses to see the difference.
3. Watch closely how the color spreads. You will notice how color diffuses faster in hot water. It will take longer to diffuse if there is more water, less food color and if the water is cooler.

Before you drop the color in, make a prediction - that's how every scientist begins!

What will you develop and learn

• What is diffusion and how it relates to osmosis
• Factors that influence diffusion
• What is Brownian motion
• How to conduct a science experiment
• That science is fun! 😊

Frequently Asked Questions

If you liked this activity and are interested in more simple fun experiments, we recommend exploring all about the heat conduction. For more cool visuals made by chemistry, check out Lava lamp and Milk polarity experiment. And if you, like us, find the water fascinating, definitely read our article about many interesting properties of water.