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Entropy Made Simple

Today, we're going to be learning about a concept that is at the heart of both Physics and Chemistry.

This is also a concept that students find extremely challenging to grasp in class.

Today, we're going to be understanding it, and learning how to apply it.

Let's get started,

For this experiment you will need

  • Ice cubes

  • Boiling water

  • 2 Glasses

  • Food colouring

  • Cold Water

Step 1: Boil some water till it bubbles.

Step 2: Put another glass of water in the refrigerator till cold but not freezing.

Step 3: Pour some water into an ice cube tray and add some food coloring.

Step 4: Take 2 Glasses and pour hot water into one and cold water into the other.

Step 5: Add an ice cube to both glasses

Step 6: Observe the rate of melting of the ice cube in both the glasses.

Here's a short clip to help visualise what you'd likely see when performing this experiment.

Now, onto the science,

What is entropy?

Let us try to analyse entropy,

Entropy is said to be

"The measure of disorder"

The most common definition that is given for entropy is that it is a direct measure of the disorder of a system.

The more unpredictable the particles of a system are, the higher it's entropy.

For example,

Imagine a room full of gas particles whizzing around fast.

In this situation, it becomes very hard to predict where the gas particles will be or at what speed they'll travel at after some time.

However, in a solid, the particles are more rigidly arranged in an orderly structure.

Predicting it's behaviour is much easier.

It's motion is less chaotic.

Thereby, a general rule of thumb is that solids have a lower entropy than liquids and liquids have a lower entropy than solids.

Entropy is generally denoted by the capital letter Q.

Qs → Ql→ Qg Increasing Order

Where Qs is the entropy of solids

Ql is the entropy of liquids and

Qg is the entropy of gases

Let us go one step further in our effort to define entropy,

We mentioned that entropy is related to the disorder in a system.

Well, when I say this, I really mean how distributed the systems energy is.

We can imagine a cup of hot coffee kept in a room. The coffee cup has greater thermal energy than the room. The systems energy is therefore concentrated in the cup. The entropy is thereby lower.

Eventually it cools down and the thermal energy is evenly distributing between the cup and the surroundings. The entropy is thereby higher than before.

Energy tends to spread itself out and it always flows from an area of high energy to an area of low energy.

Entropy measures how spread out that energy is.

If the energy is evenly distributed the entropy is high.

If the energy is concentrated in one place, the entropy is lower.

Before we get into explaining the icecube experiment, let me introduce you to the basics of another crucial concept in chemistry,

The second law of thermodynamics,

This law states that the entropy of the universe always tends to increase.

This is a universal law that predicts an increase in entropy no matter what the situation may be.

Now, let us explain the experiment,

The ice cube is cold, it thereby has low thermal energy,

In the clip, we can clearly see how much quicker the cube melts in the warm water than in the cold water.

Why does this happen?

Well, first let's look at the cold water,

The thermal energy in the cold water is low,

The thermal energy is the ice cube is lower.

The difference in the energies of the ice cube and the water is very small. The energy is almost evenly distributed.

Thereby, the system tends towards a state of higher entropy at a slow rate.

However, in the case of the hot water, the thermal energy is concentrated in the hot water.

The ice cube possess lower thermal energy.

We can say that the systems energy is not evenly distributed because there exists a large difference in energies between the ice cube and the water.

Thereby, the system tends towards higher entropy at a faster rate.

That's all from my side,

See you next week!

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