Thermodynamics

Pure Substance

Question

What is a pure substance? Explain the meaning of a pure substance with suitable examples.

Given

A substance may exist in different phases such as solid, liquid and gas. A change of phase does not necessarily mean that the chemical composition of the substance has changed.

The important condition for a pure substance is that its chemical composition remains homogeneous and invariable.

Concept Used

In thermodynamics, we are often interested in whether a material remains chemically the same throughout a system.

A substance can contain more than one phase and still be a pure substance, provided its chemical composition does not change from one phase to another.

\boxed{ \text{Phase change} \neq \text{Change in chemical composition} }

For example, liquid water and steam can exist together. Both are chemically \(H_2O\), so the system is still a pure substance.

Detailed Answer

1. Definition of Pure Substance

A pure substance is one that has a homogeneous and invariable chemical composition, even though there may be a change of phase.

\boxed{ \text{Pure substance} = \text{Homogeneous and invariable chemical composition} }

The important point is that the substance must retain the same chemical composition throughout the system.

2. What Does Homogeneous Composition Mean?

Homogeneous means that the chemical composition is the same throughout the substance.

If we take two small samples from different parts of a pure substance, they have the same chemical composition.

\boxed{ \text{Same chemical composition throughout} }

For example, pure water has the chemical composition \(H_2O\) throughout the sample.

3. What Does Invariable Chemical Composition Mean?

Invariable means that the chemical composition does not change merely because the substance changes its physical phase.

For example, water can change from liquid to steam. During this phase change, its chemical composition remains \(H_2O\).

H_2O_{\text{liquid}} \longrightarrow H_2O_{\text{vapour}}

Therefore, liquid water and water vapour can together form a pure substance.

4. Pure Substance Can Have More Than One Phase

A common mistake is to think that a pure substance must exist in only one phase. That is incorrect.

A pure substance can exist in two or more phases at the same time, as long as the chemical composition remains the same.

Example: Liquid water and steam

Suppose a container contains both liquid water and steam. There are two phases:

Liquid phase
Vapour phase

But both phases have the same chemical composition, \(H_2O\).

\boxed{ \text{Liquid water + Steam} = \text{Pure substance} }

5. Example: Ice and Water

Consider a container containing a mixture of ice and liquid water.

There are two physical phases:

Solid phase: ice
Liquid phase: water

Both are made of \(H_2O\). Therefore, the mixture of ice and water is still a pure substance.

\boxed{ \text{Ice + Liquid water} = \text{Pure substance} }

6. Example: Water and Steam

A mixture of liquid water and steam is another important example of a pure substance.

Although the physical state is different, the chemical substance is the same in both phases.

\boxed{ H_2O_{\text{liquid}} + H_2O_{\text{steam}} \Rightarrow \text{Pure substance} }

Therefore, the presence of more than one phase does not automatically make a system a mixture.

7. Example: Liquid Air and Gaseous Air

Consider a system containing a mixture of liquid air and gaseous air.

This is not considered a pure substance because air itself is a mixture of different gases.

During phase separation, the composition of the liquid and gaseous portions can differ. Hence, the chemical composition is not the same throughout the system.

\boxed{ \text{Liquid air + Gaseous air} \neq \text{Pure substance} }

8. The Most Important Rule

Do not decide whether a substance is pure only by looking at the number of phases.

Instead, check the chemical composition.

\boxed{ \begin{aligned} \text{Same chemical composition} &\Rightarrow \text{Pure substance}\\ \text{Different composition} &\Rightarrow \text{Not a pure substance} \end{aligned} }

Thus, a system may have one phase or multiple phases and still be a pure substance.

9. Pure Substance vs Mixture

The difference can be understood by comparing their chemical composition.

\boxed{ \begin{array}{c|c|c} \text{Point} & \text{Pure Substance} & \text{Mixture}\\ \hline \text{Composition} & \text{Homogeneous and invariable} & \text{Can vary}\\ \text{Phases} & \text{May be one or more} & \text{May be one or more}\\ \text{Example} & \text{Water + steam} & \text{Air} \end{array} }

The key distinction is not simply the number of phases. The chemical composition is what matters.

10. Examples at a Glance

The examples from this topic can be remembered as:

\boxed{ \begin{array}{c|c} \text{System} & \text{Pure substance?}\\ \hline \text{Liquid water} & \text{Yes}\\ \text{Liquid water + steam} & \text{Yes}\\ \text{Ice + water} & \text{Yes}\\ \text{Liquid air + gaseous air} & \text{No} \end{array} }

11. Why Water Can Have Multiple Phases

Water is a particularly useful example because its solid, liquid and vapour phases are all chemically \(H_2O\).

\boxed{ \text{Ice} = H_2O,\qquad \text{Water} = H_2O,\qquad \text{Steam} = H_2O }

Therefore, a system containing any combination of these phases can still represent a pure substance, provided there are no other substances present.

12. How to Identify a Pure Substance in an Exam

Use the following simple method:

Identify the chemical substances present.
Check whether the chemical composition is the same throughout the system.
Do not worry if the substance exists in more than one physical phase.
If the chemical composition remains homogeneous and invariable, it is a pure substance.

\boxed{ \text{Composition first, phase second} }

Final Answer

A pure substance is one that has a homogeneous and invariable chemical composition, even though there may be a change of phase.

Therefore, a pure substance can exist in more than one phase as long as the chemical composition remains the same in all phases.

Examples include:

Liquid water
Liquid water + steam
Ice + water

A mixture of liquid air and gaseous air is not a pure substance because its composition is not the same throughout.

\boxed{ \text{Pure substance} = \text{Homogeneous + Invariable chemical composition} }

Quick Revision

Definition

\boxed{ \text{Pure substance} = \text{Homogeneous and invariable chemical composition} }

Important point

\boxed{ \text{More than one phase} \not\Rightarrow \text{Not a pure substance} }

Examples

\boxed{ \begin{aligned} \text{Water} &\rightarrow \text{Pure}\\ \text{Water + Steam} &\rightarrow \text{Pure}\\ \text{Ice + Water} &\rightarrow \text{Pure}\\ \text{Liquid air + Gaseous air} &\rightarrow \text{Not pure} \end{aligned} }

Exam Tip

The biggest trap in this topic is thinking that a pure substance must have only one phase.

That is wrong. Water + steam and ice + water can both be pure substances because every phase contains the same chemical substance, \(H_2O\).

\boxed{ \text{Check chemical composition, not just number of phases} }

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