Intensive and Extensive Properties
Imagine you have a cup of hot tea. You take a sip — it's hot. Now pour half of it into another cup. Is the tea in each cup still hot? Yes, it is. The temperature didn't change just because you split the tea. Now think about the volume: each cup now has less tea than the original. The volume did change.
That's the entire intuition in one sip.
Temperature is an intensive property — it doesn't care how much tea you have. Volume is an extensive property — it cares a lot.
The Precise Definition
An intensive property is independent of the amount of matter present. An extensive property depends on the amount of matter.
If you take a system and cut it into two equal halves, any property that stays the same in each half is intensive. Any property that gets halved is extensive.
| Intensive (stays same) | Extensive (gets halved) |
|---|
| Temperature | Mass |
| Pressure | Volume |
| Density | Energy |
| Concentration | Number of moles |
| Refractive index | Heat capacity |
| Viscosity | Entropy |
Notice something: density is mass divided by volume. Both mass and volume are extensive, but their ratio is intensive. That's a powerful pattern — ratios of extensive properties are always intensive.
Why This Matters
In thermodynamics, you cannot simply add intensive properties. If you mix two buckets of water at 30∘C and 50∘C, the final temperature is not 80∘C — it's somewhere in between. But you can add extensive properties: the total mass is the sum of the two masses, the total volume is the sum of the two volumes.
A common mistake: thinking that "intensive" means "always constant." Temperature is intensive but it can change — it just doesn't change because you change the amount.
The Decisive Test
Here's a foolproof way to classify any property. Ask yourself:
If I double the amount of substance, does this property double?
- Yes → Extensive (mass, volume, energy, charge)
- No → Intensive (temperature, pressure, density, colour)
Try it on a tricky one: electrical resistance. If you double the length of a wire, resistance doubles. If you double the cross-sectional area, resistance halves. So resistance depends on shape, not just amount — it's not purely extensive. That's why it's not in the table above. The clean classification works best for homogeneous systems where "amount" means mass or moles.
One Last Intuition
Think of intensive properties as the quality of matter — how hot, how dense, how concentrated. Extensive properties are the quantity — how much, how heavy, how voluminous. You can change the quantity without changing the quality, but not the other way around.
In exam problems, if you're asked to identify which property is intensive, look for the one that is a ratio of two extensive properties (density, specific heat, molar volume) or a field-like quantity (temperature, pressure). These are almost always intensive.
Students search 'intensive and extensive properties examples', 'intensive vs extensive properties class 11 chemistry', and 'thermodynamics properties important questions' while revising this topic, grounded in the NCERT-aligned CBSE Class 11 Chemistry Thermodynamics syllabus. Correctly classifying a given property, especially tricky ratios like density, is a quick, recurring question type in board exams and JEE Main.