how can you say that one substance is different from another

You can say that one substance is different from another when it has different properties that stay the same no matter how much of it you take.

What “substance” means (in chemistry)

In chemistry, a substance is a kind of matter that has a fixed composition and characteristic properties (like pure water, pure oxygen, pure salt).

That means every sample of that substance behaves the same way under the same conditions.

Two big ways substances differ

You can tell substances apart by:

1. Physical properties – observed without changing what it is.

2. Chemical properties – how it reacts and changes into something new.

If at least one of these properties is reliably different, you’re justified in saying “this is a different substance.”

Physical properties that distinguish substances

Common physical properties you can use to tell substances apart include:

• Color, shape, and crystal form
• Odor
• Density (mass ÷ volume)
• Melting point (solid → liquid temperature)
• Boiling point (liquid → gas temperature)
• Solubility in water or other solvents
• Hardness and texture
• Electrical or thermal conductivity

Each pure substance has its own characteristic combination of these values.

For example, pure water always boils at a specific temperature at a given pressure, while ethanol boils at a lower temperature, so they are different substances.

Chemical properties that distinguish substances

Chemical properties show how a substance behaves in reactions.

Key ones include:

• Reactivity with acids or bases
• How it burns (combustibility, flame color, ease of ignition)
• Tendency to rust or oxidize
• Behavior in redox (electron-transfer) reactions

If two materials react differently with the same test (say, one fizzes in acid and the other does nothing), they are different substances.

Example: saying two substances are different

Imagine two clear, colorless liquids in identical glasses:

• Liquid A boils at 100 °C, does not burn, and has a density of about 1.0 g/mL.

• Liquid B boils at about 78 °C, burns easily with a blue flame, and has a density of about 0.79 g/mL.

Because their boiling point, density, and combustibility all differ, we say they are different substances (water vs ethanol).

How this fits pure substances vs mixtures

Chemists also ask whether something is a pure substance or a mixture :

• A pure substance has uniform composition and fixed properties throughout (like pure water or pure oxygen).

• A mixture contains more than one type of particle; its composition can vary, and its properties can change with the proportions.

So sometimes saying “this substance is different from that one” really means “this is a pure substance; that is a mixture” or “this mixture has a different composition.”

Bottom line

You say one substance is different from another when:

• It has a different composition (different elements or arrangement of atoms).

• That different composition shows up as different physical or chemical properties that are consistent and reproducible.

If you can reliably measure a property that doesn’t match, you have a good reason to claim the substances are different.

TL;DR:
Two substances are different if their composition is not the same and this is reflected in distinct, repeatable physical and/or chemical properties (like boiling point, density, or reactivity).

Information gathered from public forums or data available on the internet and portrayed here.