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We say that a homogeneous sample consists of the chemically same type of molecules whereas heterogeneous ones vary chemically, i.e. the molecules throughout are not same and also not uniform.

Why do we treat a (water + water vapour) system as heterogeneous in spite of the presence of the same kind of molecules? Also if there exists an equilibrium in the two phases would we still not call them homogeneous?

Does heterogeneity have a chemical basis or physical basis?

I do not seem to get a clear idea as to what defines heterogeneity. The definition has kept on changing since my high school and later on.

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Any classification like chemical or a physical change, homogeneous or heterogeneous substance, will eventually fail at one point. From an analytical or physical chemistry's perspective, homogeneous substance is a substance, in a single phase, whose chemical composition is uniform. A heterogeneous substance has different composition from place to place like your garden soil.

Why do we treat a water+water vapour system as heterogeneous in spite of similar molecules present? Also if there exists an equilibrium in the two phases would we still not call them homogeneous?

It is a bad example to label water+ water vapor system as heterogeneous- but with respect to what? It is better to call it a two phase system. You have to define heterogeneous with respect to some property.

For example, if we take tap water in a glass, we can say the liquid (water) is homogeneous with respect to its chemical composition. Let us put some ice cubes in water, now it is a heterogeneous phase, although chemically it is all water.

More formally, from the International Union of Pure and Applied Chemistry's Gold Book:

homogeneity (in analytical chemistry): The degree to which a property or a constituent is uniformly distributed throughout a quantity of material. A material may be homogeneous with respect to one analyte or property but heterogeneous with respect to another.

You can see the subtilty above in italics.

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  • $\begingroup$ It is a bad example to label water+ water vapor system as heterogeneous- but with respect to what? WRT near any property but chemical composition. A liquid/gas fog or solid/liquid mash is very heterogenous. $\endgroup$ – Poutnik Jan 28 at 13:28
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General concept of heterogenity says the matter properties within the given region are location dependent ( within the given resolution and tolerance of the scenario ).

The change of these properties can be the gradual ( like a concentration gradient ), abrupt ( like a phase boundary ) or combination of both.

Practically speaking, matter is homogenous, if any its point-like sample has representative properties for the whole region, otherwise it is heterogenous.

So mixture of liquid propane and n-butane is homogenous, if its macroscopic properties, like composition, temperature or density, are the same across the volume ( within the given tolerance ).

OTOH, a system containing just water in 2 or 3 phases is heterogenous, as a point-like sample from either of phases is not representative for the other phases.

See also wikipedia Homogeneity_and_heterogeneity and Homogeneous_and_heterogeneous_mixtures

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Your definition of homogeneity is wrong

Homogeneous systems are not, as you seem to assume, about the makeup or constituents of the system being uniform. Homogeneity is about the distribution of the components in the system being uniform. A solution of salt in water is homogeneous (if well mixed) despite having multiple types of molecule in it. If those molecules are evenly distributed, the mixture is homogeneous.

More examples: ethanol and water form a homogeneous mixture under all normal conditions as the two components are mutually soluble. Oil and water form a heterogeneous mixture and the best you can get is a fine dispersion of one liquid in the other with two distinct liquid phases. Air normally contains some water vapour as a homogeneous mixture. But, when too much water vapour is present, you get fog which is an heterogeneous suspension of small water droplets in air (which is saturated with water vapour). Water is present in both phases but in two different states, liquid and vapour.

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