Explanation:

* Particle Size: Smaller particles have a greater surface area to volume ratio than larger particles.

* Interparticle Spaces: When substances with different particle sizes are mixed, there are spaces between the particles, particularly if the particles are of significantly different sizes. These spaces are not filled by the particles themselves, resulting in a larger overall volume.

* Packing Efficiency: The efficiency with which particles pack together in a mixture is affected by their size. Smaller particles can fit into the spaces between larger particles, leading to a less efficient packing and a greater overall volume.

Example:

Imagine mixing sand and pebbles. The pebbles are larger than the sand grains. When mixed, the sand grains will fill in some of the spaces between the pebbles. However, there will still be empty spaces between the particles, leading to a larger overall volume than the sum of the volumes of the sand and pebbles alone.

Exceptions:

* Dissolving: When one substance dissolves in another, the particles of the dissolved substance become dispersed among the particles of the solvent. In this case, the combined volume may be close to or even less than the sum of the individual volumes.

* Compaction: Under high pressure, particles can be forced closer together, reducing the overall volume.

Conclusion:

In general, the combined volume of a mixture of substances with different particle sizes is greater than the sum of the individual volumes due to the presence of interparticle spaces and the less efficient packing of particles. However, there are exceptions to this rule, such as dissolving and compaction.