Density is one characteristic property of a substance that is taught in the very beginning of chemistry classroom practice. Students often know this property from Physic lessons as quotient of mass and volume. When learning about the particle nature of matter there is a danger of confusing the density concept. Students explain the insolubility or the immiscibility with high density: the particles are densely packed, it is hard to break the compound. This paper is discussing this issue, using concrete examples and suggesting to be attentive to it in critical phases of classroom practice.