It is normal practice to ascertain the purity of a material by first identifying the properties – physical and chemical – that it should satisfy. Therefore, “purity” is really a degree of how well the properties have been met and properties are those that can be observed when evaluating the behaviour of the material.
In the software world, the quality is about how well a system/software meets the expectations, similar to the notion of purity. Therefore, software quality should also be about satisfying properties. The act of testing is about clarifying / setting-up the expectations by identifying properties and their intended value and then assessing them.
Read the article – by T Ashok – below that highlights the notion of purity, quality and cleanliness criteria in this context.