The dynamical modelling of physical (bio-)chemical processes based on first principles considerations is analysed from a structural point of view. Based on a classification of the variables and equations that occur in such models, we propose a general framework that can help to organise the model in a transparent way and to analyse efficiently its solv- ability properties. We show that a well-known tool in the theory of nonlinear dynamical systems, the Zero Dynamics Algorithm, can be used in the analysis of higher index mod- els and also in index reduction. The symbolic computations involved in this algorithm are readily available in the form of nonlinear system analysis packages. The proposed methods are illustrated by a few simple concrete examples. Keywords : First principles modelling, differential-algebraic systems, index reduction.