  The star forming processes strongly influence the ISM chemistry. Nowadays,
there are available many high-quality databases at millimeter wavelengths.
Using them, it is possible to carry out studies that review and deepen previous
results. If these studies involve large samples of sources, it is preferred to
use direct tools to study the molecular gas. With the aim of testing these
tools such as the use of the HCN/HNC ratio as a thermometer, and the use of
H$^{13}$CO$^{+}$, HC$_{3}$N, N$_{2}$H$^{+}$, and C$_{2}$H as "chemical clocks",
we present a molecular line study towards 55 sources representing massive young
stellar objects (MYSOs) at different evolutive stages: infrared dark clouds
(IRDCs), high-mass protostellar objects (HMPOs), hot molecular cores (HMCs) and
ultracompact HII regions (UCHII). We found that the use of HCN/HNC ratio as an
universal thermometer in the ISM should be taken with care because the HCN
optical depth is a big issue that can affect the method. Hence, this tool
should be used only after a careful analysis of the HCN spectrum, checking that
no line, neither the main nor the hyperfine ones, present absorption features.
We point out that the analysis of the emission of H$^{13}$CO$^{+}$, HC$_{3}$N,
N$_{2}$H$^{+}$, and C$_{2}$H could be useful to trace and distinguish regions
among IRDCs, HMPOs and HMCs. The molecular line widths of these four species
increase from the IRDC to the HMC stage, which can be a consequence of the gas
dynamics related to the star-forming processes taking place in the molecular
clumps. Our results do not only contribute with more statistics regarding to
probe such chemical tools, useful to obtain information in large samples of
sources, but also complement previous works through the analysis on other types
of sources.
