It is usually not difficult to locate reliable data for standard gases, small organic molecules, and common inorganic substances in pure form, along with their aqueous solutions and well-known binary systems. Standard reference tools like the CRC Handbook, the NIST WebBook, and DIPPR can answer many of these basic questions. But if you are looking for a complex or proprietary material -- things like polymers, drugs, biological molecules, exotic molecules, composites, newly synthesized compounds, or commercial products -- published data often don't exist. It can also be difficult to find data covering non-standard conditions such as extreme temperatures and pressures; extrapolation of known data to such conditions may not be reliable. Engineers often rely on property values calculated by estimation programs. These are useful within their stated limits, but are outside the scope of these pages, which focus on finding published literature values.
High-quality data can be found in certain online databases, but these almost always require a subscription or a fee to use. Many printed secondary data compilations have been published over the years, of varying quality and scope. They are all arranged differently, sometimes incomprehensibly. They cover different types of compounds and properties, and they tend to be scattered in a library, making them hard to remember and locate. Data reported in the primary journal and technical report literature can be even more elusive.
This guide is not meant to be comprehensive, but aims to list the best starting points and other important resources both in print and online; to describe various data collection agencies and their publications; and to explain the vital concept of data quality.