Final answer:
It is true that we rely on indirect observations to identify properties of atoms and molecules due to their tiny size. This was initially done using macroscopic properties and has evolved with the introduction of the mole, and advanced instrumentation for direct measurement.
Step-by-step explanation:
The statement that 'atoms and molecules are too small to see; so we must rely on indirect observations related to the properties of substances to identify things' is true. Atoms are the smallest unit of elements, and they combine to form molecules, which are the smallest unit of compounds. Instruments like microscopes and more sophisticated tools are used to measure the microscopic world. However, chemists typically operate with massive numbers of atoms and molecules, requiring a unit like the mole for practical operations. The mole relates macroscopic quantities of substances in a way that we can handle.
Early discoveries of the size of atoms, such as through analysis of Brownian motion, helped establish properties like atom size (around 10-10 meters on average) and Avogadro's number. Now, we possess advanced instruments that can measure these microscopic traits directly. Previously, properties were inferred from macroscopic observations, like mass and volume measurements with balances and volumetric glassware. These macroscopic observations remain vital in understanding substances at a molecular level.