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Efforts to verify or invalidate hypotheses on the structure of liquid water have been hampered by the lack of a general theory of the liquid state. In the absence of such a theory, conclusions about the structure of water have been based on two approaches. The first involves formulating a model for liquid water, treating the model in a particular fashion — involving massive approximations — using methods of statistical mechanics, and comparing the calculated values of macroscopic properties with those that are observed. This chapter focuses on the second approach, which is to deduce aspects of the structure of the liquid from the macroscopic properties of water.

In the gaseous state, molecules are to a good approximation isolated entities traveling in space at high speed with sparse and near elastic collisions. At the other extreme, a perfect crystal has a periodic and symmetric intermolecular structure. The structure is dictated by intermolecular forces, and molecules can only perform small oscillations around their equilibrium positions. In between these two extremes, matter has many more ways of aggregation. This chapter deals with proper liquids and the liquid state. Molecular diffusion in liquids occurs approximately on the timescale of nanoseconds, to be compared with the timescale of molecular or lattice vibrations. This chapter also discusses molecular dynamics (MD), the Monte Carlo (MC) method, structural and dynamic descriptors for liquids, physicochemical properties of liquids from MD or MC simulations, simulations of enthalpy, heat capacity and density, crystal and liquid equations of state, polarisability and dielectric constants, free energy simulations, and simulation of water.

This chapter discusses several basic concepts necessary to advance from the fundamental knowledge of the structure and thermophysical properties of metallic liquids to their applications. It begins with preliminary concepts, namely the three phases of matter, phase transitions, and liquid ranges of metallic elements. It then covers approaches to the liquid state; well-known, representative models for the thermophysical properties of liquid metals; methods for assessment of models/equations; electron configuration and the periodic table of the elements; and other important matters in studying metallic liquids.

This chapter talks about the astronomers that have solved some of the riddles of how much water Mars had and still has, a century and a half after William Huggins first proved that Mars had water. It highlights that Mars' water was no longer in the liquid state on the surface as the planet had become much dryer after billions of years. It also examines significant evidence that strongly suggests that Mars experienced a more recent epoch when water carved valleys and was formed from melting snow and ice that flowed slowly into and out of chains of lakes. The chapter describes one of Mars' lakes that appears to have been widespread both north and south of the Martian equator, containing more water than Lake Ontario. It discusses Mars' layered ice deposits at the north and south polar caps, in which a thin layer is deposited each Martian winter and then sublimates in Martian spring.