Final answer:
The density of water is affected more by temperature than pressure, with the significant changes occurring in density as water cools to and below 4°C due to the properties of hydrogen bonding and thermal expansion.
Step-by-step explanation:
When considering what has a greater effect on density of water: temperature or pressure, it becomes clear that temperature has a significant impact on water's density. As the temperature of water increases, thermal expansion causes the average distance between the water molecules to increase, leading to a reduction in density. Water reaches its maximum density at 4°C. Beyond this point, as water cools further, it begins to expand as it approaches the solid crystal form of ice, which inherently possesses more empty space between molecules compared to liquid water.
Pressure, while it does influence the density of water, primarily affects it when dealing with extremely high pressures that are not common in daily life. For instance, to keep ice from expanding when it freezes, an enormous pressure is required, as indicated by a problem that calculates the pressure necessary to keep ice at 0°C from expanding.
Overall, while both temperature and pressure can affect the density of water, temperature influences the everyday changes in water's density more noticeably than pressure, with its effects being particularly relevant in environmental and biological contexts, like the behavior of lakes during winter.