Can water be compressed? We all know about hydraulic systems. Put simply, a hydraulic system transmits power using the pressure of a fluid. This gives people the impression that fluids, including water, can be compressed. But is this really the case? The answer is both yes and no. In fact, technically speaking, *everything* can be compressed with enough force. Water is abundant on Earth, covering more than 70% of its surface. As the source of life, it is indispensable to humans, animals, and plants alike. We call it "water" because it exists in a liquid state, but it can also take on solid (ice) or gaseous (water vapor) forms depending on environmental conditions and temperature. When water freezes into ice, its volume expands. An experiment illustrates this: if you put water in a plastic bag and freeze it, the bag will often burst once the water solidifies completely. Conversely, if water is compressed into a gas, its volume is likely to shrink. If humans mastered the technology to forcefully compress water, it might even be possible to create celestial bodies. While most things in the world can be compressed, there is one exception: the singularity at the center of a black hole. It is said to have an extremely high density but an infinitesimally small volume, and its unique internal structure makes it the only thing that cannot be compressed. Let’s delve deeper: Can water really be compressed? ### Bulk Modulus To understand this, we first need to examine the structure of water. Like all matter, water is composed of atoms. Specifically, water is made up of water molecules, each consisting of one oxygen atom and two hydrogen atoms. Molecules are not stationary; water molecules are in constant motion. This molecular motion is what causes variations in temperature. In theory, compressing water molecules is feasible—for example, through pressurization or cooling—assuming technical challenges are set aside. However, in practice, compressing water molecules is far from easy, as it involves overcoming numerous technical hurdles. The idea that many substances cannot be compressed often stems from our perception of volume as a fixed quantity. When a substance is subjected to intense external pressure, it may deform and shrink in volume. In reality, water has a relatively low bulk modulus compared to materials like steel or diamonds, meaning it resists compression less strongly than these solids. From an atomic perspective, water *can* be compressed, but the change is so minimal that it is not visible to the naked eye. In contrast, air—with a much lower bulk modulus—shows obvious compression effects. But you might ask: These are just theories. Is there any practical evidence? Indeed, experiments have been conducted to test this, using large diamonds as part of the setup. Specifically, scientists used a diamond anvil cell to compress water. This equipment can generate pressures as high as several dozen gigapascals (GPa), with 10 GPa being easily achievable. During the experiment, scientists observed the compression of water under a microscope. As pressure intensified, the hydrogen bonds between water molecules began to reform. Under sustained low-pressure compression, water eventually transformed into a state resembling ice—though distinct from ordinary ice. Thus, water *can* be compressed, and during this process, chemical bonds undergo rearrangement. What if the pressure exceeds what a diamond anvil cell can generate? At extremely high pressures—such as those found on Jupiter—water would transform into "metallic water." But this is not the end of the story. Pushing further, the water molecules would begin to resist compression through a phenomenon called "electron degeneracy pressure." Electrons orbit atomic nuclei in irregular patterns, and when subjected to extreme external pressure, they exert a force to counteract it. If the pressure becomes even greater, electrons are forced into the atomic nucleus. This intrusion disrupts the nucleus’s stability, altering the number of protons. At this point, the substance is no longer water—it has become a neutron state, the same state as neutron stars. Can compression continue beyond this? Absolutely. The next stage after the neutron state is a black hole. Recently, humans released the first image of a black hole, a feat that took scientists 10 years to achieve. The universe is full of wonders, and there is no shortage of fascinating phenomena to explore if we are willing to seek them. ### Atomic Perspective The compressibility of water can also be understood by examining its atomic structure. Like all matter, water is composed of particles, which are not tightly packed in a rigid arrangement but are relatively spread out. This means there is space between particles, not just minimal gaps. Compression, at its core, reduces this interparticle space, and as compression proceeds, substances can undergo phase changes. The diamond anvil experiments on water demonstrate this process. While further compression of water is theoretically possible, current human technology cannot achieve it. If we did possess such technology, what would happen? This brings us back to electron degeneracy pressure, a quantum effect. In reality, atoms consist of electrons orbiting a nucleus. When we compress atomic structures, the outer electrons experience pressure. Due to the Pauli exclusion principle, these electrons generate a counterforce known as electron degeneracy pressure. If the pressure is relatively low, the atomic structure remains intact. But if the pressure exceeds the electron degeneracy pressure, electrons are forced into the nucleus, leaving only neutrons behind. This process occurs naturally in the universe: under extreme conditions, stars can collapse into neutron stars. Neutron stars are extremely dense celestial bodies—just one teaspoon of neutron star material would weigh over a billion tons. Beyond neutron stars, the universe contains other dense objects, such as white dwarfs. These are compact stars where electron degeneracy pressure prevents further collapse. Black holes, however, are even more extreme. Their gravitational pull is so immense that it overcomes neutron degeneracy pressure, and even light cannot escape their grasp, making them the densest objects in the universe. Thus, from an atomic and particle perspective, water *can* be compressed, but it requires an enormous amount of external force—force that exceeds our current technological capabilities. Source: Guanyuge, Youmin Talks About Life