To understand that gases are readily compressible, and liquids are not.
Everything in this universe is made up of materials which scientists have termed as "Matter". Matter is defined as anything that has mass and occupies space. Everything we can see, hear, touch, taste, and smell is matter. It is made up of tiny particles that cannot be seen with the naked eye. Air and water, gold and silver, table and chair, milk, and oil etc., are all different kinds of matter, because all of them occupy space and have mass.
Based on physical properties, matter is classified as solids, liquids, and gases. Matter is classified into elements, compounds, and mixtures based on its chemical properties.
Solids have a definite shape, distinct boundaries, and fixed volumes, and thus have negligible compressibility. Solids tend to maintain their shape when subjected to outside force. Solids can be broken by force, but changing their shape is difficult, so they are rigid.
Intermolecular space: In solids, particles are tightly packed together with strong intermolecular forces. The intermolecular spaces are significantly smaller compared to liquids and gases. However, these spaces do exist, allowing for slight vibrations and movements of the particles within the solid structure.
Compressibility: Solids are considered to be incompressible, meaning their volume cannot be significantly reduced by the application of external pressure. This is due to the strong forces holding the particles in a fixed position.
Liquids adapt to the shape of their container but maintain a fixed volume, displaying fluid behaviour. Both solids and liquids can diffuse into liquids, with atmospheric gases like oxygen and carbon dioxide dissolving in water, crucial for the survival of aquatic life. This dissolved oxygen enables underwater breathing for aquatic organisms. Notably, liquids diffuse more rapidly than solids due to the increased freedom and spacing among particles in the liquid state compared to solids.
Intermolecular space: In liquids, the particles are close together but have more freedom of movement compared to solids. The intermolecular spaces are larger than those in solids, allowing the particles to move past each other, resulting in fluidity.
Compressibility: Liquids are considered incompressible compared to gases. While liquids can be compressed to a small extent, it usually requires high pressures to cause any noticeable change in volume.
Intermolecular space: Gases have large intermolecular spaces as the particles are far apart and move freely. The intermolecular forces are negligible in gases, allowing them to fill the container they are in and to be easily compressed or expanded.
Compressibility: Gases are highly compressible due to the significant space between particles. Changes in pressure can cause substantial changes in the volume of a gas, following Boyle's law, which states that the volume of a given amount of gas is inversely proportional to its pressure when temperature remains constant.