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Matter is anything that has mass and takes up space. It is the 'stuff' that makes up everything in the universe, from the air we breathe to the stars in the sky. Matter is made up of tiny particles called atoms, and groups of atoms bonded together are known as molecules.

Matter exists in different states such as solid, liquid, and gas, and can also change between these states. However, no matter the state, the composition of matter remains constant unless a chemical change occurs. In the context of physical changes, matter retains its identity; for instance, ice melting into water or water evaporating into steam both involve the same molecules of H2O before and after the change.

Chemical properties of matter describe its potential to undergo specific chemical changes and reactions due to the arrangement of atoms and the type of chemical bonds between them. These properties are based on the ability of a substance to transform into new substances with different compositions and properties.

Important chemical properties include reactivity with other chemicals, flammability, and pH level. No matter the physical shape or phase in which a substance exists—be it solid, liquid, or gas—its underlying chemical properties remain unaltered unless the substance undergoes a chemical reaction that changes its chemical structure. This is key in differentiating between physical and chemical changes.

Phase changes, also known as physical changes, involve a change in the physical state of matter without any alteration of the substance's chemical structure. Common examples include melting, freezing, boiling, condensation, sublimation, and deposition.

During melting, a solid turns into a liquid. Freezing is the opposite; a liquid becomes a solid. Boiling or evaporation occurs when a liquid transforms into a gas, whereas condensation refers to the process of a gas becoming a liquid. Sublimation and deposition are less common: sublimation is the change from a solid directly to a gas while deposition is gas to solid—without going through a liquid phase.

These changes are typically induced by changes in temperature and pressure and are reversible—meaning the chemical identity of the substance is not altered. Understanding phase changes is crucial for grasping how substances interact with their environment, and is a fundamental concept in physical science.