Because they travel so easily, which substances are difficult, if not impossible to confine and contain?

• A. Gases
• B. Vapors
• C. Liquids
• D. Dusts

Answer: (A)

Gas-phase substances move through air so readily that they don’t respect typical barriers. Their molecules are lightweight and diffuse quickly, spreading to fill the available space and leaking through tiny gaps, joints, and porous materials. Because containment relies on creating a truly gas-tight boundary, even small openings can become pathways for rapid spread, making confinement extremely difficult or effectively impossible without specialized airtight enclosures and careful ventilation control. Liquids and dusts can often be contained with barriers and filtration, and vapors behave like gases in mobility, but the statement about traveling so easily points to the gas state as the one that most readily defeats ordinary containment methods.