General Science MCQs – Page 34 (Q331-340)

The direct change of a substance from a solid to a gas, without passing through an intermediate liquid stage, is called sublimation, occurring when a solid's particles gain enough energy to escape directly into the gaseous state under specific temperature and pressure conditions. Solid carbon dioxide, commonly known as dry ice, is a well-known example, sublimating directly into carbon dioxide gas at room temperature and normal atmospheric pressure, producing its characteristic dramatic fog effect often used in theatrical and special effects applications. Naphthalene mothballs and solid iodine are other common substances that readily sublimate under normal conditions.

Solid carbon dioxide, commonly known as dry ice, is well known for undergoing sublimation at room temperature, transforming directly from a solid into carbon dioxide gas without passing through a liquid phase, since at standard atmospheric pressure, carbon dioxide cannot exist as a liquid, only as a solid below minus 78.5 degrees Celsius or as a gas above that temperature. This property makes dry ice extremely useful for refrigeration and cooling applications where a liquid residue is undesirable, as well as for producing the dramatic fog effect commonly seen in theatrical productions and concerts, caused by the cold carbon dioxide gas condensing surrounding water vapor into a visible mist.

The change of a substance from a gaseous state directly to a liquid state is called condensation, occurring when gas particles lose enough thermal energy, typically by cooling, that intermolecular attractive forces become strong enough to pull them into the closer, more ordered arrangement characteristic of a liquid. This process is commonly observed as water droplets forming on the outside of a cold glass on a humid day, or as morning dew forming on grass overnight as air cools and can no longer hold as much water vapor. Condensation is also a key stage in the water cycle, forming clouds from rising, cooling water vapor.

The change of a substance from a liquid state to a solid state is generally called freezing, occurring when a liquid loses enough thermal energy, typically through cooling, that its particles slow down sufficiently for intermolecular or ionic forces to lock them into a fixed, ordered crystalline or amorphous solid structure. Freezing point is generally the same temperature as melting point for a given pure substance, since both describe the same equilibrium transition temperature between solid and liquid states, just approached from opposite directions, with water reliably freezing and melting at 0 degrees Celsius at standard atmospheric pressure.

The direct change of a substance from a gas to a solid, skipping the intermediate liquid stage entirely, is called deposition, essentially the reverse process of sublimation, occurring when gas particles lose enough energy that they transition directly into the ordered, fixed arrangement of a solid. A common everyday example is frost formation, where water vapor in cold air deposits directly onto surfaces like grass, windows, or car windshields as ice crystals overnight, without first condensing into liquid water. This same deposition process contributes to snowflake formation high in cold clouds, where water vapor deposits directly as intricate ice crystal structures.

At standard atmospheric pressure, pure water boils at approximately 100 degrees Celsius, or 212 degrees Fahrenheit, the temperature at which water's vapor pressure equals the surrounding atmospheric pressure, allowing vapor bubbles to form throughout the liquid rather than just at its surface. This boiling point decreases at higher altitudes, where atmospheric pressure is lower, which is why food takes longer to cook properly in high-altitude regions, since water boils at a lower, less effective cooking temperature there. Conversely, pressure cookers raise internal pressure above normal atmospheric levels, allowing water to reach higher temperatures before boiling and cooking food significantly faster.

At standard atmospheric pressure, pure water freezes at approximately 0 degrees Celsius, or 32 degrees Fahrenheit, the reference point originally used to define the Celsius temperature scale devised by Anders Celsius in the eighteenth century. Dissolved substances like salt lower this freezing point through a phenomenon called freezing point depression, which is why salt is commonly spread on icy roads and sidewalks in winter, since it lowers the temperature at which water freezes, helping to melt existing ice or prevent new ice from forming even below the normal 0 degree Celsius threshold.

Unlike most substances, which become denser as they solidify, water has the unusual property of expanding and becoming less dense when it freezes into ice, due to the specific hydrogen-bonded crystalline structure ice molecules form, which spaces water molecules farther apart than in liquid water. This is precisely why ice floats on liquid water rather than sinking, an anomaly with profound ecological importance, since ice forming on the surface of lakes and ponds insulates the liquid water below, allowing aquatic life to survive winter rather than freezing solid from the bottom up. This expansion is also why frozen water pipes can burst.

Because it can dissolve such an exceptionally wide range of substances, from salts and sugars to many gases and organic compounds, water is often referred to as the universal solvent, a property arising from its polar molecular structure, which allows it to surround and separate charged ions and other polar molecules effectively. This remarkable dissolving ability makes water essential for biological processes, since it enables nutrients, gases, and waste products to be transported and dissolved within living organisms, and it is also why natural water sources readily pick up dissolved minerals and, unfortunately, pollutants as they flow through soil and rock.

A chemical bond formed by the complete transfer of one or more electrons from one atom to another is called an ionic bond, typically occurring between a metal, which readily loses electrons to form a positive cation, and a nonmetal, which readily gains those electrons to form a negative anion, with the resulting oppositely charged ions then held together by strong electrostatic attraction. Ionic compounds like sodium chloride typically form hard, brittle crystalline solids with high melting points, and they conduct electricity when dissolved in water or melted, since the ions become free to move and carry charge, unlike in their fixed solid crystal form.