In chemistry, what is commonly referred to as dry ice?

Introduction / Context:
Dry ice is a popular term used in everyday life as well as in chemistry, especially for demonstrations involving fog and cooling. It is used as a refrigerant and for creating special effects because it sublimates directly from solid to gas. Knowing what substance dry ice is made from helps link common names to chemical identities and clarifies its physical properties and safety aspects.

Given Data / Assumptions:

• The term dry ice refers to a solid substance used for cooling and producing fog.
• Options include solid carbon dioxide, solid nitrogen dioxide, solid sulfur dioxide, ordinary ice, and solid ammonia.
• We assume standard definitions used in chemistry and industry.
• The focus is on the widely accepted commercial and chemical meaning of dry ice.

Concept / Approach:
Dry ice is the solid form of carbon dioxide, obtained by compressing and cooling carbon dioxide gas until it liquefies, then allowing it to expand and freeze. It is called dry because it does not melt into a liquid under normal atmospheric pressure but sublimates directly into carbon dioxide gas. Ordinary ice is solid water and becomes liquid on melting, so it does not behave like dry ice. Other gases such as nitrogen dioxide and sulfur dioxide have different solid forms and are not called dry ice in common usage.

Step-by-Step Solution:
Step 1: Recall that dry ice is very cold and creates dense white fog when placed in warm water, used in stage effects and cooling. Step 2: Its chemical identity is solid carbon dioxide, so its formula is CO2 in solid state. Step 3: When dry ice warms, it does not form a liquid at ordinary atmospheric pressure but directly sublimates to carbon dioxide gas. Step 4: This behaviour is why it is called dry ice, since no liquid water is formed, and the surroundings remain dry. Step 5: Solid water, which is ordinary ice, melts to form liquid water, and therefore does not fit this description. Step 6: Nitrogen dioxide, sulfur dioxide, and ammonia have their own solid forms but are not used or named as dry ice in everyday practice.

Verification / Alternative check:
Commercial suppliers and safety data sheets describe dry ice as solid carbon dioxide used for refrigeration and transport of perishable goods. Chemistry demonstrations that show sublimation of dry ice always explain that the fog seen is water vapour condensing in cold carbon dioxide gas. Reference books and exam guides consistently define dry ice as the solid form of carbon dioxide. These observations confirm that solid carbon dioxide is the correct answer.

Why Other Options Are Wrong:
Solid nitrogen dioxide: Nitrogen dioxide is a brown gas that can form dimeric solids, but these are not called dry ice and are not used for common cooling applications.
Solid sulfur dioxide: Sulfur dioxide is a gas with a pungent smell and is not sold as dry ice; its solid form is not widely used as a refrigerant.
Solid water (ice): Ordinary ice is simply frozen water and melts to liquid, so it does not behave like dry ice and is not given that name.
Solid ammonia: Solid ammonia exists at very low temperatures and is not the substance referred to as dry ice in common or industrial usage.

Common Pitfalls:
Some learners may confuse ordinary ice with dry ice because both are cold solids. Others might guess any gas that sounds familiar without remembering the exact definition. To avoid these mistakes, link the name dry ice firmly with carbon dioxide and recall the key property that it sublimates directly from solid to gas without forming a liquid at normal pressure.

Final Answer:
Dry ice is the solid form of carbon dioxide.