Understanding Solid $CO_2$ and States of Matter

This question asks us to identify the correct statements regarding the properties of solid carbon dioxide ($CO_2$), commonly known as dry ice, and the general principles governing states of matter.

Analyzing the Statements about Solid $CO_2$

• Statement (i): Solid $CO_2$ is called dry ice.

  This statement is correct. Solid carbon dioxide is widely referred to as "dry ice" because it does not form a liquid when it sublimes (changes directly from solid to gas), unlike water ice which melts into liquid water.

• Statement (ii): Solid $CO_2$ changes directly into gas at 1 atm pressure.

  This statement is correct. At standard atmospheric pressure (1 atm), solid $CO_2$ undergoes sublimation. This means it transitions directly from the solid state to the gaseous state without becoming a liquid. The sublimation temperature of $CO_2$ at 1 atm is approximately $-78.5^\circ C$ (or $194.65 K$).

• Statement (iii): Pressure and temperature can change the state of matter.

  This statement is correct. Changes in temperature and pressure are fundamental factors that influence the state (solid, liquid, gas, plasma) of a substance. For example, increasing temperature can cause melting or boiling, while increasing pressure can cause condensation or freezing.

• Statement (iv): Solid $CO_2$ melts into liquid $CO_2$ before becoming gas.

  This statement is incorrect under the conditions specified implicitly by statement (ii) (i.e., at 1 atm pressure). As explained above, at 1 atm, solid $CO_2$ sublimes directly into gas. Melting into liquid $CO_2$ only occurs at pressures above carbon dioxide's triple point (approximately $5.185$ atm or $525.4 kPa$).

Conclusion on Correct Statements

Based on the analysis, statements (i), (ii), and (iii) are correct descriptions of the properties of solid $CO_2$ and the principles of states of matter at typical atmospheric conditions.

Summary Table

Therefore, the combination of correct statements is (i), (ii), and (iii).