Question

State whether each of the following is a physical or a chemical change: (a) zinc metal produces a colorless gas in acid. (b) nickel alloys with copper in a 5 \& coin. (c) liquid helium boils at \(-269^{\circ} \mathrm{C}\). (d) sodium metal reacts with chlorine gas.

Short answer

(a) Chemical change (b) Physical change (c) Physical change (d) Chemical change

Step-by-step solution

Analyze the nature of the change for zinc

When zinc metal produces a colorless gas in acid, a reaction occurs where zinc reacts with the acid to form a new substance, typically hydrogen gas and a zinc salt. This indicates that a chemical reaction is taking place, since new substances are formed.

Determine the type of change for nickel alloying

Alloying involves dissolving one metal into another. When nickel alloys with copper, the process involves physically combining the two metals to form a new mixture, but no new substances are formed. Therefore, this is a physical change.

Examine the process of boiling for helium

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Examine the process of boiling for helium

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Analyze the process of boiling liquid helium

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Analyze the process of boiling liquid helium

Boiling is a phase change from liquid to gas. Liquid helium boiling at -269^{ systdisaeaiup6}COccurs without forming new chemical substances, indicating that this is a physical change.

Examine the reaction of sodium and chlorine

When sodium metal reacts with chlorine gas, a reaction occurs to form sodium chloride, a new compound. This indicates that a chemical change has taken place because a new substance is formed.

Key concepts

Chemical Reactions

A chemical reaction takes place when substances interact to form new products. Unlike physical changes, chemical reactions result in the creation of new substances with different properties. For example, when zinc reacts with acid, a colorless gas is produced — typically hydrogen gas as a byproduct. This change signifies a chemical reaction, as zinc participates in transforming into a zinc salt and produces hydrogen gas.

• New substances are formed
• Often involves energy change (e.g., light, heat, sound)
• Can be reversible or irreversible

Phase Changes

Phase changes refer to the transitions between solid, liquid, and gas states of matter. These changes occur due to variations in temperature and pressure, altering the physical arrangement but not the chemical identity of a substance. Boiling, for instance, is a type of phase change where a liquid turns into gas. When liquid helium boils at -269^{ ext{°C}}, it shifts from liquid to gaseous form without changing its chemical makeup.

• Only physical properties are altered
• Chemical identity remains the same
• Reversible processes

Alloying

Alloying is the process of mixing metals to form an alloy. It is a physical change because it involves the physical combination of metals without creating new chemical substances. When nickel is alloyed with copper, as in the case of a 5 & coin, they dissolve into one another to yield a uniform mixture. This changes the physical properties like hardness and color, while maintaining the original metallic properties.

• Mixture of two or more metals
• Changes physical properties such as strength and ductility
• No new chemical substance is formed

Boiling

Boiling is a process where a liquid turns into vapor when heated to its boiling point. Like other phase changes, boiling is a physical change as there is no alteration in the substance's chemical structure. For example, when water boils, it changes from liquid water to steam, but stays as H₂O. The boiling point, such as -269^{ ext{°C}} for helium, is the temperature at which this transition happens under a given pressure.

• Physical change from liquid to gas
• Temperature-dependent
• Retains the molecular formula