Question

Which of the following is evidence for a chemical reaction? (a) Dropping sodium metal into water gives a yellow flame. (b) Dropping iron metal into acid produces gas bubbles.

Short answer

Both scenarios are evidence of a chemical reaction.

Step-by-step solution

Understanding Chemical Reactions

A chemical reaction involves a change in the composition of substances, resulting in new substances. Evidence for a chemical reaction includes temperature change, color change, gas production, formation of a precipitate, or emission of light.

Analyzing the Sodium and Water Reaction

When sodium metal is dropped into water, it reacts vigorously. The yellow flame indicates an exothermic reaction and light emission, suggesting a new substance is formed. This is evidence of a chemical reaction.

Analyzing the Iron and Acid Reaction

Dropping iron into acid results in the production of gas bubbles, likely hydrogen gas. The formation of gas bubbles indicates a new substance is being formed, thus confirming a chemical reaction.

Comparing Both Reactions

Both reactions involve observable indicators of chemical reactions: light emission in the sodium and water reaction, and gas production in the iron and acid reaction. Both provide evidence of chemical reactions occurring.

Key concepts

Evidence of Chemical Reactions

Chemical reactions are processes where substances interact to form new products. You can tell a chemical reaction has occurred through various types of evidence. Some common indicators include:

• Color Change: A noticeable change in color signals a chemical reaction as new substances form.
• Temperature Change: Reactions can either release or absorb heat, causing a change in temperature.
• Light Emission: Some reactions emit light, showcasing energy transformation.
• Gas Production: The formation of bubbles or fumes indicates chemical change.
• Precipitate Formation: This occurs when a solid forms in a previously clear solution.

Noticing these signs during an experiment is often a direct indication that a chemical reaction is taking place.

Exothermic Reactions

Exothermic reactions are chemical processes that release energy in the form of heat or light. When you see elements reacting and giving off heat or light, such as in a burning flame, you are witnessing an exothermic reaction.

How Exothermic Reactions Occur

During a reaction, bonds between atoms in the reactants are broken, and new bonds are formed to create the products. If the internal energy of the products is lower than that of the reactants, the excess energy is released as heat or light. In the example of sodium and water, the yellow flame indicates that energy is being released. This release of energy not only signals an exothermic reaction but also suggests a substantial shift from reactants to new substances.

Gas Production

The presence of gas bubbles is a strong indicator of a chemical reaction occurring. When a reaction produces gas, bubbles will often appear as the gas escapes into the atmosphere. This is particularly evident in reactions involving metals and acids.

Understanding Gas Production

In the case of iron reacting with an acid, the appearance of gas bubbles occurs due to the release of a gas, such as hydrogen. Hydrogen gas is created as a result of the acid reacting with the metal, breaking down and forming new substances. Gas production is a clear and tangible sign of chemical reactions and can be easily observed in laboratory experiments as visual evidence.

New Substance Formation

A fundamental outcome of any chemical reaction is the creation of substances that differ from the initial reactants. This formation of new substances characterizes the transformation process.

Identifying New Substances

When observing reactions, like sodium in water or iron in acid, the outcome is new chemical products formed from the reactants.

• In sodium and water: the formation of sodium hydroxide and hydrogen gas signals new substance creation.
• In iron and acid: the presence of iron salt and hydrogen gas are new products resulting from the reaction.

Understanding the formation of new substances helps confirm that a chemical reaction has indeed occurred. It's this transformation that underpins the study and understanding of chemistry.