Question

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

Short answer

Both (a) and (b) provide evidence of a chemical reaction.

Step-by-step solution

Identify the Characteristics of Chemical Reactions

To determine if a chemical reaction has occurred, look for indicators such as color change, gas formation, temperature change, or the production of light. These indicators imply the formation of new substances.

Analyze Reaction (a): Potassium in Water

Dropping potassium metal into water results in a violet flame. The flame indicates a temperature change and the emission of light, suggesting a chemical reaction. Potassium reacts with water to produce potassium hydroxide and hydrogen gas, which burns with a violet flame.

Analyze Reaction (b): Zinc in Acid

Dropping zinc metal into acid results in the production of gas bubbles. The formation of gas bubbles is a key indicator of a chemical reaction. Zinc reacting with acid produces hydrogen gas, which forms bubbles in the liquid.

Conclusion from Analysis

Both scenarios show evidence of a chemical reaction. In (a), the violet flame is evidence of a chemical reaction; in (b), the gas bubbles indicate a chemical reaction. Both involve reactants forming new products and displaying characteristic signs of chemical reactions.

Key concepts

Evidence of Chemical Reactions

When trying to identify whether a chemical reaction has occurred, we should look for specific signs or evidence. A chemical reaction involves substances known as reactants transforming into new substances, called products.

There are several crucial pieces of evidence that indicate a chemical reaction has taken place:

• **Color change:** The appearance of new colors can indicate that new substances have been formed.
• **Gas formation:** The appearance of gas bubbles or the emergence of a new gas is a common piece of evidence.
• **Temperature change:** A reaction may produce or absorb heat resulting in a temperature change.
• **Emission of light or flames:** Some reactions release energy in the form of light or flames, which is also clear evidence.
• **Precipitate formation:** The appearance of a solid from a solution, known as a precipitate, is another indicator.

These evidences typically imply that a new substance with different properties from the reactants has been formed, signaling a chemical change.

Indicators of Chemical Change

Indicators of chemical change are specific alterations or signs that help us recognize when a chemical reaction has occurred. These indicators make it easier to distinguish simple physical changes from chemical transformations.

Let's delve deeper into a few important indicators:

• **Color Change:** If you notice an unexpected change in color, it is often an indicator of a new substance being formed. For instance, copper turning green when exposed to the air is an example of color change due to a chemical reaction.
• **Gas Bubbles:** When zinc metal is dropped in acid and gas bubbles appear, it indicates the formation of hydrogen gas – evidence that a chemical reaction has taken place.
• **Heat or Light Production:** Sometimes, reactions release energy in the form of heat or light. For example, when potassium metal is placed in water, it can cause a violet flame due to the emission of light during the reaction.

Recognizing these indicators helps us conclude that a chemical reaction has indeed occurred and aids us in understanding the nature of the substances involved.

Reaction Analysis

Reaction analysis involves carefully examining a reaction to determine whether a chemical change has occurred and what characteristics emerged during the reaction. This often involves identifying new products and understanding the transformation process.

When analyzing reactions, you can take steps like:

• **Observing Physical Changes:** Keep an eye out for changes such as color shifts, formation of gas bubbles, or solid precipitates.
• **Identifying Energy Changes:** Determine if the reaction caused temperature changes or emitted light, like a flame.
• **Understanding Product Formation:** Clarify what new substances were produced from the reactants. In our original analysis, potassium reacting with water forming potassium hydroxide and hydrogen gas was a product-driven analysis.

Comprehending these aspects allows you to confirm which chemicals are involved, what they're turning into, and how confidently asserting whether a chemical reaction occurred.