Question

Daisy was conducting an investigation using two liquids. Both liquids were clear and were at the same temperature, \(25^{\circ} \mathrm{C}\). When she combined the liquids in a large beaker, gas bubbles were produced and the liquid turned blue. The student took the temperature of the beaker, which now measured \(27^{\circ} \mathrm{C}\). All of the following show that a chemical reaction may have occurred EXCEPT (A) a temperature change occurred. (B) a color change occurred. (C) the two liquids mixed together. (D) a gas was produced.

Short answer

The correct answer is (C) the two liquids mixed together, as this option does not necessarily indicate a chemical reaction has occurred. The mixing of two liquids can be a physical change and not always a chemical change.

Step-by-step solution

Analyze Option (A)

Option (A) mentions a temperature change occurred. In a chemical reaction, the breaking and forming of chemical bonds are generally associated with the absorption or release of energy, often in the form of heat. Therefore, a temperature change may indicate that a chemical reaction has taken place.

Analyze Option (B)

Option (B) points out a color change occurred. A color change can be a result of a new substance being formed during a chemical reaction, suggesting that this option also indicates that a chemical reaction may have occurred.

Analyze Option (C)

Option (C) states that the two liquids mixed together. This option does not necessarily indicate a chemical reaction has occurred, as physical mixing of liquids doesn't always lead to chemical changes. Two liquids can be mixed together, and sometimes a mixture can be formed without changes in their chemical bonds.

Analyze Option (D)

Option (D) tells us a gas was produced. Production of a gas, such as the formation of gas bubbles, is a sign that a chemical reaction might have taken place. A new substance, in this case, a gaseous one, is formed as a result of the reaction.

Choose the Correct Answer

Based on our analysis, Option (C) is the one that does not necessarily indicate that a chemical reaction has occurred. The mixing of two liquids can be a physical change, and not always a chemical change. Therefore, the correct answer is (C) the two liquids mixed together.

Key concepts

Temperature Change

When Daisy combined the two liquids, she observed a temperature change from \(25^{\circ} \mathrm{C}\) to \(27^{\circ} \mathrm{C}\). This is an important clue hinting that a chemical reaction may have occurred. During chemical reactions, energy is either absorbed or released.

This change in energy often appears as a change in temperature. For instance, if the reaction releases heat, we call it an exothermic reaction, which raises the temperature of the mixture. On the other hand, if the reaction absorbs heat, it's an endothermic reaction, leading to a temperature drop. In Daisy's experiment, the temperature increase suggests an exothermic process took place.

Temperature changes can be quick indicators of a reaction and help identify whether energy changes are significant enough to alter the physical conditions of the substances involved. It serves as a practical observation method in many experiments.

Color Change

In Daisy's experiment, the clear liquids turned blue when mixed. This kind of color change is one of the most straightforward signs of a chemical reaction.

Why does this happen? A color change occurs because new substances form that possess different color properties than the original substances. When a reaction entails the formation of new compounds, these often have different arrangements of atoms, affecting how they absorb and reflect light.

Color changes are usually quick and easy to observe without any special equipment, allowing for immediate conclusions about the occurrence of a reaction. They are significant because they show new products have formed, confirming the occurrence of a chemical chemical process.

Gas Production

As Daisy combined the two liquids, she noticed gas bubbles emerging from the mixture. Producing gas is another strong indicator that a chemical reaction is happening. Here’s why it's significant:

When substances react to produce a gas, the atoms rearrange to create a new compound in a gaseous state. This is especially noticeable when reactions involve liquids or solids, since the formation of a gaseous product often causes bubbling or fizzing, as seen in Daisy's experiment. Examples of such reactions include vinegar and baking soda, which produce carbon dioxide gas.

These visible gas bubbles suggest the formation of a completely new substance that wasn't present before. Gas production can often be paired with other reaction clues, such as a temperature or color change, which together strongly suggest that a chemical change has occurred.