Question

How do we know when a chemical reaction is taking place? Can you think of an example of how each of the five senses (sight, hearing, taste, touch, smell) might be used in detecting when a chemical reaction has taken place?

Short answer

A chemical reaction is a process that transforms chemical substances into new products and can be detected using our five senses. Sight can be used by observing changes in color, gas production, or precipitate formation, like the yellow precipitate in the potassium iodide and lead nitrate reaction. Hearing can be used in reactions like the explosive reaction between hydrogen and oxygen gas, which produces water and a loud "pop" sound. Taste can detect changes in flavor, such as the sour to salty shift when vinegar reacts with baking soda, but should only be used with safe substances. Touch can detect changes in temperature or texture, like the exothermic reaction of calcium chloride and water. Finally, smell can be used to identify new substances with distinct odors, like the oxidation of a sliced apple exposed to air, which results in browning and a different smell.

Step-by-step solution

Define a Chemical Reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. This typically involves the rearrangement of the molecular or ionic structure of the reactants, producing one or more products. Chemical reactions can be observed through a change in color, the production of a gas, the formation of a precipitate, or a change in energy (e.g., heat, light).

Sight (Visual Observation)

Observing a change in color, the appearance of a precipitate (solid), or the production of a gas (bubbles) are visual indicators that a chemical reaction is taking place. Example: When a solution of potassium iodide is mixed with a solution of lead nitrate, it forms a yellow precipitate of lead iodide, indicating a chemical reaction has occurred.

Hearing (Auditory Observation)

Detecting a change in sound during a chemical reaction might be less common, but it can still provide evidence of a reaction. Example: Hydrogen gas reacts explosively with oxygen gas in the presence of a flame or spark, producing water and a loud “pop” sound, indicating a chemical reaction.

Taste (Gustatory Observation)

Taste can sometimes be used to detect a chemical reaction, but it is essential to exercise caution and only use this method with safe, non-toxic substances. Example: Vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide gas and a change in taste from sour (acidic) to salty (neutral).

Touch (Tactile Observation)

Some chemical reactions may produce a noticeable change in temperature or texture that can be detected by touch. Example: When adding water to calcium chloride, the mixture becomes noticeably warmer, indicating an exothermic chemical reaction has taken place.

Smell (Olfactory Observation)

Many chemical reactions produce new substances that have distinct smells, which can be used to detect the occurrence of a chemical reaction. Example: When a sliced apple is exposed to air (oxygen), it undergoes a chemical reaction called oxidation, which turns the apple's surface brown and produces a different smell as the apple's compounds break down.

Key concepts

Visual Indicators in Chemistry

When observing a chemical reaction, the sense of sight is often the most straightforward but powerful tool we possess. A chemical reaction can be visually detected through various changes:

• Color Change: When two reactants mix, and there's a noticeable change in color, it's a classic sign that a chemical reaction is occurring. For instance, the mixing of potassium iodide with lead nitrate results in a vivid yellow precipitate of lead iodide.


• Formation of a Precipitate: This refers to the creation of a solid from a solution. The solid emergence, like the yellow lead iodide, is a visual indicator that molecules are rearranging into a new substance.


• Gas Production: The appearance of bubbles indicates gas being formed, evident in reactions like vinegar with baking soda, which creates carbon dioxide bubbles.


• Change in Energy Output: The emission of light or heat, such as in combustion reactions, can also be visibly observed.

Auditory Indicators in Chemistry

While sound isn’t as often discussed in chemical reactions as sight, auditory indicators can be quite telling. Here are some sound-related clues:

• Sound of Reaction: Some reactions are accompanied by sound, such as fizzing, popping, or cracking. For instance, the loud 'pop' heard when hydrogen gas reacts explosively with oxygen signifies a new substance formation.


• Consistent Sound Patterns: Bubbles in a liquid may crackle or pop, which is a result of gas production, like the effervescence of carbon dioxide gas in fizzy drinks.

Using auditory clues can be especially helpful when visual indicators are minimal or indirect.

Gustatory Indicators in Chemistry

Gustatory or taste-based observations can sometimes indicate chemical reactions, but they must be approached with caution due to safety concerns:

• Change in Flavour: A chemical reaction can alter the taste of substances. For example, mixing vinegar with baking soda changes the sour taste of vinegar into a salty flavor, as the reaction neutralizes the acetic acid.


• Presence of New Flavors: Be cautious with tasting, as not all reaction products are safe or palatable.


• Food Preparation: In cooking, chemical reactions often improve flavor, like how the caramelization of sugar enhances sweetness.

While taste can be informative, prioritize safety and rely on gustatory indicators only when the substances are non-toxic and safe for consumption.

Tactile Indicators in Chemistry

Touching or feeling can provide substantial evidence of a reaction though it should be done safely:

• Temperature Change: Reactions often involve heat absorption or release. The warmth in your hand when water is added to calcium chloride demonstrates an exothermic reaction, where heat energy is released, increasing the mixture's temperature.


• Change in Texture: Sometimes, chemical reactions alter the texture of materials, such as gelatinization or solidification.


• Presence of New Textures: Reactions can produce foam, slime, or other textures depending on the substances involved.

Tactile observations aid in understanding energy changes during reactions, but ensure you’re touching safe substances or use protective equipment.

Olfactory Indicators in Chemistry

The sense of smell often provides subtle yet significant clues about chemical reactions:

• Formation of New Odors: Many chemical reactions release gases that have unique smells. The oxidation of a sliced apple, producing a distinct smell as it turns brown, is one example.


• Distinctive Reaction Smells: Some reactions are notorious for their strong odors, such as sulfur compounds reminiscent of rotten eggs.


• Safety Concerns: Always conduct smell tests cautiously, as some gases can be harmful or abrasive, even at low concentrations.

Utilizing olfactory indicators can be useful for identifying changes in the composition and properties of substances during reactions, just remember to prioritize safety.