Problem 22 - Chemistry The Central Science In Si Units

Question

A match is lit to light a candle. The following observations are made: (a) The candle burns. (b) Some wax melts. (c) Melted wax solidifies on the candleholder. (d) Soot (carbon) is produced by the burning of the match and the candle. Which of these occurrences are due to physical changes, and which are due to chemical changes?

Step-by-Step Solution

Answer

(a) and (d) are chemical changes; (b) and (c) are physical changes.

Step 1: Define Physical and Chemical Changes

Physical changes are changes in the state of matter or appearance that do not alter the chemical composition. Common examples include melting, freezing, evaporating, and dissolving. Chemical changes, on the other hand, involve the formation of new substances through chemical reactions, such as burning or rusting.

Step 2: Analyze Candle Burning (a)

The burning of a candle involves the combustion of wax, which is a chemical reaction. New substances, including carbon dioxide, water vapor, and possibly soot, are produced. Therefore, the burning of the candle is a chemical change.

Step 3: Analyze Melting of Wax (b)

When wax melts, it changes from a solid to a liquid, without changing its chemical composition. This is an example of a physical change as it only involves a change in state.

Step 4: Analyze Solidification of Melted Wax (c)

Just like melting, when wax solidifies, it's changing from a liquid back into a solid, but remains chemically the same. This process is a physical change due to the change in state.

Step 5: Analyze Production of Soot (d)

Soot is produced when the match and candle burn, resulting from incomplete combustion of carbon in the wax and match stick. This indicates a chemical change since new products (soot) are formed.

Key Concepts

Combustion

Combustion is a chemical reaction that involves the burning of a substance in the presence of oxygen. In the case of the lit match and candle, combustion leads to the production of new substances. These include carbon dioxide, water vapor, and soot. When a candle burns, the wax, typically made of carbon and hydrogen, reacts with oxygen in the air. This transforms the chemical composition, resulting in combustion.

Some important aspects of combustion include:

• Combustion is an exothermic reaction, meaning it releases energy, often as heat and light.
• The efficiency of combustion influences the amount of soot produced. Incomplete combustion usually results in more soot.
• This process is essential for many everyday applications, such as heating and running engines.

Recognizing combustion as a chemical change helps understand how new substances form and energy emits from burning materials.

Melting and Solidification

Melting and solidification are physical changes. These processes change the state of a substance without altering its chemical composition. In the example of a candle, when it is lit, the heat causes the solid wax to melt into a liquid. This is simply a phase change.

Here are some key features:

• Melting occurs when a solid absorbs heat and becomes a liquid.
• Solidification is the reverse; it happens when a liquid loses heat and turns back into a solid.
• No new substances are formed during these changes.

For the candle, once the flame is extinguished, the liquid wax cools, losing heat and solidifying again. This illustrates a physical change due to the alternation between melting and solidification.

Chemical Reactions

Chemical reactions occur when substances interact and produce one or more new substances. These reactions involve breaking and forming of chemical bonds. In the context of a lit match and candle, several chemical reactions take place.

• When a candle burns, the main reaction is the combustion of wax, forming carbon dioxide and water vapor.
• The soot creation is another example, resulting from incomplete burning of carbon compounds.
• Burning of the match involves further reactions, breaking down its chemicals to add oxygen and produce flame.

Understanding chemical reactions is crucial because they change the identity of substances, unlike physical changes which only alter states or appearance without changing the makeup. This knowledge helps one to distinguish between transformations where the chemical content remains unchanged and those resulting in new product formation.