Question

For food storage, what is the advantage of placing a metal lid on a glass jar? (Hint: Why does running the metal lid under hot water for a minute help you open such a jar?)

Short answer

Answer: The advantage of using a metal lid on a glass jar for food storage is that the metal lid expands and contracts more than the glass jar in response to temperature changes, creating an airtight seal that prevents the entry of air and bacteria, as well as preserving the food's moisture content. Running the lid under hot water causes the metal to expand faster than the glass, loosening the airtight seal and making it easier to open the jar.

Step-by-step solution

Examine the materials' properties

Consider the properties of metal and glass as materials. Metals are good conductors of heat, whereas glass is a poor conductor. This difference in heat conductivity is crucial in understanding the advantage of having a metal lid on a glass jar.

Understand thermal expansion

When objects are heated, they expand due to an increase in the energy of their atoms that causes them to move and vibrate more. This increase in energy corresponds to a growth in the size of the object. Different materials expand at different rates when heated, and metals typically expand more rapidly than glass.

Apply thermal expansion to the jar and lid

Due to the properties of metal and glass, a metal lid will expand more rapidly than a glass jar when they are heated. This difference in expansion rates is crucial to understanding the advantage of having a metal lid on a glass jar.

Explain the advantage of using a metal lid

By having a metal lid on the glass jar, the lid will expand and contract more than the glass jar in response to temperature changes. Specifically, when the jar is filled with hot content and sealed, the lid will expand, creating an airtight seal as the jar's content cools down. This seal helps keep the food fresh by preventing the entry of air and any associated bacteria. Additionally, the airtight seal minimizes the loss of moisture from the stored food, which helps to maintain its quality.

Explain the effect of hot water on opening the jar

If a jar with a metal lid is tightly sealed, it can sometimes be challenging to open due to the strong airtight seal. Running the lid under hot water for a minute heats the metal and causes it to expand more quickly than the glass jar. As a result, the seal loosens, which makes it easier to open the jar.