Question

Classify each change as physical or chemical. a. Sugar burns when heated on a skillet. b. Sugar dissolves in water. c. A platinum ring becomes dull because of continued abrasion. d. A silver surface becomes tarnished after exposure to air for a long \(\quad\) period of time.

Short answer

a. Chemical change, b. Physical change, c. Physical change, d. Chemical change.

Step-by-step solution

Identify Change Type for Sugar Burning

To determine if sugar burning is a physical or chemical change, consider whether the substance's chemical composition is altered. Burning sugar results in caramelization and combustion, producing new substances such as carbon and water vapor. Since new chemicals are formed, it's a chemical change.

Identify Change Type for Sugar Dissolving in Water

Dissolving sugar in water involves mixing sugar molecules with water molecules. The chemical structure of sugar does not change; it is just dispersed in water. Thus, this is a physical change because no new substances are formed.

Determine Change Type for a Platinum Ring Becoming Dull

A platinum ring becoming dull due to abrasion is an example of a physical change. The dullness comes from the physical wearing away of the metal, not from any change in the chemical composition of the platinum.

Determine Change Type for Silver Tarnishing

When silver tarnishes, it reacts with sulfur compounds in the air to form silver sulfide, a black substance on the surface of the silver. This process changes the chemical makeup of the silver's surface, indicating that it is a chemical change.

Key concepts

Chemical Change

When we talk about a chemical change, we're referring to a process where the substances involved undergo a transformation that leads to the formation of new substances with different chemical properties. This type of change is fundamental, and the original substances lose their previous characteristics and form new compounds with their own unique properties. For example, when sugar is heated on a skillet, it doesn't just melt; it undergoes caramelization and combustion, creating entirely new substances like carbon and water vapor. This is clearly a chemical change because the sugar's molecular structure is altered, resulting in something new and different.

Indicators of Chemical Changes

• Change in color
• Formation of gas
• Change in temperature
• Formation of a precipitate

In the case of sugar burning, you can see a change in color to dark brown or black, possibly the release of gases, and a change in temperature, all indicating a chemical change.

Physical Change

A physical change differs from a chemical change as it involves a change in a substance's form or physical properties without altering its chemical identity. When sugar dissolves in water, the molecules spread out within the water but remain chemically the same. No new substances are created; the sugar can even be retrieved by evaporating the water. The same can be said for a platinum ring becoming dull due to abrasion. The ring may look different, but it is still made of platinum atoms. The substance's essence remains unchanged, making it a quintessential example of a physical change.

Characteristics of Physical Changes

• Reversible changes
• No new substances formed
• Change in state or shape

Physical changes include phase changes like ice melting into water or water evaporating into vapor, as well as dissolving, where substances like salt or sugar dissolve in liquids without chemical reactions.

Dissolving Sugar in Water

Understanding the concept of sugar dissolving in water can be quite simple. Imagine tiny sugar crystals entering the water and separating from each other, spreading out amongst the water molecules. The sugar is still sugar, and the water is still water; they are just mixed. This mixing is a perfect example of a physical change because no new product results from the sugar's interaction with water. The solution we get is physically different - it’s sweeter and maybe more viscous - but chemically it’s the same water and sugar, just in different forms.

Reversibility of Dissolving

Since dissolving is a physical change, it’s typically reversible, although sometimes it requires specific conditions. For sugar in water, simply evaporating the water will leave behind the sugar crystals. This reversibility further underscores the fact that no chemical change has taken place during the dissolving process.

Tarnishing of Silver

Tarnishing of silver is an interesting phenomenon that illustrates a chemical change. Over time, silver objects exposed to air will develop a dull, gray, or black coating. This is because silver reacts with sulfur-containing substances in the air, particularly hydrogen sulfide, to form silver sulfide. This new compound creates a surface layer that's chemically different from the underlying silver. The appearance of tarnish is an indicator of a chemical change - the silver's shiny surface has been transformed into a substance with different properties. Unlike the dissolving of sugar in water, tarnish cannot be simply washed away to restore the silver. The process of restoring tarnished silver often involves a chemical reversal called 'polishing,' which essentially removes the layer of silver sulfide, restoring the silver's lustrous appearance.

Prevention and Treatment of Tarnish

• Store silver in airtight containers to prevent exposure to air.
• Regularly clean silver with a polish that chemically reverses tarnishment.
• Use silica gel packets or other absorbents to minimize humidity and sulfur compounds in storage areas.

Treatment after tarnishing does not just change the appearance but involves a chemical process to convert the silver sulfide back to silver, showcasing the fundamental distinction between physical and chemical changes.