Question

Classify the following as physical or chemical changes. a. Mothballs gradually vaporize in a closet. b. A French chef making a sauce with brandy is able to burn off the alcohol from the brandy, leaving just the brandy flavoring. c. Hydrofluoric acid attacks glass, and is used to etch calibration marks on glass laboratory utensils. d. Calcium chloride lowers the temperature at which water freezes, and can be used to melt ice on city sidewalks and roadways. e. An antacid tablet fizzes and releases carbon dioxide gas when it comes in contact with hydrochloric acid in the stomach. f. Baking soda fizzes if mixed with vinegar. g. Chemistry majors usually get holes in the cotton jeans they wear to lab because of the acids used in many experiments. h. Whole milk curdles if vinegar is added to it. i. A piece of rubber stretches when you pull on it. j. Rubbing alcohol evaporates quickly from the skin. k. Acetone is used to dissolve and remove nail polish.

Short answer

a. Physical b. Chemical c. Chemical d. Physical e. Chemical f. Chemical g. Chemical h. Chemical i. Physical j. Physical k. Physical

Step-by-step solution

Example a: Mothballs gradually vaporize in a closet.

This is a physical change because the mothballs only change from a solid to a gas, but their chemical composition remains the same.

Example b: A French chef making a sauce with brandy can burn off the alcohol from the brandy, leaving just the brandy flavoring.

This is a chemical change because the alcohol undergoes a combustion reaction to produce new substances (carbon dioxide and water), changing its chemical composition.

Example c: Hydrofluoric acid attacks glass, and is used to etch calibration marks on glass laboratory utensils.

This is a chemical change because the reaction between hydrofluoric acid and glass results in the formation of silicon tetrafluoride and water, changing the chemical composition of the glass surface.

Example d: Calcium chloride lowers the temperature at which water freezes, and can be used to melt ice on city sidewalks and roadways.

This is a physical change because calcium chloride only alters the freezing point of water, but there is no change in the chemical composition of the water or calcium chloride.

Example e: An antacid tablet fizzes and releases carbon dioxide gas when it comes in contact with hydrochloric acid in the stomach.

This is a chemical change because the reaction between the antacid and stomach acid produces new substances (water, salt, and carbon dioxide), changing their chemical compositions.

Example f: Baking soda fizzes if mixed with vinegar.

This is a chemical change because the reaction between baking soda and vinegar produces new substances (carbon dioxide, water, and sodium acetate), changing their chemical compositions.

Example g: Chemistry majors usually get holes in the cotton jeans they wear to lab because of the acids used in many experiments.

This is a chemical change because the acids react with the cellulose in the cotton fabric, resulting in the degradation of the fabric's chemical composition.

Example h: Whole milk curdles if vinegar is added to it.

This is a chemical change because the reaction between the milk and vinegar produces new substances (curdled milk proteins and whey), changing their chemical compositions.

Example i: A piece of rubber stretches when you pull on it.

This is a physical change because the rubber only changes its shape when stretched, but its chemical composition remains the same.

Example j: Rubbing alcohol evaporates quickly from the skin.

This is a physical change because the rubbing alcohol only changes from liquid to gas, but its chemical composition remains the same.

Example k: Acetone is used to dissolve and remove nail polish.

This is a physical change because the acetone only changes the state of the nail polish by dissolving it, but the chemical compositions of both substances remain the same.

Key concepts

vaporization

Vaporization is the process where a liquid or solid substance changes into a gas. This is a physical change because, during this process, the chemical identity of the substance doesn't change. A common example is when mothballs vaporize in a closet. The solid mothballs transition into a gaseous state, yet their chemical composition remains unchanged.

Vaporization includes both evaporation and boiling. While evaporation occurs at the surface of a liquid and at temperatures below its boiling point, boiling happens throughout the liquid once reaching a specific temperature and pressure. The key point in vaporization is that despite changing states, the substance still retains its molecular structure.

combustion reactions

Combustion reactions are chemical changes that involve the burning of a substance in the presence of oxygen, producing energy, light, and new products. For instance, when brandy is flambéed in cooking, the alcohol undergoes combustion. It reacts with oxygen, generating heat and resulting in new substances like carbon dioxide and water.

Combustion is a type of exothermic reaction, characterized by releasing energy typically as heat or light. These reactions are critical for various applications, from engines in cars to power plants generating electricity. It's important to differentiate combustion from simply heating a substance, as combustion always results in chemical transformation.

freezing point depression

Freezing point depression is a physical phenomenon where the freezing point of a liquid is lowered by adding another substance. This process doesn't involve a change in the chemical structure of the substance, making it a physical change.

An example is the use of calcium chloride to melt ice on sidewalks. When added to water, calcium chloride disrupts the formation of a solid structure by introducing additional particles between water molecules. This interference makes it more difficult for the molecules to organize into a solid, thus lowering the freezing point of the solution and allowing ice to melt at a lower temperature.

acid-base reactions

Acid-base reactions are chemical processes where an acid reacts with a base, often producing water and a salt. Among the examples is when stomach acid comes in contact with an antacid tablet, leading to a fizzing reaction as carbon dioxide is released.

These reactions are characterized by the transfer of protons ( ext{H}^+) from the acid to the base. A common household example is the reaction between vinegar (acid) and baking soda (base), resulting in the production of carbon dioxide gas along with water and sodium acetate. Acid-base reactions are essential in various applications, from digestive processes in our bodies to balancing pH levels in different environments.