Question

Classify each of the following as a physical or chemical change or property. a. Milk curdles if a few drops of lemon juice are added to it. b. Butter turns rancid if it is left exposed at room temperature. c. Salad dressing separates into layers after standing. d. Milk of magnesia neutralizes stomach acid. e. The steel in a car has rust spots. f. A person is asphyxiated by breathing carbon monoxide. g. Sulfuric acid spilled on a laboratory notebook page causes the paper to char and disintegrate. h. Sweat cools the body as the sweat evaporates from the skin. i. Aspirin reduces fever. j. Oil feels slippery. k. Alcohol burns, forming carbon dioxide and water.

Short answer

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

Step-by-step solution

a. Milk curdles if a few drops of lemon juice are added to it.

This situation describes a chemical change, as adding lemon juice to milk causes a change in the milk's chemical composition, producing new substances (curd).

b. Butter turns rancid if it is left exposed at room temperature.

This example is a chemical change because the exposure to air and temperature results in the butter's chemical composition changing, producing new substances and altering its properties.

c. Salad dressing separates into layers after standing.

This is a physical change because the separation of the layers does not alter the individual components' chemical composition in the salad dressing.

d. Milk of magnesia neutralizes stomach acid.

This is a chemical change because milk of magnesia reacts with stomach acid, producing new substances with different chemical properties.

e. The steel in a car has rust spots.

This case involves a chemical change because rust is the result of a chemical reaction between the steel and oxygen, producing a new substance (iron oxide or rust).

f. A person is asphyxiated by breathing carbon monoxide.

This is a chemical property since it involves the toxic nature and effect of carbon monoxide on the human body when chemically reacting with hemoglobin.

g. Sulfuric acid spilled on a laboratory notebook page causes the paper to char and disintegrate.

This is a chemical change because the sulfuric acid reacts with the paper, leading to the formation of new substances and the paper's decomposition.

h. Sweat cools the body as the sweat evaporates from the skin.

This is a physical change because the evaporation of sweat does not involve any change in the chemical composition of the sweat, only a change of state from liquid to gas.

i. Aspirin reduces fever.

This situation reflects a chemical property as aspirin's fever-reducing ability is a result of a chemical reaction between aspirin and enzymes in the body.

j. Oil feels slippery.

This example is a physical property because the feeling of slipperiness does not involve any chemical changes. It is an inherent characteristic of oil.

k. Alcohol burns, forming carbon dioxide and water.

This is a chemical change because burning alcohol involves a chemical reaction between alcohol and oxygen, producing new substances like carbon dioxide and water.

Key concepts

Chemical Properties

Chemical properties describe a substance's ability to undergo a specific chemical change. These properties can only be observed when a substance undergoes a chemical reaction. They highlight how a substance interacts with other substances, potentially forming new compounds.
A good example is carbon monoxide's toxic nature. When carbon monoxide is inhaled, it reacts with hemoglobin in the blood. This is a chemical reaction that significantly impacts health, demonstrating the danger associated with this gas.
Similarly, aspirin's ability to reduce fever is a chemical property. The effect occurs due to its interaction with enzymes in the human body. This chemical interaction is what makes aspirin effective as a medication.

Physical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing its chemical identity. These properties encompass aspects like appearance, texture, color, odor, and melting point.
For instance, the slippery feel of oil is a physical property. It does not require any transformation at the molecular level of the oil to be experienced. It's simply an inherent characteristic of the substance.
Similarly, the separation of salad dressing into layers demonstrates a physical property. The act of separation does not involve any change in the chemical constituents of the dressing’s ingredients.

Classification of Changes

In chemistry, changes are classified as either physical or chemical based on whether they alter the chemical composition of the substances involved.
Chemical changes result in the formation of new substances. An example is milk curdling when lemon juice is added. This process changes the milk’s chemical makeup, resulting in the formation of curds.
On the other hand, physical changes do not produce new substances. The evaporation of sweat, which cools the body, is a physical change. It involves a state change from liquid to gas without altering the chemical structure of the substance.

Educational Chemistry

Educational chemistry emphasizes understanding the core principles that govern chemical and physical phenomena. By learning about chemical and physical properties, students can comprehend how substances interact in the world around them.
This knowledge is essential for recognizing changes and their classifications, such as distinguishing between the processes of rust formation on steel as a chemical change and the layering of salad dressing as a physical change.

• Helps predict reactions between different materials.
• Guides safe handling and usage of chemicals.
• Enhances understanding of everyday reactions.

Overall, educational chemistry aims to build foundational scientific literacy, encouraging students to explore and question the natural processes they observe.