Question

Which of these describe physical and which describe chemical properties? (a) Iron has a tendency to rust. (b) Rainwater in industrialized regions tends to be acidic. (c) Hemoglobin molecules have a red color. (d) When a glass of water is left out in the sun, the water gradually disappears. (e) Carbon dioxide in air is converted to more complex molecules by plants during photosynthesis.

Short answer

(a) Chemical property, (b) Chemical property, (c) Physical property, (d) Physical property, (e) Chemical property

Step-by-step solution

Analyze statement A

(a) Iron has a tendency to rust. This describes a chemical property, as the iron reacts with oxygen in the air to form a new substance, namely rust. The process of rusting involves a change in the chemical composition of the iron.

Analyze statement B

(b) Rainwater in industrialized regions tends to be acidic. This again represents a chemical property, as acidity indicates the presence of excess H+ ions which is a sign of chemical change.

Analyze statement C

(c) Hemoglobin molecules have a red color. This is a physical property. The color of a substance can be determined without inducing any chemical change in it.

Analyze statement D

(d) When a glass of water is left out in the sun, the water gradually disappears. This describes a physical process. When water evaporates, it changes from a liquid to a gas, but it still remains water. It's a physical change - no new substance is formed.

Analyze statement E

(e) Carbon dioxide in air is converted to more complex molecules by plants during photosynthesis. This is a chemical property as photosynthesis is a chemical reaction of converting carbon dioxide into sugars by plants.

Key concepts

Physical Properties

Physical properties of a substance are characteristics that can be observed or measured without changing the substance's identity. These properties are usually visible and can be measured without altering the chemical composition. For example, attributes like color, odor, boiling point, melting point, and density are considered physical properties.
They help in identifying and describing substances because they are measurable and comparable between different materials.
In the exercise, the red color of hemoglobin molecules is a physical property. The color can be observed without changing what hemoglobin is, and it remains unchanged during assessments and handling.

Chemical Changes

Chemical changes involve converting one or more substances into new substances with different properties. This happens through chemical reactions, where the chemical structure of the original substance is altered.
Rusting of iron is a good example of a chemical change. Iron reacts with oxygen in the air to form iron oxide, commonly known as rust. This reaction leads to a change in the chemical composition of iron. During chemical changes, new substances are formed, and different properties may emerge from the original components.

Photosynthesis

Photosynthesis is a crucial natural process carried out by plants, algae, and some bacteria to convert light energy into chemical energy. During photosynthesis, chlorophyll in plant cells captures light energy, usually from the sun, to transform carbon dioxide and water into glucose, a sugar, which serves as food for the plant.
Oxygen is released as a by-product of this reaction. This process is fundamental for life on Earth as it supplies oxygen for respiration and organic matter that forms the basis of most food chains. In the context of the exercise, photosynthesis illustrates a chemical change because carbon dioxide is converted into a more complex substance.

Rusting

Rusting is a specific type of corrosion that affects iron and its alloys. It's a slow process where iron reacts with moisture and oxygen from the environment to form iron oxide, commonly known as rust.
This reaction not only changes the appearance of the metal but also affects its structural integrity, making it weaker over time. The process of rusting is a chemical change since it results in the formation of a new substance, altering the original material's chemical makeup. Preventing rust involves protective coatings such as paint or using stainless steel, which resists rust formation.

Evaporation

Evaporation is a common physical process where a liquid turns into a gas. It occurs when molecules at the surface of a liquid gain enough energy to transform into vapor. This is a crucial part of the water cycle, where water from oceans, lakes, and other bodies evaporates into the atmosphere.
In the exercise, leaving a glass of water out in the sun results in evaporation. Although the water changes its state from liquid to vapor, it remains chemically the same (as water). Evaporation is a physical change because it doesn't involve altering the chemical structure of the substance.
This phenomenon occurs at temperatures below the boiling point of the liquid, and factors such as temperature, air pressure, and surface area of the liquid can influence the rate of evaporation.