Which of the following statements describe physical properties 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.

Understanding the nature of matter is foundational in science. Matter has intrinsic characteristics known as properties, which can be categorized into two main types: physical and chemical properties. Physical properties are those that can be observed or measured without changing the composition of the substance; these include aspects such as state (solid, liquid, gas), color, melting point, boiling point, and density. For example, the red color of hemoglobin is a physical property because it describes its appearance without altering its structure.

On the other hand, chemical properties involve the substance's ability to form new substances and describe how a material reacts with other matter. A classic example is iron's tendency to rust when exposed to oxygen and moisture; here, the iron undergoes a transformation into a different compound, iron oxide. Recognizing whether a property is physical or chemical is central to understanding how substances will interact with the environment or change under different conditions.

Chemical reactions are processes where the molecular structure of a substance is altered, resulting in the formation of one or more different substances. These reactions involve the breaking and forming of bonds between atoms, and they are often accompanied by observable signs such as energy changes, color changes, or the formation of a precipitate or gas.

For instance, when rainwater becomes acidic, it's a consequence of chemical reactions between water and pollutants in the air, such as sulfur dioxide and nitrogen oxides. This reaction produces acids that dissolve in the rain. Similarly, photosynthesis is a complex chemical reaction where plants convert carbon dioxide and water into glucose and oxygen with the aid of sunlight. Chemical reactions are at the heart of countless processes both in nature and industry, encompassing everything from the metabolism in our bodies to the synthesis of new materials.

When a substance undergoes a physical change, its form or appearance may be altered, but its chemical composition remains constant. Some commonplace examples of physical changes include changes in state, such as ice melting into water or water evaporating into steam. When the textbook mentions the evaporation of water when left out in the sun, it demonstrates a physical change—from liquid to gas—without any change in its chemical identity, as the molecules remain H₂O.

Physical changes are typically reversible, and the original properties of the material can be restored, like refreezing melted ice. Being able to distinguish physical changes from chemical ones is key in understanding material interactions and predicting the outcomes of specific conditions on substances.

Photosynthesis is not only a fascinating chemical reaction but also an essential biological process that sustains the life of plants and, by extension, the ecosystems dependent on them. Through photosynthesis, plants manage to capture energy from the sun and use it to transform carbon dioxide and water into glucose and oxygen. This complex process, which predominantly takes place in the chloroplasts of plant cells, consists of multiple steps including the light-dependent reactions and the Calvin cycle.

It's important to note that photosynthesis is not merely a mechanism for plants to produce food for themselves but also serves as the basis for the flow of energy through food chains. When we consume plants, or consume animals that have eaten plants, we are tapping into the energy initially captured during photosynthesis. Therefore, understanding photosynthesis provides insight into energy transfer in ecosystems and the crucial role plants play in maintaining life on Earth.