Question

A friend of yours reads that the process of water freezing is exothermic. This friend tells you that this can't be true because exothermic implies "hot," and ice is cold. Is the process of water freezing exothermic? If so, explain this process so your friend can understand it. If not, explain why not.

Short answer

Yes, the process of water freezing is an exothermic process. This means it releases heat energy during the formation of ice, which makes the surroundings warmer. However, the ice itself stays cold. The term exothermic refers to the release of heat, not the actual temperature of the substance. In the case of water freezing, energy is released as weak liquid hydrogen bonds are replaced by stronger solid hydrogen bonds, transitioning water molecules from a liquid state to a solid state.

Step-by-step solution

Understand Exothermic Reactions

Exothermic reactions are chemical reactions that release energy, usually in the form of heat. This means that during an exothermic reaction, the energy of the products is lower than the energy of the reactants.

Identify Water Freezing as an Exothermic Reaction

When water freezes, it turns into ice. This process involves the release of energy since the water molecules slow down and form bonds in the ice structure. In other words, energy is released as weak liquid hydrogen bonds are replaced by stronger solid hydrogen bonds. Therefore, the process of water freezing is an exothermic process since energy is released in the form of heat.

Explain the Concept to Your Friend

It is true that the process of water freezing is exothermic, meaning it releases energy. However, this doesn't imply that the ice itself is hot, as your friend assumed. During the freezing process, the water releases heat energy, which makes the surroundings warmer while the water itself becomes colder as it turns into ice. Tell your friend that exothermic means "releases heat" and not necessarily "hot." The energy released during the freezing process is responsible for creating stronger hydrogen bonds between the water molecules, thus transitioning the water molecules from a liquid state to a solid state. The released heat energy is absorbed by the surroundings, and the ice remains cold.

Key concepts

Chemical Reactions

A chemical reaction is a process where substances, referred to as reactants, transform into different substances called products. In everyday life, chemical reactions are everywhere: from the rusting of iron, the baking of a cake, to the very metabolic processes that keep us alive.

For students delving into chemistry, understanding a chemical reaction involves observing how atoms and molecules interact, break their old bonds, and form new ones. This transformation can result in a variety of changes, such as the release or absorption of energy, a change in color, or the formation of a gas or precipitate.

The process of water turning into ice is an example of a chemical reaction. Importantly, in this case, the reaction is exothermic because it releases energy to the surroundings. The water molecules, when cooled down enough, slow their movement and rearrange into a structured lattice forming solid ice. This rearrangement involves making new bonds that are more stable, releasing energy in the process.

Energy Release

The concept of energy release in chemical reactions is tied to the exothermic or endothermic nature of the reaction. An exothermic reaction is one in which energy is released to the surroundings, usually as heat, light, or sound. Examples include combustion, like a burning candle, or the rusting of iron.

In contrast, an endothermic reaction absorbs energy from the surroundings. Photosynthesis in plants is an endothermic reaction as it requires energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

The release or absorption of energy occurs because of changes in the arrangements of electrons within the chemical bonds. When water freezes, the energy released is due to the formation of more stable bonds in the ice - as these new bonds lock into place, they release energy, thus defining this process as exothermic. This is observable in the environment - for example, the latent heat of fusion for water, which effectively warms the surroundings as water turns to ice.

States of Matter

When discussing the states of matter, we must focus on the common forms that matter takes: solids, liquids, and gases. These are the forms that every high school student encounters in science class. Each state is defined by distinct characteristics such as shape, volume, and the movement of their particles.

Solids have fixed shapes and volumes due to tightly packed particles that vibrate in place. Liquids have a fixed volume but take the shape of their container because their particles can slide past one another. Gases have neither a fixed volume nor shape, as their particles move freely and rapidly. A plasma is a fourth state of matter, existing at high temperatures as seen in stars, and less commonly discussed in basic chemistry classes.

The transition between these states is known as a phase change, and it is accompanied by the absorption or release of energy. When water freezes into ice, it changes from liquid to solid. As per this phase change, the energetic and freely moving liquid water molecules slow and shift into a more ordered structure, solid ice, releasing energy in the process - an exothermic transition exemplifying the interplay between energy and states of matter.