Physical & Chemical Changes: Videos & Practice Problems

Chemistry focuses on the study of matter and its transformations, which can be classified into physical and chemical changes. Physical changes involve alterations in the physical state without changing the substance's composition, preserving the identity of the material. Examples include dissolving a solute, mixing substances, and altering the shape or form of an object, such as crushing a can. Conversely, chemical changes result in the formation of new chemical bonds, creating new substances with different identities. This includes processes like rusting, burning, and metabolism, which involve breaking and forming bonds, leading to a change in chemical composition.

Matter can undergo reversible changes, where the original substance can be restored, typically through phase changes, dissolving, or other physical changes. These include transitions between solid, liquid, and gas states, such as melting (fusion), freezing, vaporization, condensation, sublimation, and deposition. Irreversible changes, on the other hand, are permanent and cannot be undone under normal conditions. These are often chemical changes where new substances are formed, such as the reaction of nitrogen and hydrogen gases to form ammonia: N₂+3H₂→2NH₃ which cannot be easily reversed to yield the original reactants.

Understanding the distinction between reversible and irreversible changes is crucial in chemistry, as it informs the potential

Physical Changes

Chemistry is fundamentally the study of matter and the transformations it undergoes, which can be categorized into physical changes and chemical changes. Physical changes refer to alterations in the physical state of a substance without any change in its chemical composition. This means that the identity of the original substance remains unchanged throughout the process.

Common examples of physical changes include the dissolving of a solute, which is the substance that dissolves in a liquid. Other significant keywords associated with physical changes are mixing, chopping, cutting, tearing, breaking, and crushing. For instance, when you crush a soda can, it may appear different, but it remains a soda can. This illustrates that while the physical form of the substance changes, its identity does not; it is still the same material in essence.

Understanding these concepts is crucial as they lay the groundwork for distinguishing between physical and chemical changes, which is essential for further studies in chemistry.

Physical Change Example

Understanding the distinction between physical and chemical changes is crucial in chemistry. A physical change is characterized by a transformation that does not alter the original identity of a substance. For instance, when wood burns, it undergoes a chemical change; the resulting ash and soot are entirely different substances, indicating that the identity of the wood has changed. Similarly, iron rusting is a chemical change where iron reacts with oxygen to form iron oxide, fundamentally altering the metal's identity.

Another example is the explosion of dynamite, which also represents a chemical change. The explosion results in new substances formed from the breakdown of chemical bonds, making it impossible to revert to the original materials.

In contrast, dissolving sugar in water exemplifies a physical change. In this process, sugar (the solute) is mixed with water (the solvent) to form a solution. Importantly, the sugar retains its identity; it can be recovered by evaporating the water, demonstrating that no chemical transformation has occurred. Thus, physical changes are defined by the preservation of the original substance's identity, allowing for its recovery in the same form.

Chemical Changes

Chemical changes refer to alterations in the chemical composition of a substance, resulting in the formation of new chemical bonds and the creation of new products. This transformation fundamentally changes the identity of the original substance. A classic example of a chemical change is the rusting of metals, such as iron, which reacts with oxygen in the air to form iron oxide, a metal oxide.

Another common instance of chemical change is the burning of materials. Whether through baking or cooking, these processes involve breaking down old bonds and forming new ones, thereby altering the identity of the original substance. Additionally, the metabolism of food exemplifies a chemical change, as the body processes nutrients through the digestive system, transforming them into different substances essential for bodily functions.

Chemical reactions are central to these changes, where the original substances undergo transformations to create new chemical bonds. Observable signs of chemical changes include color shifts, such as a clear liquid turning blue, and odor changes, where a substance may emit a pleasant smell that becomes unpleasant or vice versa. These visual and olfactory indicators serve as evidence of the underlying chemical transformations occurring.

In summary, a chemical change is characterized by the alteration of a substance's identity through the modification of its chemical bonds and connections, leading to the formation of new substances.

Chemical Change Example

Understanding the distinction between physical and chemical changes is crucial in chemistry. A chemical change occurs when the identity of a substance is altered, resulting in the formation of a new substance. In contrast, a physical change does not affect the chemical composition of the material.

For example, consider the melting of wax. This process transitions wax from a solid to a liquid state, but the substance remains wax throughout, indicating a physical change rather than a chemical one.

In contrast, cooking an egg exemplifies a chemical change. The process involves breaking and forming new chemical bonds, transforming the egg from a clear liquid to a solid with a different appearance and texture. This alteration signifies a change in the substance's identity.

Another example is the condensation of water vapor. When water vapor cools and turns into liquid water, it remains H₂O, regardless of its state. Thus, this process is a physical change, as the identity of the substance does not change.

Lastly, carving a piece of wood is also a physical change. Although the shape of the wood changes, it remains wood throughout the process. No new substances are formed, and the chemical identity is preserved.

In summary, among the examples provided, only cooking an egg represents a chemical change, as it results in a new substance with different properties.

Which of the following is a physical change?

• A. Milk sours when left out of the refrigerator
• B. Tums (containing CaCO₃) neutralizes stomach acid.
• C. Sugar caramelizing when heated on a skillet.
• D. Tearing a piece of paper.

Answer: (D)

Reversible Changes in Matter

Reversible and irreversible changes are fundamental concepts in understanding the behavior of matter. A reversible change is defined as a transformation that can be undone, allowing the original substance to be restored. Common examples of reversible changes include phase changes, dissolving of compounds in liquids, and various physical changes. For instance, carbon dioxide (CO₂) can transition from a solid state (denoted as s) to a gaseous state (denoted as g) without altering its chemical identity; it remains carbon dioxide throughout the process.

Phase changes are a prevalent type of reversible change, involving transitions between different states of matter. When discussing bond formation, molecules come closer together, leading to states such as condensation, where a gas transforms into a liquid. This is commonly observed when water vapor condenses on cold surfaces. Another example is freezing, where a liquid turns into a solid, such as water in an ice tray. Additionally, a gas can directly convert into a solid through a process known as deposition.

Conversely, breaking bonds can also occur through phase changes. For example, melting (or fusion) describes the transition from solid to liquid, while evaporation or vaporization refers to the change from liquid to gas. Sublimation is another process where a solid directly transforms into a gas. It is crucial to note that during these reversible changes, the identity of the substance remains unchanged, allowing for the original material to be restored.

In contrast, irreversible changes result in the formation of new substances that cannot be reverted to their original state. Understanding these concepts is essential for grasping the principles of chemistry and the behavior of matter in various conditions.

Irreversible Changes in Matter

Irreversible changes are permanent transformations that cannot be undone, meaning the original structure cannot be restored. In the context of chemistry, these changes often involve chemical reactions that alter the identity of the substances involved. A prime example is the reaction between nitrogen gas (N₂) and hydrogen gas (H₂), which combine to form ammonia (NH₃). This process can be represented by the equation:

$$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$$

In this reaction, the nitrogen and hydrogen gases chemically bond to create ammonia, resulting in a new substance. Once this transformation occurs, it is not possible to revert to the original nitrogen and hydrogen gases under normal conditions, highlighting the irreversible nature of the process. Thus, irreversible changes create entirely new materials and permanently alter the original substances, emphasizing the one-way direction of these chemical reactions.

Reversible vs Irreversible Example

Understanding the distinction between reversible and irreversible processes is crucial in the study of physical and chemical changes. Reversible changes are typically associated with physical changes, where the identity of the original material remains intact. For instance, when sugar is dissolved in water, it represents a physical change. This process is reversible because the sugar can be recovered by boiling off the water, leaving the original sugar behind.

In contrast, irreversible changes are linked to chemical changes, where the original substances undergo a transformation that alters their identity. A clear example of this is the addition of lemon juice to cabbage juice, which causes a color change from purple to deep red. This color change signifies a chemical reaction, making it an irreversible process.

Another example involves adding citrus to baking powder. This interaction produces fizzing and bubbling, indicating a chemical reaction is occurring. Since this reaction results in new substances being formed, it is classified as an irreversible change.

On the other hand, mixing oil and water exemplifies a physical change. These two substances do not chemically interact; instead, they maintain their individual identities. This lack of interaction means that the process is reversible, as the original substances can be separated without any change to their chemical structure.

In summary, physical changes, such as dissolving and mixing, are generally reversible, while chemical changes, characterized by color changes and reactions, are irreversible. Recognizing these key terms and concepts is essential for understanding the nature of changes in matter.

Which of the following represents a reversible change?

• A. Melting a chocolate bar then re-solidifying it in the freezer.
• B. Changing water vapor into oxygen and hydrogen gas.
• C. Turning a potato into french fries.
• D. Baking a cake from a recipe.

Answer: (A)

Here’s what students ask on this topic:

What is the difference between physical and chemical changes?

Physical changes and chemical changes are two types of processes that materials can undergo. A physical change involves a change in the physical properties of a substance, but its chemical identity remains the same. Examples of physical changes include changes in state (like ice melting into water), tearing a piece of paper, or stretching a rubber band. These changes are generally reversible.

On the other hand, a chemical change results in the formation of one or more new substances with different chemical properties. This type of change involves making or breaking chemical bonds, and it often results in a change in color, the production of a gas, a change in temperature, or the formation of a precipitate. Examples include rusting iron, burning wood, or baking a cake. Chemical changes are typically not easily reversible.

Understanding the difference between these changes is important in chemistry because it helps in predicting the behavior of materials and the conservation of matter in different reactions and processes.

What are physical changes in chemistry?

Physical changes in chemistry refer to alterations in the form or appearance of a substance without changing its chemical composition. These changes are generally reversible. Examples include changes in state (like ice melting into water), size (such as cutting a piece of paper), shape (bending a metal rod), or texture (crushing a can). During a physical change, the molecules of the substance rearrange, but the actual structure of the molecules—the atoms they're made of and the way those atoms are bonded together—remains the same. This means that a physical change affects only physical properties, such as density, color, ductility, hardness, or conductivity. Since the identity of the substance doesn't change, physical changes are often used in separation techniques like filtration, distillation, or chromatography to isolate or purify substances.

What is a chemical change in chemistry?

A chemical change in chemistry refers to a process where substances combine or break apart to form new substances with different chemical properties. This transformation involves the making or breaking of chemical bonds, leading to a change in the molecular composition of the substances involved. Unlike a physical change, which only affects the form of a material without altering its underlying structure, a chemical change results in the production of one or more new substances.

Examples of chemical changes include the rusting of iron, the combustion of gasoline, the baking of bread, and the digestion of food. These changes are often irreversible and can be accompanied by observable signs such as the release of gas, a change in color, the formation of a precipitate, or the emission or absorption of heat. Identifying a chemical change is crucial in understanding chemical reactions, which are fundamental to the study of chemistry and the processes that occur in living organisms and the environment.