Question

State whether each of the following represents a chemical change or a physical change: (a) A steak is cooked on a grill until well done. (b) In the lab, students firepolish the end of a glass rod. The jagged edge of the glass becomes smooth. (c) Chlorine bleach is used to remove a coffee stain on a white lab coat. (d) When two clear and colorless aqueous salt solutions are mixed together, the solution turns cloudy and yellow. (e) One gram of an orange crystalline solid is heated in a test tube, producing a green powdery solid whose volume is 10 times the volume of the original substance. (f) In the lab, a student cuts a 20 -cm strip of magnesium metal into \(1-\mathrm{cm}\) pieces.

Short answer

a) Chemical change, b) Physical change, c) Chemical change, d) Chemical change, e) Chemical change, f) Physical change.

Step-by-step solution

Analyze Cooking of Steak

Cooking a steak involves chemical reactions that change the molecular structure of the proteins and fats. This process is a chemical change as new substances are formed.

Evaluate Firepolishing Glass

Firepolishing glass is a process where heat is applied to a glass rod to smooth out its jagged edges. This is a physical change as the glass's composition remains the same and only its shape changes.

Assess Bleach Removing Stains

Using chlorine bleach to remove a coffee stain involves a chemical reaction where the bleach breaks down the molecules causing the stain. This is a chemical change as the bleach reacts with the stain to produce new substances.

Inspect Mixing Aqueous Salt Solutions

When two clear and colorless aqueous salt solutions are mixed and the resulting solution turns cloudy and yellow, this indicates a chemical reaction where a new substance (possibly a precipitate) is formed. Thus, it is a chemical change.

Determine Heating Crystalline Solid

Heating an orange crystalline solid that results in a green powdery solid with significantly more volume is a chemical change. This is due to a reaction and the formation of new substances with different properties.

Examine Cutting Magnesium Metal

Cutting a strip of magnesium metal into smaller pieces is a physical change as it alters the shape and size but not the composition of the magnesium itself.

Key concepts

Chemical Change

Understanding the concept of a chemical change is fundamental in chemistry, as it involves the transformation of substances into new and different products. A key characteristic of a chemical change is the reorganization of atoms and the creation of new chemical bonds.

Examples include the cooking of a steak, where the heat induces chemical reactions, transforming the nutrients and proteins into different compounds. Similarly, when chlorine bleach acts on a stain, it's not just removing the color but chemically altering the molecules of the stain, making it invisible or turning it into different substances that can be washed away.

In both instances, the identity of the original matter changes and new substances are formed, a hallmark of chemical change. This process often involves energy changes, color changes, the formation of gas or precipitate, or an irreversible reaction. Emphasizing this concept in educational materials ensures students can identify and differentiate chemical changes from physical ones in various contexts.

Physical Change

A physical change contrasts with a chemical change by tweaking a substance's physical properties without affecting its chemical identity. These adjustments might include changes in the state of matter, such as solid to liquid, or alterations in size or shape.

For example, firepolishing glass or cutting magnesium into smaller pieces exemplify physical changes. The glass's edges become smooth, and the magnesium is divided, yet their internal structures remain unaltered. These processes are reversible, and no new substances form as a result. The magnesium’s composition remains constant, albeit in smaller pieces.

In chemistry education, understanding these changes aids students in recognizing that not all changes are inherently chemical. Engaging with concepts like melting, boiling, dissolving, or breaking materials helps solidify their grasp of physical properties and the nature of substances.

Chemistry Education

The pedagogical approach to chemistry education centers on imparting a clear and thorough understanding of the concepts of chemical and physical changes. Educators strive to make these abstract ideas tangible through experiments, demonstrations, and real-life examples.

Effective chemistry education uses exercises like the textbook problem above to encourage analytical thinking. When students examine the outcomes of cooking a steak or mixing two solutions, they can draw a connection between theory and practice. This intersection is where genuine comprehension occurs. Emphasizing the importance of observation and critical analysis when exploring the properties of matter can ensure that pupils not only memorize facts but also develop a deeper understanding of chemical reactions and processes. This foundational knowledge is crucial for students as they progress in their study of chemistry.

Properties of Matter

The properties of matter are integral to distinguishing between chemical and physical changes. These properties fall into two broad categories: physical properties, such as color, density, and melting point, and chemical properties, related to a substance's ability to form new substances.

For example, the heating of an orange crystalline solid hints at both physical and chemical changes; the solid becomes a green powder with a different volume - a clear indication of a chemical change. The alteration of chemical properties results in a substance with new characteristics.

Incorporating this concept into chemistry education can ensure that learners understand how and why different materials behave under various conditions. Knowledge of matter's properties also sets the stage for topics like stoichiometry, thermodynamics, and reaction kinetics, establishing a solid foundation for students in advanced chemistry topics.