Problem 32 - Chemistry The Molecular Science

Question

In each case, describe the change as a chemical or physical change. Give a reason for your choice. (a) Salt dissolves when you add it to water. (b) Food is digested and metabolized in your body. (c) Crystalline sugar is ground into a fine powder. (d) When potassium is added to water there is a purplishpink flame and the water becomes basic (alkaline).

Step-by-Step Solution

Answer

(a) Physical change; (b) Chemical change; (c) Physical change; (d) Chemical change.

Step 1: Analyze Salt Dissolving in Water

When salt (sodium chloride) is dissolved in water, the ionic bonds between sodium and chloride ions are broken, and these ions interact with water molecules. However, no new substances are formed. Therefore, this is a physical change because the chemical identity of the salt remains unchanged.

Step 2: Analyze Digestion and Metabolism

During digestion and metabolism, complex molecules in food are broken down into simpler molecules through chemical reactions, often involving enzymes. New substances are formed in this process, such as glucose and amino acids. Hence, this is a chemical change.

Step 3: Analyze Grinding of Sugar

When crystalline sugar is ground into a fine powder, the sugar particles become smaller, but the chemical structure of sucrose does not change. Therefore, this change is physical as no new chemical substances are produced.

Step 4: Analyze Reaction of Potassium with Water

When potassium is added to water, a chemical reaction occurs where potassium hydroxide and hydrogen gas are produced, and a purplish-pink flame is observed. The chemical identity of the potassium changes, demonstrating a chemical change.

Key Concepts

Chemical Reactions

When we talk about chemical reactions, we are focusing on processes where one or more substances change into new substances. This transformation involves breaking of old bonds and formation of new ones. During a chemical reaction:

• The chemical identity of the reacting substances is changed.
• New substances are formed with different properties from the original substances.
• Energy is often either absorbed or released, sometimes in the form of heat, light, or sound.

In our example of digestion and metabolism, complex molecules like carbohydrates are broken down through various chemical reactions. As they break down, new molecules such as glucose are formed. This process involves enzymes, which act as catalysts to accelerate the reaction.

Physical Changes

Physical changes are a type of transformation that affects the form of a chemical substance, but not its chemical composition. With these changes, the substances remain the same even though their physical state might change. Here are some characteristics of physical changes:

• The chemical identity of the substance remains unchanged.
• Only the physical properties such as shape, size, color, and state might change.
• These changes are often reversible.

For instance, when salt dissolves in water, the ionic bonds break, but no new substances are formed. Similarly, grinding sugar into powder changes its texture but not its chemical composition. These examples clarify why both are considered physical changes.

Chemical Changes

Chemical changes are transformations that alter the chemical structure of substances involved, leading to the creation of new chemical entities. During a chemical change:

• New substances with new chemical formulas and properties are created.
• This usually involves a visible sign such as color change, gas production, or heat/light emission.
• Energy changes are often noticeable as a part of the process.

An example of a chemical change is when potassium is added to water. It reacts vigorously, producing potassium hydroxide and hydrogen gas. The purplish-pink flame is a visible indicator of this chemical reaction. Thus, the identity of potassium changes, and new substances are formed.

Ionic and Covalent Bonds

In chemistry, ionic and covalent bonds are the two main types of atomic connections that determine how substances interact and transform. These bonds significantly influence the properties of compounds:

• Ionic Bonds are formed when electrons are transferred from one atom to another, creating charged ions. These ions are then attracted to each other, like in table salt (sodium chloride).
• Covalent Bonds occur when atoms share electrons, resulting in the formation of molecules like water (H₂O).

Understanding these bonds helps explain why substances behave the way they do during chemical reactions. For instance, when salt dissolves in water, the ionic bonds break, leading to the interaction and dispersion of sodium and chloride ions in the water, but without forming a new chemical substance.