Problem 9 - Biology: The Unity and Diversity of Life

Question

A(n) _______ substance repels water. a. acidic b. basic c. hydrophobic d. polar

Step-by-Step Solution

Answer

Option c. hydrophobic is the correct answer.

Step 1: Understanding the Concept

To solve this exercise, we need to understand the properties of each type of substance mentioned. An acidic substance has a low pH and can donate protons. A basic substance has a high pH and can accept protons. Hydrophobic substances repel water, typically not mixing with or absorbing it. Polar substances have a distribution of electric charge that allows them to interact well with water.

Step 2: Analyzing the Problem

The exercise asks for a type of substance that repels water. This indicates a property of lacking affinity for water, which is characteristic of hydrophobic substances. While acidic, basic, and polar substances all interact with water, hydrophobic substances do not.

Step 3: Choosing the Correct Option

Given that a hydrophobic substance is defined by its tendency to repel and not mix with water, option c. hydrophobic is the correct choice. Options a, b, and d, involve substances that interact with water in some way.

Key Concepts

Acidic Substances

Acidic substances are those that have a sour taste and a pH value less than 7. The lower the pH, the stronger the acidity. Acids are known for their ability to donate a proton ($H^+$ ion) to other substances in a chemical reaction.

Some common examples of acidic substances include:

• Lemon juice
• Vinegar
• Battery acid

Acids play a significant role in various chemical processes. They can break down complex molecules, such as proteins and fats, making them essential in both natural and industrial processes.

It's crucial to handle acidic substances with care, as they can be corrosive and harmful to skin and other materials. In the context of interactions with water, acids dissolve easily, which often results in a release of heat (exothermic reaction). This dissolution is due to their polar nature, which means they have regions of positive and negative charge that water molecules can interact with.

Basic Substances

Basic substances, often called alkalis, exhibit a slippery feel and a bitter taste. They have a pH greater than 7. A base is a substance that can accept protons ($H^+$ ions) or donate an electron pair in reactions. This property is what makes them capable of combating acidity.

Examples of basic substances include:

• Soap
• Baking soda
• Ammonia

Just like acids, bases play a crucial role in both industrial and household domains. They are used in cleaning products due to their ability to neutralize acids and dissolve organic matter.

When bases are mixed with water, they can dissociate into hydroxide ions ($OH^-$). This process increases the concentration of hydroxide ions in the solution making it more alkaline. Despite their slippery nature, caution is advised when handling strong bases as they can cause chemical burns.

Polar Substances

Polar substances are characterized by having an uneven distribution of electric charges across their molecules. This feature means that one end of the molecule is slightly positive, while the other end is slightly negative. Such a polar structure allows these substances to interact very well with water, a polar solvent.

Water itself is the most common example of a polar substance, with other examples including:

• Alcohol
• Ammonia
• Glucose

The ability of polar substances to dissolve in water is due to the attractive forces between the positive and negative charges of the molecules. This solubility is crucial for many biological processes, as many molecules within living organisms are water-soluble polar compounds.

It's this interaction that allows polar substances to dissolve ionic compounds like salts, facilitating chemical reactions necessary for life. Polar molecules, with their water affinity, contrast sharply with hydrophobic substances which repel water.