Bases Introduction: Videos & Practice Problems

Bases are substances that neutralize acids and can be categorized as ionic or covalent. Ionic bases consist of a metal cation paired with basic anions such as hydroxide (OH^-), hydride (H^-), amide (NH₂^-), and oxide (O^2-). Examples include lithium hydroxide (LiOH), calcium hydride (CaH₂), sodium amide (NaNH₂), and barium oxide (BaO). Covalent bases are neutral amines, composed of nitrogen and hydrogen or carbon, nitrogen, and hydrogen. When dissolved in water, bases ionize, releasing cations and basic anions. They are characterized by a bitter taste and slippery feel, akin to soap. Additionally, bases turn red litmus paper blue, indicating their basicity. Understanding these properties of bases, including their reactions and characteristics in aqueous solutions, is important in the study of acid-base chemistry.

Bases and Their Types

A base is defined as any substance capable of neutralizing an acid through a chemical reaction. Bases can be categorized into two main types: ionic bases and covalent bases. Ionic bases are ionic compounds that consist of a metal cation, which is a positively charged metal ion, paired with a basic anion. The common basic anions include hydroxide ions (OH^-), hydride ions (H^-), amide ions (NH₂^-), and oxide ions (O^2-).

For example, when combining these anions with metal cations, we can form various ionic bases. If we take lithium ion (Li⁺), it pairs with hydroxide to form lithium hydroxide (LiOH). In the case of calcium ion (Ca²⁺), which has a different charge, we crisscross the charges to yield calcium hydride (CaH₂). Similarly, sodium ion (Na⁺) combines with amide to create sodium amide (NaNH₂), and barium ion (Ba²⁺) pairs with oxide to form barium oxide (BaO).

On the other hand, covalent bases are primarily represented by neutral amines. Amines are compounds that consist of nitrogen and hydrogen, and may also include carbon. This distinction is important as it highlights the different structures and bonding characteristics of bases. In summary, when identifying a base, one can classify it as either an ionic base, characterized by metal cations and basic anions, or a covalent base, which is typically a neutral amine.

Base Structure Example

In chemistry, bases can be classified into two main categories: ionic bases and covalent bases. Understanding the structure of these bases is crucial for identifying their properties and behaviors.

For example, consider the compound COH₄. This structure does not qualify as an ionic base because it lacks a metal cation; it consists solely of nonmetals. Additionally, it does not represent a neutral base, as it is not an amine. Therefore, this compound is not a suitable example of a base.

Next, we examine an amine, which is characterized by the presence of nitrogen and hydrogen. Neutral amines are typically classified as bases, while positively charged amines tend to exhibit acidic properties. Thus, if the amine in question is positively charged, it would not be considered a base.

On the other hand, the compound represented by option C is a valid example of a base. It contains potassium (a metal) and the hydroxide ion (OH^-), forming potassium hydroxide (KOH). This combination exemplifies a typical ionic base.

Lastly, option D does not qualify as a base either, as it is a covalent compound that begins with hydrogen, indicating it is more likely to behave as an acid.

In summary, among the options provided, only option C accurately represents a typical base, specifically an ionic base, due to its composition of a metal cation and hydroxide ion.

Base Properties

Bases, like acids, exhibit distinct characteristics when dissolved in aqueous solutions. When bases are placed in water, they ionize, releasing hydroxide ions (OH^-) and cations. For instance, sodium hydride (NaH) dissociates into sodium ions (Na⁺) and hydride ions (H^-), while strontium hydroxide (Sr(OH)₂) breaks down into strontium ions (Sr²⁺) and two hydroxide ions (OH^-).

In terms of sensory properties, bases are known for their bitter taste, which can be remembered by associating the letter "B" in "bases" with "bitter." Additionally, bases feel slippery to the touch, a characteristic that can be exemplified by soap, which is a common basic substance used in everyday life.

Litmus paper serves as a useful indicator for identifying bases. When red litmus paper is introduced to a basic solution, it changes color to blue, indicating the presence of hydroxide ions. This reaction is similar to how litmus paper is used to test for acids, but in this case, the transition from red to blue signifies a basic environment.

Understanding these properties of bases—ionization in water, sensory characteristics, and litmus paper reactions—provides a foundational knowledge of their behavior in chemical contexts.

Strong Bases Example

When identifying characteristics of a strong base, it's essential to understand the general properties that define all bases. Unlike acids, which turn blue litmus paper red, bases do not exhibit this behavior; instead, they typically turn red litmus paper blue. A strong base does not release hydrogen ions (H⁺) in solution; rather, it dissociates to produce hydroxide ions (OH^-). For instance, lithium hydroxide (LiOH) ionizes to yield OH^- ions, contributing to its basic properties.

Additionally, bases are known for their distinct taste and tactile sensations. They generally have a bitter taste and feel slippery to the touch, which is a characteristic that can be observed in cleaning supplies, such as soap. This slippery feel is due to the presence of hydroxide ions, which interact with fats and oils, making bases effective in cleaning applications.

In summary, a strong base is characterized by its ability to produce hydroxide ions in solution and its use in cleaning products, along with its bitter taste and slippery feel.

Which of the following compounds will turn a piece of red litmus paper to a bluish color? 
a) HI                                         b) C₆H₅NH₂                               c) CH₃COOH                            d) HOCN

• A. HI
• B. C₆H₅NH₂
• C. CH₃COOH
• D. HOCN

Answer: (B)

Here’s what students ask on this topic:

How does litmus paper identify a base?

Litmus paper is a simple indicator used to determine whether a substance is acidic or basic. When you dip a strip of litmus paper into a solution, the paper changes color due to a chemical reaction between the dye on the paper and the substance in the solution.

For a base (also known as an alkaline substance), litmus paper turns blue. This happens because litmus paper contains dyes derived from lichens that are sensitive to pH changes. In the presence of a base, the molecular structure of the dye changes, resulting in a color shift. If the substance is strongly basic, the change will be immediate and very noticeable. For weaker bases, the color change might be less pronounced, but it will still shift towards blue. Conversely, if the substance were acidic, the litmus paper would turn red. Neutral substances, with a pH of around 7, will not change the color of the paper.

How can a strong base be identified?

A strong base is a substance that can completely dissociate into its ions in an aqueous solution. This means that when a strong base is dissolved in water, it releases hydroxide ions (OH^-) readily, resulting in a high pH value, typically above 7. To identify a strong base, you can look for certain characteristics:

1. Chemical Composition: Strong bases often contain hydroxide ions. Examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
2. Reactivity: They readily react with acids in a neutralization reaction to form water and a salt. This reaction is usually vigorous and exothermic (releases heat).
3. Conductivity: In solution, strong bases are good conductors of electricity due to the high concentration of ions.
4. pH Level: Using pH indicators or a pH meter, a strong base will show a high pH, usually between 12 and 14.
5. Litmus Test: A strong base will turn red litmus paper blue, indicating its basic nature.

Remember, strong bases are corrosive and should be handled with care, using proper safety equipment like gloves and goggles.

How can a base be identified?

A base can be identified through several characteristics and tests. Chemically, bases are substances that can accept hydrogen ions (protons) or donate a pair of valence electrons to form a bond. Here are some common ways to identify a base:

1. pH Level: Bases have a pH greater than 7. Using pH paper or a pH meter can help determine the pH level of a solution.
2. Litmus Test: Bases turn red litmus paper blue, which is a quick and straightforward test.
3. Texture: Many bases feel slippery to the touch due to the saponification reaction with the fats on your skin.
4. Reaction with Acids: Bases react with acids to form salt and water in a neutralization reaction. This is often accompanied by a temperature change.
5. Conductivity: Aqueous solutions of bases conduct electricity because they contain ions.
6. Taste: Though not recommended as a test due to potential hazards, bases generally have a bitter taste.

Always handle bases with care, as they can be corrosive and harmful to skin and eyes. Use appropriate safety equipment when conducting tests involving bases.