Acid-Base Introduction: Videos & Practice Problems

Acids are covalent compounds with a hydrogen ion and a nonmetal or polyatomic anion, typically ionizing in water to release H⁺ ions, resulting in a sour taste and a reaction that turns blue litmus paper red. Bases, ionic compounds with metal cations and hydroxide ions (OH^-), ionize in water, producing a bitter taste and slippery feel, turning red litmus paper blue. Understanding these properties is essential for recognizing acid-base indicators and their roles in chemical reactions.

Acid-Base Introduction Concept 1

Now recall that acids are covalent compounds that have a hydrogen ion connected to a nonmetal anion, so a negative ion, or a polyatomic ion. Generally, the hydrogen ion is found at the beginning of the compound except for acetic acid. Here we have examples of some common types of acids: hydrochloric acid, hydrosulfuric acid, hydrocyanic acid, phosphoric acid, and acetic acid. Remember, when we talked about acids, we noted that acetic acid is one example where the hydrogen is not at the beginning but at the end of the compound.

Acid-Base Introduction Example 1

Which of the following does not represent the possible structure of an acid? So remember, we said that generally speaking, when it comes to an acid, it's a covalent compound that has the hydrogen ion located at the beginning of the compound. The exception to this is acetic acid. If we take a look here, this is covalent with the hydrogen in the beginning, and that's because it's hydrobromic acid. Here we have a covalent compound with hydrogen in the beginning, so here this is iodic acid, and over here we have hypochlorous acid, covalent compound with a hydrogen in the beginning.

The answer is c. This is CH₄, also known as methane. It is not an acid. The hydrogen is not found in the beginning of this covalent compound, and by the definition we used earlier, it would not be classified as an acid. Now, there are other technicalities that make it not an acid, but just go by that simple definition.

Acid-Base Introduction Concept 2

Now, acids belong to a distinct class of covalent compounds because of their characteristics in aqueous solutions. Some characteristics of acids include dissolution, taste, reactivity, and how they react with litmus paper.

For dissolution, acids ionize when placed in water. An example is hydrochloric acid, which ionizes in water breaking up into its H+ ion, which is aqueous, plus the chloride ion, which is also aqueous. For sulfuric acid, containing two H+ ions in this compound, it breaks up into 2 H+, aqueous, plus SO42-, aqueous.

For taste, the presence of + ions gives acids a sour taste, as can be observed in natural citrus fruits like lemons and oranges, which have a sour taste because of the natural acids found within them.

Concerning reactivity, acids react with metals to form H₂ gas. A good example is hydrochloric acid reacting with magnesium solid. In this reaction, magnesium and chlorine react together to form MgCl₂, and the hydrogen becomes H₂ gas.

Finally, for litmus paper, a type of paper that changes colors in response to an acid or a base, an acid will change blue litmus paper to red. By taking blue litmus paper and dipping it into an acidic solution, it shows that it's acidic as the litmus paper changes to red. These are some of the common characteristics found with different types of acids.

Which of the following compounds would produce the greatest concentration of hydrogen ions when dissolved?

• A. HNO₂
• B. HBrO
• C. H₃PO₄
• D. H₂SO₃

Answer: (C)

Acid-Base Introduction Concept 3

Generally speaking, we're going to say that bases represent ionic compounds containing a metal cation, so a positive metal ion, connected to the basic anion of hydroxide ion, which is OH^-. Now bases can also be represented by nitrogen containing covalent compounds called amines. We'll talk about that later on, but these are the basic ideas when it comes to a base. So most common types of bases possess a metal connected to OH. So if you can spot that, that is a good indication that you have a base.

Acid-Base Introduction Example 3

Here it says, "Which of the following represents the possible structure of a base?" So remember, the definition we went over was that most common types of bases possess a metal cation connected to the hydroxide ion. If we take a look here, we have hydroxide ion in two places. Here's an OH and here's an OH. So B is out, and D is out. In A, it's a carbon connected to OH. We said it's going to be a metal, so that would be out. Carbon is a nonmetal. Here the best structure for base is C, potassium hydroxide. Potassium is a metal and OH is connected to it, so that represents a typical type of base.

Acid-Base Introduction Concept 4

Now like acids, bases share in common certain characteristics when placed in aqueous solutions. So here we're going to take a look at their dissolution, their taste and feel, as well as their reaction with litmus paper. Just like acids, bases ionize when placed in water. So here we have sodium hydroxide. When I place it in water, it becomes sodium ion aqueous plus hydroxide ion aqueous. When I take strontium hydroxide and I place it in water, it becomes strontium ion aqueous, plus 2 hydroxide ions aqueous.

What about taste and feel? Well, we're going to say bases have a bitter taste. So bitter b bases b. And they are slippery to the touch. A great example of a base is in the manufacture of soaps. Soaps can be slippery when they're wet. Now litmus paper. Litmus paper reacts to the presence of the basic anion, the hydroxide ion. If I take red litmus paper and I dip it in a basic solution, it will change the red litmus paper into blue. Okay? So b base, blue, both of them have b. So that's a great way to remember the color changes associated with the base. Just remember, these are the most basic characteristics of bases when you're discussing them in aqueous solution.

Acid-Base Introduction Example 4

Here it says, which of the following is a characteristic of a strong base? Alright. So it doesn't matter if it's a strong base or a weak base. These are characteristics of all bases. "It turns blue litmus paper red." No. That wouldn't be a base. That would be an acid. "It releases H⁺ ions in a solution." No, that would also be an acid. Acids release H⁺ ions in a solution. We never talked about bases removing H⁺ from a solution. "It can be used in the production of cleaning supplies." We said that bases can be used in the formation of soaps. Soaps are an instrument to clean. Right? So, by extension, we can assume that bases can be used to make other cleaning supplies. So here, the best answer would be option D.

Which of the following is true in regards to LiOH?

• A. It has a sour taste.
• B. It produces H⁺ ions in water
• C. It conducts an electrical current in solution.
• D. It decreases the OH^– ions in water.

Answer: (C)

Here’s what students ask on this topic:

What are the common characteristics of acids in aqueous solutions?

Acids in aqueous solutions exhibit several key characteristics. They ionize in water, releasing H⁺ ions. For example, hydrochloric acid (HCl) dissociates into H⁺ and Cl^- ions. Acids have a sour taste, which is evident in citrus fruits like lemons and oranges. They react with metals to produce hydrogen gas (H₂), such as when hydrochloric acid reacts with magnesium to form magnesium chloride (MgCl₂) and H₂ gas. Additionally, acids turn blue litmus paper red, indicating their acidic nature.

How do bases behave in aqueous solutions?

Bases in aqueous solutions also ionize, releasing hydroxide ions (OH^-). For instance, sodium hydroxide (NaOH) dissociates into Na⁺ and OH^- ions. Bases have a bitter taste and a slippery feel, which is why soaps, which are basic, feel slippery. They turn red litmus paper blue, indicating their basic nature. For example, strontium hydroxide (Sr(OH)₂) dissociates into Sr²⁺ and 2 OH^- ions in water.

What is the difference between acids and bases in terms of their ionization in water?

Acids and bases differ in their ionization in water. Acids ionize to release hydrogen ions (H⁺), such as HCl dissociating into H⁺ and Cl^-. Bases ionize to release hydroxide ions (OH^-), like NaOH dissociating into Na⁺ and OH^-. This difference in ionization leads to their distinct properties: acids have a sour taste and turn blue litmus paper red, while bases have a bitter taste, feel slippery, and turn red litmus paper blue.

Why do acids have a sour taste?

Acids have a sour taste due to the presence of hydrogen ions (H⁺) in their aqueous solutions. These H⁺ ions interact with taste receptors on the tongue, producing the characteristic sour flavor. This is why citrus fruits like lemons and oranges, which contain natural acids, taste sour. The sour taste is a common property of all acids, resulting from their ability to release H⁺ ions when dissolved in water.

How do litmus papers indicate the presence of acids and bases?

Litmus paper is a pH indicator used to identify acids and bases. When blue litmus paper is dipped into an acidic solution, it turns red, indicating the presence of H⁺ ions. Conversely, when red litmus paper is dipped into a basic solution, it turns blue, indicating the presence of OH^- ions. This color change occurs due to the chemical reactions between the litmus dye and the ions in the solution, making litmus paper a simple and effective tool for identifying acids and bases.