7 th grade science What is pH?

Some of our favorite foods make our tongue curl up because they are SOUR. Some of our favorite foods make our tongue curl up because they are SOUR. Acids and Bases

Bitter! Some foods have a “bite” of their own because they’re somewhat bitter. Some foods have a “bite” of their own because they’re somewhat bitter. WHY? WHY?

Acidic/Basic There is a scientific reason for this: There is a scientific reason for this: These foods are either acidic or basic. These foods are either acidic or basic. The degree of acidity or alkalinity (basic) is important in organisms. The degree of acidity or alkalinity (basic) is important in organisms.

Acids and Bases Chemicals may be classofied as acids or bases. Chemicals may be classofied as acids or bases. Things that are neither acids nor bases are neutral. Things that are neither acids nor bases are neutral. pH measures how acidic or basic a solution is. pH measures how acidic or basic a solution is.

Acids Often taste sour Often taste sour Strong acids can burn skin & eyes Strong acids can burn skin & eyes Strong acids can dissolve metals Strong acids can dissolve metals Examples: Examples: Lemon juice Lemon juice Vinegar Vinegar Car battery acid Car battery acid (dangerous!)

Bases Can taste bitter, sweetish, or salty Can taste bitter, sweetish, or salty Strong bases can burn skin & eyes Strong bases can burn skin & eyes Bases are often used for cleaning Bases are often used for cleaning Examples: Examples: Milk Milk Baking soda Baking soda Soap Soap Drain cleaner Drain cleaner (dangerous!)

Some substances are not really an acid or a base: For example, pure water Some substances are not really an acid or a base: For example, pure water

pH A special name is given to the acid or base characteristic that a substance has: A special name is given to the acid or base characteristic that a substance has: It is called: pH It is called: pH “p” stands for potential and “H” stands for hydrogen; hence, the potential of a substance to attract hydrogen ions “p” stands for potential and “H” stands for hydrogen; hence, the potential of a substance to attract hydrogen ions

Acid Any substance which has a pH of value of less than 7 is considered an acid Any substance which has a pH of value of less than 7 is considered an acid Proton donors Proton donors This means that the number of hydronium (H 3 O+ ) ions is greater than the number of hydroxide ions (OH-). This means that the number of hydronium (H 3 O+ ) ions is greater than the number of hydroxide ions (OH-) Acid Neutral Base Acid Neutral Base

Base Any substance which has pH value greater than 7 is a base Any substance which has pH value greater than 7 is a base Proton acceptors Proton acceptors This means that the number of hydroxide (OH-) ions in a solution is greater than the number of hydronium (H 3 O+ ) ions. This means that the number of hydroxide (OH-) ions in a solution is greater than the number of hydronium (H 3 O+ ) ions Acid Neutral Base Acid Neutral Base

pH 7 A pH of 7 is called neutral—neither acid nor base. A pH of 7 is called neutral—neither acid nor base Acid Neutral Base Acid Neutral Base

The pH Scale Compares the relative concentration of H 3 O+ ions and OH- ions Compares the relative concentration of H 3 O+ ions and OH- ions Scale ranges from 0 to 14; 0-3 is very acidic; 7 is neutral; is very basic or alkaline Scale ranges from 0 to 14; 0-3 is very acidic; 7 is neutral; is very basic or alkaline Litmus paper, phenolphthalein, pH paper, & other indicators that change color can be used to measure pH Litmus paper, phenolphthalein, pH paper, & other indicators that change color can be used to measure pH

pH Using litmus paper: An acid turns blue litmus paper red and a base turns red litmus paper blue. Using litmus paper: An acid turns blue litmus paper red and a base turns red litmus paper blue.

pH scale Scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution Scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution Scale ranges from 0 to 14 Scale ranges from 0 to 14

pH The lower the pH the stronger the acid The lower the pH the stronger the acid The higher the pH the stronger the base The higher the pH the stronger the base pH 7.0 is neutral pH 7.0 is neutral

Buffers Control of pH is important to organisms Control of pH is important to organisms Enzymes function only within a narrow pH range; usually neutral Enzymes function only within a narrow pH range; usually neutral Buffers neutralize acids or bases in organisms to help control pH Buffers neutralize acids or bases in organisms to help control pH