AP BIOLOGY

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BIOCHEMISTRY

PROPERTIES OF WATER

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
Reading Passage:In water (H2O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxide ions (OH-). The “lost” hydrogen ions join up with water molecules to form hydronium ions (H+). In pure water, there are an equal number of hydrogen ions and hydroxide ions. The solution is neither acidic or basic. An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic.A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the base “soaks up” hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is alkaline. Question:When added to water, how does an acid affect the pH and H+ concentration?
A
Both pH and H+ decrease.
B
Both pH and H+ increase.
C
The pH decreases while the H+ increases. The pH decreases while the H+ increases.
D
The pH increases while the H+ decreases.
Explanation: 

Detailed explanation-1: -As H+ ions are formed, they bond with H2O molecules in the solution to form H3O+ (the hydronium ion). This is because hydrogen ions do not exist in aqueous solutions, but take the form of the hydronium ion, H3O+. A reversible reaction is one in which the reaction goes both ways.

Detailed explanation-2: -Splitting of H2O molecules into hydrogen ions and oxygen in the presence of light and grana is photolysis.

Detailed explanation-3: -Water Dissociation A small percentage of water molecules (about 1 in 10, 000, 000) break apart in pure water at room temperature. Positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-) are formed in this dissociation.

Detailed explanation-4: -Water and pH This means that the molar concentrations of H + and OH-are approximately 10-7 each in pure water. Therefore, a H + concentration of 10-7 M has a pH of 7.0. In pure water, there are always equal numbers of H + and OH-, and the pH is defined to be neutral.

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