Tooth decay begins when the pH of the mouth falls below about
- A.7.0
- B.6.5
- C.5.5
- D.3.5
Correct answer
Explanation
The correct answer is C, 5.5. Tooth enamel is made of calcium phosphate, the hardest substance in the body, and it begins to corrode when the pH in the mouth drops below about 5.5. Bacteria in the mouth break down leftover sugar into acid, which is why sweets left on the teeth cause decay and why cleaning the teeth with a mildly basic toothpaste neutralises the excess acid. Option A is wrong because 7.0 is the neutral point, at which enamel is perfectly safe. Option B is wrong because at 6.5 the mouth is only slightly acidic and the enamel still holds. Option D is wrong because 3.5 is far more acidic than the threshold; damage would already have begun well before that value is reached. Note the neighbouring figure often confused with this one: rain is called acid rain when its pH falls below about 5.6.
Read the full article: Acids, Bases and Salts: pH Scale and Common Salts
Practice Questions
View allWhich of the following contains citric acid?
- A.Tomato
- B.Orange
- C.Tamarind
- D.Sour milk
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Citrus fruits such as orange, lemon, lime and grapefruit owe their sharp sour taste to citric acid, which is the acid textbooks name for this family and the one examiners expect here.
Read the full explanation →The pH of a solution is 3. When its pH changes to 6, then the hydrogen ion concentration
- A.Increases 2 times
- B.Increases three times
- C.Decreases 100 times
- D.Decreases 1000 times
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The pH is the negative logarithm of the hydrogen ion concentration, so each rise of one unit means the concentration has fallen to one tenth.
Read the full explanation →When litmus is added to a solution of borax, it turns _______.
- A.Pink
- B.Orange
- C.Red
- D.Blue
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Borax, sodium tetraborate, is the salt of a strong base, sodium hydroxide, and a weak acid, boric acid, so its solution in water is basic.
Read the full explanation →What is the chemical formula of baking soda?
- A.Na2CO3
- B.NaHCO3
- C.NaOH
- D.CaOCl2
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Baking soda is sodium hydrogen carbonate, a mild, non corrosive base used as an antacid, as an ingredient of baking powder along with a mild edible acid such as tartaric acid, and in soda acid fire extinguishers, where it releases carbon dioxide.
Read the full explanation →The acid present in the sting of an ant is
- A.Formic acid
- B.Citric acid
- C.Lactic acid
- D.Tartaric acid
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The sting of an ant and the hairs of the nettle plant inject formic acid, also called methanoic acid, which is why the skin burns; rubbing on a mild base such as baking soda or the leaf of the dock plant relieves it by neutralising the acid.
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