Which one of the following is a major effect of long term consumption of drinking water containing little (less than 0.5 ppm) or no fluoride ?
- A.Cavity of tooth
- B.Erosion of nail
- C.Deformation of bone
- D.Mottling of tooth
Correct answer
Explanation
The correct answer is A, Cavity of tooth. Too little fluoride in drinking water leaves tooth enamel weak, so teeth decay more easily and develop cavities, called dental caries.
Fluoride becomes part of the enamel and turns hydroxyapatite into the harder, acid-resistant fluorapatite. It also helps damaged enamel to remineralise and slows down the bacteria that make acid in the mouth. That is why a small amount, about 1 ppm, is considered good for teeth, and many places add fluoride to water or toothpaste. India's drinking water standard sets 1 mg per litre as acceptable and 1.5 mg per litre as the limit.
B is wrong because nail erosion is not linked to fluoride intake. C is wrong because bone deformity is a sign of skeletal fluorosis, which comes from too much fluoride over many years, not too little. D is wrong because mottling, the brown stains on enamel, is dental fluorosis, also caused by excess fluoride above about 1.5 ppm.
Exam tip: low fluoride = dental caries; high fluoride = mottled teeth, then skeletal fluorosis.
Practice Questions
View allThe potential difference between the terminals of an electrical heating device is 40 V, when it draws a current of 2 A from the source. Which one of the following is the correct value of current drawn by the heating device if the potential is increased to 140 V?
- A.7 A
- B.5 A
- C.4 A
- D.2 A
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By Ohm's law the heater's resistance is 40 V divided by 2 A, which is 20 ohms, so at 140 V it draws 140 divided by 20, or 7 A. Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the potential difference across…
Read the full explanation →Consider the following statements in the light of octane number and cetane number of fuels: 1. Cetane number is used for gasoline and octane number is used for diesel. 2. Octane number of n-hexadecane is 100 and cetane number of isooctane is 100. Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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Both statements swap the two scales: octane number rates petrol, cetane number rates diesel, and their reference fuels are the other way round.
Read the full explanation →Which one among the following statements related to human eye is correct?
- A.For a person suffering from short-sight defect, the image of a distant object is focused behind the retina.
- B.Short-sight defect can be corrected by a converging spectacle lens.
- C.For a person suffering from long-sight defect, the image of a near object is focused behind the retina.
- D.Long-sight defect can be corrected by a diverging spectacle lens.
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In long-sightedness (hypermetropia) the eye cannot see nearby objects clearly because their image is formed behind the retina.
Read the full explanation →When sunlight (white light) falls on a triangular glass prism, a band of colours called a spectrum is obtained. Which one among the following represents this effect appropriately?
- A.Reflection
- B.Diffraction
- C.Refraction
- D.Dispersion
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The splitting of white light into its seven component colours by a prism is called dispersion. White light is a mixture of colours, and glass bends each colour by a different amount because its refractive index depends on wavelength.
Read the full explanation →Which one of the following statements is correct?
- A.The number of electrons in alpha particle is 2.
- B.The coordination number of EDTA is 6.
- C.The charge of beta (β) particle is +2.
- D.Both carbon and silicon form ionic compounds.
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EDTA is a hexadentate ligand: one EDTA ion binds a metal ion through six donor atoms, so it forms complexes with coordination number 6.
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