Which one of the following statements about the law of conservation of mass is correct?
- A.A given compound always contains exactly same proportion of elements.
- B.When gases combine in a reaction, they do so in a simple ratio by volume, provided all gases are at room temperature.
- C.Matter can neither be created nor destroyed.
- D.Equal volumes of all gases at same temperature and pressure contain equal number of molecules.
Correct answer
Explanation
The correct answer is C, Matter can neither be created nor destroyed. The law of conservation of mass, stated by Antoine Lavoisier in 1789, says that in a chemical reaction mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products; atoms are only rearranged, not made or lost. This is why a chemical equation must be balanced, with the same number of each kind of atom on both sides. Option A is wrong because it states the law of definite (constant) proportions, given by Joseph Proust: a compound always contains the same elements in the same fixed ratio by mass, as water always has hydrogen and oxygen in a 1:8 mass ratio. Option B is wrong because it is a form of Gay-Lussac's law of gaseous volumes, which deals with volume ratios of reacting gases, not with mass. Option D is wrong because it is Avogadro's law, which links the volume of a gas to the number of its molecules. Exam tip: conservation of mass = Lavoisier; definite proportions = Proust; gaseous volumes = Gay-Lussac; equal volumes, equal molecules = Avogadro.
Practice Questions
View allChlorophyll in photosynthetic prokaryotic bacteria is associated with
- A.plastids
- B.membranous vesicles
- C.nucleoids
- D.chromosomes
Show answer
Photosynthetic bacteria have no chloroplasts; their pigments sit on infoldings of the cell membrane that form vesicles or sacs, often called chromatophores.
Read the full explanation →Fat in human body is stored in
- A.Adipose tissue
- B.Red blood cells
- C.Muscle tissue
- D.Epithelial tissue
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Adipose tissue is a loose connective tissue whose cells, called adipocytes, are built to store fat. Each cell holds a large droplet of fat that pushes the nucleus and cytoplasm to one side.
Read the full explanation →Which one of the following is a metalloid ?
- A.Germanium
- B.Coke
- C.Iodine
- D.Sugar
Show answer
Germanium is a metalloid, an element whose properties lie between those of metals and non-metals. It is a hard, shiny, grey-white solid like a metal, yet it is brittle and conducts electricity far less well than a metal, which makes it a semiconductor.
Read the full explanation →A photon of X-ray has energy of 1 keV. A photon of visible radiation has energy of 3 eV. In this context, which one of the following statements is not correct?
- A.The wavelength of X-ray photon is less than the wavelength of visible radiation photon.
- B.Both the photons have different energies.
- C.The speeds of both the photons in vacuum are different.
- D.The frequency of X-ray photon is higher than the frequency of visible radiation photon.
Show answer
This statement is wrong because every photon, whatever its energy, travels in vacuum at the same speed of light, about 3 × 10^8 metres per second.
Read the full explanation →Which of the following substances cause temporary hardness in water?
1. Mg(HCO3)2
2. Ca(HCO3)2
3. CaCl2
4. MgSO4
Select the correct answer using the code given below.
- A.3 and 4
- B.2 and 3
- C.1 and 4
- D.1 and 2
Show answer
Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out.
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