Science Facts and Famous Scientists CBSE Class 9 Chemistry Worksheets Set 05

Questions with Answers

Question 1. What is the symbol of tungsten and where does it come from?
Answer:
The symbol of tungsten is W. It comes from its German name Wolfram.
In simple words: W, from Wolfram.

Teacher's Note:
1. Symbols of sodium (Na) and potassium (K) also come from Latin names natrium and kalium.
2. Tungsten is used for bulb filaments because of its high melting point.

Question 2. What was Dalton's main idea about the atom?
Answer:
John Dalton proposed in 1803 that matter is made of tiny, indivisible particles called atoms, which can neither be created nor destroyed in a chemical reaction. Atoms of one element are alike, and atoms of different elements differ. Compounds form by combining atoms in simple whole-number ratios.
In simple words: Atoms are indivisible and neither created nor destroyed.

Teacher's Note:
1. Today we know atoms are made of protons, neutrons and electrons.
2. Dalton's theory explains the laws of chemical combination.

Question 3. Name the element that was named after the native country of its discoverer.
Answer:
Polonium, discovered by Marie Curie and named after Poland, her native country.
In simple words: Polonium.

Teacher's Note:
1. Marie Curie also discovered radium.
2. Polonium is radioactive.

Question 4. How did the term 'empirical formula' get its name?
Answer:
The word empirical comes from the Greek empeirikos, meaning 'experience' or 'by trial'. An empirical formula is found from experimental results (percentage composition) and shows the simplest whole-number ratio of atoms.
In simple words: From the Greek word for experience: it is found by experiment.

Teacher's Note:
1. Empirical formula of glucose is CH₂O.
2. Molecular formula is a multiple of the empirical formula.

Question 5. Name two elements named after their discoverers.
Answer:
Einsteinium (symbol Es, atomic number 99) is named after Albert Einstein and fermium (symbol Fm, atomic number 100) after Enrico Fermi. (They are named after scientists, but were discovered in the debris of a nuclear explosion.)
In simple words: Einsteinium and fermium.

Teacher's Note:
1. Both are artificial elements.
2. Mendelevium (Md) is named after Mendeleev.

Question 6. What is the empirical formula of hydrogen peroxide?
Answer:
The molecular formula of hydrogen peroxide is H₂O₂. The ratio of hydrogen to oxygen atoms is 1 : 1, so the empirical formula is HO.
In simple words: HO, while the molecular formula is H₂O₂.

Teacher's Note:
1. Empirical formula = simplest ratio.
2. Molecular formula = n × empirical formula, here n = 2.

Question 7. An element bears the name of the messenger of the gods. Name it. Why is it so called?
Answer:
Mercury, named after the Roman messenger god. The liquid metal runs quickly and does not wet glass, like a swift messenger.
In simple words: Mercury.

Teacher's Note:
1. Its symbol Hg comes from the Latin hydrargyrum, meaning liquid silver.
2. It is also called quicksilver.

Question 8. Why are noble gases not called inert gases any more?
Answer:
Noble gases were once thought to form no compounds, but chemists have prepared compounds of xenon (for example XeF₂, XeF₄), krypton and radon. So they are better called noble gases than inert gases.
In simple words: Some of them form compounds, so they are not completely inert.

Teacher's Note:
1. Helium and neon form no known compounds.
2. Their outer shells are full.

Question 9. Which noble gas was first detected in the spectrum of the Sun?
Answer:
Helium. It was seen as a bright yellow line in the Sun's spectrum in 1868 before it was found on the earth. Its name comes from Helios, the Greek word for the Sun.
In simple words: Helium.

Teacher's Note:
1. Helium is the second most abundant element in the universe.
2. It is used in balloons and for cooling magnets.

Question 10. Why can no substance have a molecular mass less than 2?
Answer:
Hydrogen is the lightest element, with atomic mass 1, and its molecule H₂ has a molecular mass of 2. Any molecule contains at least one atom of some element, and the lightest molecule is H₂.
In simple words: The lightest molecule is H₂ with mass 2.

Teacher's Note:
1. A single hydrogen atom has mass 1, but it is not a stable molecule.
2. Helium atoms have mass 4.

Question 11. What does 'metathesis' mean in chemistry?
Answer:
Metathesis is another name for double decomposition, where two compounds exchange their ions to form two new compounds, for example \( AgNO_3 + NaCl \rightarrow AgCl + NaNO_3 \).
In simple words: It is a double decomposition reaction.

Teacher's Note:
1. A precipitation reaction is a double decomposition reaction.
2. Ions are exchanged between the reactants.

Question 12. Which scientist was responsible for the general acceptance of Avogadro's hypothesis?
Answer:
Stanislao Cannizzaro, who explained its importance at the Karlsruhe Congress of 1860, 49 years after Avogadro proposed it in 1811. He used it to fix atomic masses.
In simple words: Cannizzaro.

Teacher's Note:
1. Avogadro's hypothesis separates atoms from molecules.
2. The mole concept grew from it.

Question 13. Which scientist was guillotined during the French Revolution, and who introduced quantitative methods into chemistry?
Answer:
Antoine Lavoisier, who was executed during the French Revolution. He used the balance carefully in experiments and so introduced quantitative methods; he also gave the law of conservation of mass.
In simple words: Lavoisier.

Teacher's Note:
1. He named oxygen and hydrogen.
2. He showed that burning is a combination with oxygen.

Question 14. Who was the schoolmaster who became a famous chemist?
Answer:
John Dalton, an English schoolteacher who proposed the atomic theory. He was colour-blind, and colour-blindness is sometimes called Daltonism.
In simple words: John Dalton.

Teacher's Note:
1. Dalton also studied the weather.
2. His atomic theory is the basis of modern chemistry.

Question 15. Which of the following sublimes on heating: common salt, phosphorus, nitre or iodine? Which substance is called sal ammoniac?
Answer:
Iodine sublimes on heating, forming violet vapour. Sal ammoniac is the old trade name for ammonium chloride, which also sublimes.
In simple words: Iodine sublimes; sal ammoniac is ammonium chloride.

Teacher's Note:
1. Sublimation separates a sublimable solid from impurities.
2. Ammonium chloride is used in dry cells.

Question 16. What is the formula of barium chlorate?
Answer:
Chloric acid is HClO₃, so the chlorate ion is ClO₃⁻. Barium has a valency of 2, so the formula is Ba(ClO₃)₂.
In simple words: Ba(ClO₃)₂.

Teacher's Note:
1. Cross over the valencies to write a formula.
2. Barium chlorate gives a green flame.

Question 17. Which is the first Indian scientific satellite and when was it launched?
Answer:
Aryabhata, launched on 19 April 1975. It was built in India and launched from the Soviet Union.
In simple words: Aryabhata, 19 April 1975.

Teacher's Note:
1. India's first satellite launched from India was Rohini in 1980.
2. ISRO is India's space agency.

Question 18. Name one Indian-born American scientist who won the Nobel Prize.
Answer:
Har Gobind Khorana won the Nobel Prize in Physiology or Medicine in 1968 for work on the genetic code and the synthesis of nucleic acids.
In simple words: Har Gobind Khorana.

Teacher's Note:
1. Subrahmanyan Chandrasekhar won the Nobel Prize in Physics in 1983.
2. C. V. Raman won the Nobel Prize in Physics in 1930 for the Raman effect.

Question 19. Which planet is nearest to the earth, and which scientist explained correctly the nuclear fission reaction?
Answer:
Venus is the planet which comes nearest to the earth. Nuclear fission was discovered by the German scientists Otto Hahn and Fritz Strassmann in 1938 and explained by Lise Meitner and Otto Frisch.
In simple words: Venus; Hahn, Strassmann, Meitner and Frisch.

Teacher's Note:
1. Mercury is, on average, nearest to the earth over time.
2. Fission is the splitting of a heavy nucleus.

Question 20. What is the science dealing with X-rays and radioactivity called?
Answer:
Radiology is the branch of science which deals with X-rays and radioactivity and their use, especially in medicine.
In simple words: Radiology.

Teacher's Note:
1. X-rays were discovered by Roentgen in 1895.
2. Radioactivity was discovered by Becquerel in 1896.

Question 21. Who invented dynamite? Which is the hardest known natural substance? What does TNT stand for?
Answer:
Dynamite was invented by Alfred Nobel of Sweden, after whom the Nobel Prizes are named. Diamond is the hardest known natural substance. TNT stands for trinitrotoluene, a powerful explosive.
In simple words: Alfred Nobel; diamond; trinitrotoluene.

Teacher's Note:
1. Nobel gave his fortune to the prizes.
2. Diamond is a form of carbon.

Question 22. For which discovery was Sir C. V. Raman awarded the Nobel Prize?
Answer:
Raman received the Nobel Prize in Physics in 1930 for his discovery of the scattering of light, known as the Raman effect. When monochromatic light passes through a transparent substance, a small part is scattered with a changed frequency, which depends on the substance.
In simple words: The Raman effect: change of frequency of scattered light.

Teacher's Note:
1. The change in frequency is the fingerprint of the molecules.
2. 28 February is celebrated as National Science Day.

Question 23. What is a lodestar?
Answer:
A lodestar is a star used for guidance, especially the pole star. The word is used for anything that serves as a guide.
In simple words: A guiding star, such as the pole star.

Teacher's Note:
1. The pole star shows the north.
2. Lode means way or course.

Question 24. Which radioactive isotope is used in the treatment of cancer? What is solder?
Answer:
Cobalt-60 gives gamma rays used in radiation treatment of cancer. Solder is an alloy of lead and tin (in varying proportions) with a low melting point, used to join metal parts.
In simple words: Cobalt-60; solder is a lead-tin alloy.

Teacher's Note:
1. Radiation treatment kills cancer cells.
2. Radioisotopes are used in medicine.

Question 25. Name a contrivance in which mechanical energy is converted into electrical energy.
Answer:
A dynamo or generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field (electromagnetic induction).
In simple words: Dynamo or generator.

Teacher's Note:
1. A motor does the opposite.
2. Faraday discovered electromagnetic induction.

Question 26. What is the coolant commonly used in domestic refrigerators?
Answer:
Older refrigerators used Freon (chlorofluorocarbons, CFCs), which was found to damage the ozone layer. Today HFCs and hydrocarbons such as isobutane are used instead.
In simple words: Freon (CFC) in the past; HFCs or hydrocarbons now.

Teacher's Note:
1. The coolant takes heat inside and gives it out at the back.
2. CFCs damage the ozone layer.