Chemistry— Concepts, Formulas & Shortcuts
- Air is roughly 78% nitrogen, 21% oxygen and 0.93% argon by volume.
- Common names: common salt NaCl, baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O, quicklime CaO, chalk CaCO₃.
- pH below 7 is acidic, 7 is neutral, above 7 is basic; pure water at 25 °C has pH 7.
- Rusting is the oxidation of iron in the presence of oxygen and moisture.
- Atomic number = number of protons; mass number = protons + neutrons.
- Noble gases (helium, neon, argon) are inert; argon fills incandescent bulbs and neon glows in signs.
Chemistry Practice Questions with Answers
Attempt each question first, then open the explanation. All 16 questions below are free to read and require no signup.
Q1.The chemical formula of common salt is:
Easy- ANaCl
- BKCl
- CCaCl₂
- DNaHCO₃
Chemistry question 1 of 16+Show Answer & Explanation
Answer: A. NaCl
Explanation: Common salt is sodium chloride, NaCl.
Q2.The most abundant gas in the earth's atmosphere is:
Easy- AOxygen
- BNitrogen
- CCarbon dioxide
- DArgon
Chemistry question 2 of 16+Show Answer & Explanation
Answer: B. Nitrogen
Explanation: Nitrogen makes up about 78% of the atmosphere by volume.
Q3.The pH of pure water at 25 °C is:
Easy- A0
- B5
- C7
- D14
Chemistry question 3 of 16+Show Answer & Explanation
Answer: C. 7
Explanation: Pure water is neutral, with equal H⁺ and OH⁻ concentrations, giving pH 7.
Q4.Which inert gas is commonly filled in electric bulbs?
Moderate- AHelium
- BNeon
- CArgon
- DKrypton
Chemistry question 4 of 16+Show Answer & Explanation
Answer: C. Argon
Explanation: Argon is inert and inexpensive, so it protects the tungsten filament from oxidation.
Q5.The rusting of iron is an example of:
Easy- AReduction
- BOxidation
- CNeutralisation
- DSublimation
Chemistry question 5 of 16+Show Answer & Explanation
Answer: B. Oxidation
Explanation: Iron loses electrons to oxygen in the presence of moisture, forming hydrated iron(III) oxide.
Q6.The chemical name of baking soda is:
Easy- ASodium carbonate
- BSodium bicarbonate
- CCalcium carbonate
- DSodium hydroxide
Chemistry question 6 of 16+Show Answer & Explanation
Answer: B. Sodium bicarbonate
Explanation: Baking soda is sodium bicarbonate, NaHCO₃. Washing soda is sodium carbonate.
Q7.The atomic number of an element equals the number of:
Easy- ANeutrons
- BProtons
- CElectrons and neutrons
- DNucleons
Chemistry question 7 of 16+Show Answer & Explanation
Answer: B. Protons
Explanation: The atomic number is the proton count, which fixes the element's identity.
Q8.The hardest naturally occurring substance is:
Easy- AGraphite
- BQuartz
- CDiamond
- DCorundum
Chemistry question 8 of 16+Show Answer & Explanation
Answer: C. Diamond
Explanation: Diamond, an allotrope of carbon with a rigid tetrahedral lattice, ranks 10 on the Mohs scale.
Q9.The chemical formula of water is:
Easy- AH₂O
- BHO₂
- CH₂O₂
- DOH
Chemistry question 9 of 16+Show Answer & Explanation
Answer: A. H₂O
Explanation: Water consists of two hydrogen atoms bonded to one oxygen atom.
Q10.Which acid is present in the human stomach?
Easy- ASulphuric acid
- BHydrochloric acid
- CNitric acid
- DAcetic acid
Chemistry question 10 of 16+Show Answer & Explanation
Answer: B. Hydrochloric acid
Explanation: Gastric juice contains dilute hydrochloric acid, which activates pepsin.
Q11.Which gas is responsible for the greenhouse effect in largest measure?
Moderate- AOxygen
- BNitrogen
- CCarbon dioxide
- DHelium
Chemistry question 11 of 16+Show Answer & Explanation
Answer: C. Carbon dioxide
Explanation: Carbon dioxide is the principal long-lived greenhouse gas driving warming.
Q12.The pH of a strongly acidic solution is:
Easy- ABelow 7
- BExactly 7
- CAbove 7
- DExactly 14
Chemistry question 12 of 16+Show Answer & Explanation
Answer: A. Below 7
Explanation: Acidic solutions have pH below 7; basic ones above 7.
Q13.Which element is the most abundant in the earth's crust?
Moderate- AIron
- BSilicon
- COxygen
- DAluminium
Chemistry question 13 of 16+Show Answer & Explanation
Answer: C. Oxygen
Explanation: Oxygen makes up roughly 46% of the crust by mass, ahead of silicon.
Q14.Common salt is chemically known as:
Easy- ASodium chloride
- BPotassium chloride
- CCalcium carbonate
- DSodium carbonate
Chemistry question 14 of 16+Show Answer & Explanation
Answer: A. Sodium chloride
Explanation: Table salt is sodium chloride, NaCl.
Q15.Which gas is used to inflate weather balloons?
Moderate- AOxygen
- BHelium
- CCarbon dioxide
- DChlorine
Chemistry question 15 of 16+Show Answer & Explanation
Answer: B. Helium
Explanation: Helium is lighter than air and, unlike hydrogen, non-flammable.
Q16.The process of converting a solid directly into a gas is called:
Moderate- AEvaporation
- BCondensation
- CSublimation
- DFusion
Chemistry question 16 of 16+Show Answer & Explanation
Answer: C. Sublimation
Explanation: Sublimation skips the liquid phase — camphor and dry ice both sublime.
Chemistry — Frequently Asked Questions
Which gas is most abundant in the earth's atmosphere?+
Nitrogen, at about 78% by volume, followed by oxygen at about 21%. Argon and carbon dioxide make up most of the remainder.
Why is argon used in electric bulbs?+
It is chemically inert, so it does not react with the hot tungsten filament, and it reduces filament evaporation — extending bulb life compared with a vacuum.
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