Class 10 Chemistry Important Questions - All Chapters (CBSE NCERT Board Exam Prep)
Covers: Chemical Reactions and Equations • Acids, Bases and Salts • Metals and Non-metals • Carbon and its Compounds
- Hydrogen gas combines with nitrogen to form ammonia.
- Hydrogen sulfide gas burns in air to give water and sulphur dioxide.
- Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
- Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
- Hydrogen gas is passed over heated copper oxide to form copper and water.
A. N2(g) + 3H2(g) → 2NH3(g)
B. 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
C. 3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(s)↓
D. 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)↑
E. H2(g) + CuO(s) → Cu(s) + H2O(l) (this is also an example of a displacement/reduction reaction)
- ZnO + C → Zn + CO
- MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
A reaction in which oxidation (loss of electrons / gain of oxygen / loss of hydrogen) and reduction (gain of electrons / loss of oxygen / gain of hydrogen) occur simultaneously is called a redox reaction.
A. ZnO + C → Zn + CO — ZnO loses oxygen and is reduced to Zn; carbon gains oxygen and is oxidised to CO.
B. MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 — MnO2 is reduced to MnCl2 (loss of oxygen), while HCl is oxidised to Cl2 (loss of hydrogen).
2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
This is a thermal decomposition reaction, where a single compound breaks down into two or more simpler substances on heating.
The gas is carbon dioxide (CO2).
Limited CO2: Ca(OH)2(aq) + CO2(g) → CaCO3(s)↓ + H2O(l) — white precipitate of calcium carbonate.
Excess CO2: CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq) — soluble calcium bicarbonate, so the precipitate dissolves.
An acid ionises in water to release H+ ions. A free H+ ion cannot exist alone in water — it combines with a water molecule to form a hydronium ion.
HCl + H2O → H3O+ + Cl−
Here H+ + H2O → H3O+ (hydronium ion).
On dilution, the number of H3O+ ions per unit volume decreases because the same amount of acid is now spread over a larger volume of water. This decreases the acidic strength of the solution and its pH value moves closer to 7.
Yes, basic solutions also contain a small amount of H+ ions. However, the concentration of OH− ions in such solutions is much higher than the concentration of H+ ions. Since the OH− ion concentration dominates, the solution behaves as basic in nature.
Sodium (Na) — atomic number 11, electronic configuration 2,8,1. It has 1 valence electron, shown as a single dot around the symbol Na.
Oxygen (O) — atomic number 8, electronic configuration 2,6. It has 6 valence electrons, shown as three pairs of dots and two single dots around the symbol O.
Magnesium (Mg) — atomic number 12, electronic configuration 2,8,2. It has 2 valence electrons, shown as two dots around the symbol Mg.
(Draw these as dots placed around the chemical symbol on all four sides for the exam diagram.)
Ionic compounds are made up of oppositely charged ions arranged in a rigid, closely packed three-dimensional lattice. There is a strong electrostatic force of attraction between these ions. A considerable amount of energy is required to overcome this force and break the lattice, so ionic compounds have high melting and boiling points.
- They are very low in reactivity, so they do not corrode or react easily with air, moisture, or acids.
- They have a bright lustre that lasts a long time.
- They are highly malleable and ductile, so they can be shaped into thin sheets and wires for ornaments.
Hexanal (C6H12O, an aldehyde): CH3–CH2–CH2–CH2–CH2–CHO
Butanone (C4H8O, a ketone): CH3–CO–CH2–CH3
Ethanoic acid (C2H4O2, a carboxylic acid): CH3–COOH
(In your answer sheet, draw the full structural formula showing every bond, with –CHO for hexanal, C=O for the ketone group of butanone, and –COOH for ethanoic acid.)
When ethanol is treated with an oxidising agent such as alkaline potassium permanganate or acidified potassium dichromate, an oxygen atom is added to it to form ethanoic acid.
CH3CH2OH + 2[O] → CH3COOH + H2O
Since the reaction involves the addition of oxygen, it is classified as an oxidation reaction.
Saturated hydrocarbons are fairly unreactive and undergo substitution reactions in the presence of sunlight, where a hydrogen atom is replaced one by one by another atom, usually a halogen.
CH4 + Cl2 --(sunlight)--> CH3Cl + HCl
With excess chlorine, further substitution can occur, eventually forming CCl4.
Hard water contains dissolved calcium (Ca2+) and magnesium (Mg2+) salts. Soap molecules (sodium salts of fatty acids) react with these ions to form insoluble calcium and magnesium salts of fatty acids, which appear as a white, sticky substance called scum, instead of lather. As a result, soap does not lather well and gets wasted in hard water.
Metal oxides are comparatively easier to reduce to free metals than metal carbonates or sulphides. So carbonate ores are first converted to oxides by calcination (heating in limited air) and sulphide ores are converted to oxides by roasting (heating in excess air), before the oxide is reduced with a suitable reducing agent to obtain the pure metal.