Comprehensive Notes on Grade 10 Science: Classification of Elements, Chemical Substances, and Metallurgy

Classification of Elements and the Periodic Table

• There are 118 discovered elements, with 92 being naturally occurring and 26 being artificial. Elements are pure matter, while compounds and mixtures are the other forms of matter.
• Classification is the process where elements with similar properties are placed in one group and elements with different properties are placed into different groups.
• Dmitri Mendeleev, a Russian scientist, created the first periodic table based on atomic weight.
• Mendeleev's Periodic Law states: "The physical and chemical properties of elements are the periodic functions of their atomic weight."
• Limitations of Mendeleev's periodic table included lack of space for isotopes ($C-12$, $C-13$, and $C-14$), which are forms of the same element with different atomic weights due to varying neutron counts.
• Henry Moseley, an English scientist, discovered in 1913 AD that element properties depend on atomic number rather than atomic weight.
• Modern Periodic Law states: "The physical and chemical properties of elements are the periodic function of their atomic number."
• The modern periodic table consists of 7 horizontal rows called periods and 18 vertical columns called groups.

Structure and Periodic Trends in the Modern Periodic Table

• Elements in the same period have the same number of valence shells. Elements in the same group have similar chemical properties.
• The 7 periods are classified by the number of elements they contain:
  • First: 2 elements (Very short)
  • Second: 8 elements (Short)
  • Third: 8 elements (Short)
  • Fourth: 18 elements (Long)
  • Fifth: 18 elements (Long)
  • Sixth: 32 elements (Very long)
  • Seventh: 32 elements (Very long)
• Metals are located on the left side, non-metals on the right, and metalloids in the middle.
• Transition metals (Groups 3 to 12) are placed between metals and non-metals.
• Lanthanides (atomic numbers 57 to 71) and Actinides (atomic numbers 89 to 103) are kept separately below the main block.
• Valency Trends: In a period (left to right), the valency increases from 1 to 4 and then decreases to 0 (specifically 1, 2, 3, 4, 3, 2, 1, 0). In a group, the valency remains the same.
• Atomic Size Trends:
  • Across a period: Atomic size decreases because the number of protons and electrons increases, leading to a stronger nuclear attraction that contracts the shell.
  • Down a group: Atomic size increases because the number of shells increases.
• Electropositivity (Metallic Character) and Electronegativity (Non-metallic Character):
  • Across a period: Electropositivity decreases and electronegativity increases.
  • Down a group: Electropositivity increases and electronegativity decreases.
• Chemical Reactivity:
  • Metals: Reactivity decreases across a period and increases down a group.
  • Non-metals: Reactivity increases across a period and decreases down a group.

Subshell Electronic Configuration and Aufbau's Principle

• Shells ($K$, $L$, $M$, $N$) are divided into subshells ($s$, $p$, $d$, $f$).
• The maximum number of electrons per subshell are:
  • $s = 2$
  • $p = 6$
  • $d = 10$
  • $f = 14$
• Aufbau's Principle states that electrons are filled in subshells in the increasing order of their energy level.
• The order of filling subshells is: $1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p$.
• Electronic configuration examples for specific elements:
  • Hydrogen ($H$): $1s^1$
  • Sodium ($Na$): $1s^2, 2s^2, 2p^6, 3s^1$
  • Chlorine ($Cl$): $1s^2, 2s^2, 2p^6, 3s^2, 3p^5$

Carbon Dioxide ($CO_2$)

• Van Helmont discovered Carbon Dioxide in 1630 AD by burning wood. Joseph Black prepared it in 1755 AD by burning magnesium carbonate. Lavoisier proved it is a compound of carbon and oxygen.
• Molecular Weight of $CO_2$ is $44$. Atmospheric air contains approximately $0.03\%$ $CO_2$ by volume.
• Laboratory Preparation: Reacting Limestone ($CaCO_3$) with dilute Hydrochloric acid ($HCl$).
  • Reaction: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$
  • Collection: Collected in an upright gas jar by the upward displacement of air because it is 1.5 times heavier than air.
• Physical Properties: Acidic oxide, colorless, odorless, sour taste (when dissolved in water as carbonic acid), and can be changed to solid "dry ice" at $-78^\circ C$.
• Chemical Properties:
  • It is neither combustible nor a supporter of combustion (it extinguishes a burning matchstick).
  • Burning Magnesium in $CO_2$: $2Mg(s) + CO_2(g) \rightarrow 2MgO(s) + C(s)$.
  • Reaction with Lime Water: Turns lime water milky due to $CaCO_3$ formation; disappears with excess $CO_2$ forming soluble $Ca(HCO_3)_2$.
  • Photosynthesis: $6CO_2 + 6H_2O + \text{Solar Energy} \rightarrow C_6H_{12}O_6 + 6O_2$.
• Uses: Soft drinks, fire extinguishers, dry ice for food preservation, purification of sugar (carbonation), and manufacturing urea ($NH_2CONH_2$).

Ammonia ($NH_3$)

• Molecular Weight of Ammonia is $17$, making it lighter than air.
• Laboratory Preparation: Heating a mixture of Ammonium chloride ($NH_4Cl$) and Calcium hydroxide ($Ca(OH)_2$) in a $2:1$ ratio.
  • Reaction: $2NH_4Cl(s) + Ca(OH)_2(s) \rightarrow CaCl_2(s) + 2H_2O(l) + 2NH_3(g)$
  • Collection: Collected by downward displacement of air because it is lighter than air and highly soluble in water.
  • Pure and dry ammonia is obtained by passing the gas through a Lime Tower containing Calcium Oxide ($CaO$).
• Physical Properties: Colorless, pungent odour like rotten egg, basic (turns red litmus blue), and highly soluble in water.
• Chemical Properties:
  • Formation of liquor ammonia: $NH_3 + H_2O \rightarrow NH_4OH$.
  • Reaction with HCl (white fumes): $NH_3 + HCl \rightarrow NH_4Cl$.
  • Urea Production: $2NH_3 + CO_2 \xrightarrow{1500^\circ C, P} NH_2-CO-NH_2 + H_2O$.
• Uses: Fertilizers (Ammonium sulphate, urea), nitric acid manufacture, washing soda production, cooling agent in refrigerators, and cleaning agent for grease stains.

Greenhouse Effect and Acid Rain

• Greenhouse Effect is the process of trapping solar energy inside the atmosphere, increasing surface temperature.
• Greenhouse gases include $CO_2$, $CH_4$, $N_2O$, $CFCs$, and $O_3$.
• Impacts: Global warming, climate change, melting glaciers, sea level rise, and desertification.
• Artificial Greenhouses are structures made of glass or plastic used to grow plants in cold climates or seasons.
• Acid Rain: Caused when industrial gases like $SO_2$, $CO_2$, and $NO_x$ mix with moisture to form acids like $H_2SO_4$ and $HNO_3$.
  • pH of rain water is usually $6$, but acid rain ranges from $3$ to $5$.
  • Reaction affecting marble: $CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(s) + CO_2(g) + H_2O(l)$.

Metals, Minerals, and Metallurgy

• Minerals are naturally occurring inorganic, solid crystalline elements or compounds found in the earth's crust. Ores are minerals from which a metal can be extracted easily and economically.
• All ores are minerals, but all minerals are not ores.
• Chief Ores:
  • Iron ($Fe$): Hematite ($Fe_2O_3$) is the main ore (contains $75\%$ iron), Magnetite ($Fe_3O_4$), Siderite ($FeCO_3$), Limonite ($2Fe_2O_3.3H_2O$), and Iron pyrite ($FeS_2$).
  • Aluminium ($Al$): Bauxite ($2Al_2O_3.2H_2O$) is the main ore ($40-60\%$ aluminium), Cryolite ($Na_3AlF_6$), and Corundum ($Al_2O_3$).
  • Copper ($Cu$): Copper pyrite (Chalcopyrite, $CuFeS_2$) is the main ore ($34.5\%$ copper), Chalcocite ($Cu_2S$), and Cuprite ($Cu_2O$).
  • Silver ($Ag$): Argentite ($Ag_2S$) is the main ore ($87\%$ silver), Horn silver ($AgCl$), and Ruby silver ($Ag_3SbS_3$).
• Steps of Metallurgy:
  1. Grinding: Crushing ore into small particles using rollers.
  2. Concentration: Removing gangue (mud, sand). Methods include hydraulic separation, magnetic separation, or froth floatation.
  3. Oxidation: Converting ore to oxide via Roasting (heating in oxygen for sulfide ores) or Calcination (heating without oxygen for carbonate ores).
  4. Reduction: Removing oxygen from metal oxides using carbon, carbon monoxide, or electrolysis for reactive metals.
  5. Refining: Purifying metal via Distillation or Electro-refining (electrolysis using a voltameter where impure metal is the anode and pure metal is the cathode).

Hydrocarbons and Organic Compounds

• Organic Compounds are carbon compounds composed of $C$ and $H$, and often $O$, $N$, halogens, $S$, and $P$. Simple carbon oxides, carbonates, and carbides are excluded from this category.
• Saturated Hydrocarbons (Alkanes): Single covalent bonds only. General formula is $C_nH_{2n+2}$. They are stable and known as paraffins. Examples: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$), and Butane ($C_4H_{10}$).
• Unsaturated Hydrocarbons: Contain double or triple bonds. Known as olefins.
  • Alkenes: Double bond ($C=C$). General formula $C_nH_{2n}$. Example: Ethene ($C_2H_4$), Propene ($C_3H_6$).
  • Alkynes: Triple bond ($C\equiv C$). General formula $C_nH_{2n-2}$. Example: Ethyne ($C_2H_2$), Propyne ($C_3H_4$).
• Alkyl Radical ($R-$): A group formed by removing one hydrogen atom from an alkane. General formula $C_nH_{2n+1}$. Example: Methyl ($-CH_3$), Ethyl ($-C_2H_5$).
• IUPAC Nomenclature: Uses word roots based on carbon count (Meth-, Eth-, Prop-, But-, Pent-, Hex-, Hept-, Oct-, Non-, Dec-) plus suffixes (-ane, -ene, -yne).

Alcohols

• Alcohols are formed by replacing one or more hydrogens of an alkane with a hydroxyl group ($-OH$).
• Classification by number of $-OH$ groups:
  • Monohydric: One ($-OH$). Examples: Methyl alcohol (Methanol, $CH_3OH$), Ethyl alcohol (Ethanol, $C_2H_5OH$).
  • Dihydric: Two ($-OH$). Example: Glycol (Ethane-1, 2-diol, $CH_2OHCH_2OH$).
  • Trihydric: Three ($-OH$). Example: Glycerol (Propane-1, 2, 3-triol, $C_3H_5(OH)_3$).
• Uses of Glycerol: Protects skin from dryness, used in high-quality soaps, cosmetics, and as a sweetening agent in food.
• Uses of Ethyl Alcohol: Alcoholic beverages, sterilization of syringes, preserving biological specimens, and as an organic solvent.

Chemicals Used in Daily Life

• Food Preservatives:
  • First Class: Natural (sugar, salt, vinegar, honey, edible oils).
  • Second Class: Chemically derived (sodium benzoate, sulphur dioxide, potassium nitrate). These must be used in limited amounts.
• Natural Cleansing Agents:
  • Reetha (Soapberry): Contains saponin, used in shampoos.
  • Peena (Mustard seed cake): Used to clean hair.
  • Sajiban (Jatropha curcas): Leaves for compost, seed oil for biodiesel and glycerine.
• Soap: Sodium or potassium salts of long-chain fatty acids. Made via saponification (hydrolysis of fat with alkali).
• Detergent: Soapless soaps made from hydrocarbons. Non-biodegradable, effective in hard water, but cause chemical pollution.
• Pesticides: Poisonous substances to control pests. Classified by target (insecticides, fungicides, herbicides, rodenticides, miticides) or mode (contact, internal, systemic, fumigants).
• DDT and BHC are banned in Nepal due to high persistence and toxicity in the ecosystem, affecting organism reproduction and health.