Chemical Reactions Class 10 PPT & Notes 46 Slides Free

• Change in colour (for example green ferrous sulphate turning reddish-brown on heating)
• Change in state (for example a candle wax melting and burning)
• Evolution of a gas (for example bubbles of hydrogen when zinc meets dilute acid)
• Change in temperature: the mixture gets hot (exothermic) or cold (endothermic)

Any one of these signs means a chemical reaction has taken place.

Clean a magnesium ribbon with sandpaper, hold it with tongs and burn it over a flame (keep it away from your eyes). It burns with a dazzling white flame and leaves white magnesium oxide ash. Magnesium gains oxygen, so it is oxidised. This is a combination and exothermic reaction.

A chemical equation must be balanced because of the law of conservation of mass: mass is never created or destroyed in a chemical reaction, so atoms must be equal on both sides. When balancing, change only the coefficients (numbers before the formulae), and never the subscripts inside a formula (H₂O is correct, H₂O₄ is wrong).

1. Draw boxes around each formula (do not change anything inside the boxes)
2. List the atoms of each element on the left and the right
3. Balance the element with the most atoms first (oxygen in this example): put 4 before H₂O
4. Balance hydrogen: put 4 before H₂ on the right
5. Balance iron: put 3 before Fe on the left
6. Check every atom on both sides
7. Add the physical states

Fe: 3 = 3, H: 8 = 8, O: 4 = 4. Balanced.

• (s) solid, (l) liquid, (g) gas, (aq) aqueous solution (dissolved in water)
• Conditions are written above or below the arrow, never inside the equation: heat (Δ), sunlight, electricity, a catalyst, or high pressure (for example CO + 2H₂ → CH₃OH at 340 atm)

In a combination reaction two or more substances combine to form one single product. When water is added to quicklime (calcium oxide, a white solid), a large amount of heat is released and slaked lime forms. Touch the beaker: it is very hot.

Other combination reactions: C + O₂ → CO₂ (burning of coal) and 2H₂ + O₂ → 2H₂O. All of these are also exothermic: heat is released.

• Burning of natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O
• Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The energy released keeps our body warm and active
• The decomposition of vegetable matter into compost is also exothermic

On day 1 slaked lime is applied to the wall, giving a dull, chalky finish. On days 2 to 3 the carbon dioxide from the air reacts with the calcium hydroxide, and by day 3 a layer of calcium carbonate forms, giving a shiny, hard finish. (Marble is also calcium carbonate.)

In a decomposition reaction a single reactant breaks down into two or more simpler products. It needs energy as heat, light or electricity.

Activity 1.5: green ferrous sulphate crystals turn reddish-brown (Fe₂O₃) on heating, and the smell of burning sulphur comes from SO₂ and SO₃. Activity 1.6: white lead nitrate gives yellow PbO and brown fumes of NO₂ (toxic, do not inhale) and colourless O₂.

When electricity is passed through water containing a few drops of dilute sulphuric acid, hydrogen collects at the cathode (−) and oxygen at the anode (+). The volume of hydrogen is double that of oxygen (H₂ : O₂ = 2 : 1), because each water molecule has 2 hydrogen atoms and 1 oxygen atom. Hydrogen burns with a pop sound, and oxygen relights a glowing splinter.

White silver chloride turns grey in sunlight because silver metal forms and chlorine gas is released. This photolytic decomposition is how black and white photography works: where light hits, silver forms (dark areas), and where there is no light it stays white.

Activity: mix 2 g of barium hydroxide with 1 g of ammonium chloride in a test tube. The temperature drops, the test tube feels icy cold, and a pungent smell of ammonia is detected. Energy is absorbed from the surroundings, so the reaction is endothermic.

A more reactive element displaces a less reactive element from its compound. When an iron nail is dipped in blue copper sulphate solution for about 20 minutes, a brown deposit of copper forms on the nail, and the blue colour fades and turns light green (Fe²⁺ replaces Cu²⁺). Iron is more reactive than copper.

In a double displacement reaction two compounds exchange their ions: AB + CD → AD + CB. If an insoluble solid is formed, it is called a precipitate and the reaction is also a precipitation reaction.

Two colourless solutions give a bright yellow precipitate of lead iodide (PbI₂), and KNO₃ stays dissolved (Activity 1.2).

Sodium sulphate and barium chloride give a white precipitate of barium sulphate (Activity 1.10): SO₄²⁻ meets Ba²⁺ and precipitates, while Na⁺ and Cl⁻ stay in solution as NaCl.

• Heat copper powder in air: it turns black (copper oxide). 2Cu + O₂ → 2CuO
• Pass hydrogen over the hot CuO: the brown copper appears again. CuO + H₂ → Cu + H₂O
• Oxidation is the gain of oxygen or loss of hydrogen (H₂ gains oxygen and becomes H₂O). Reduction is the loss of oxygen or gain of hydrogen (CuO loses oxygen and becomes Cu)
• Oxidation and reduction occur simultaneously in the same reaction, which is called a redox reaction. Memory trick: OIL RIG: Oxidation Is Loss, Reduction Is Gain (of oxygen or electrons)

More redox examples: ZnO + C → Zn + CO (ZnO is reduced and C is oxidised) and MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (MnO₂ is reduced and HCl is oxidised).

When a metal is attacked by substances around it (moisture, acids, gases), it is corroded. Iron rusts: iron + oxygen + water → hydrated iron(III) oxide (Fe₂O₃·xH₂O, reddish-brown rust). Silver tarnishes by forming a black coating of silver sulphide (silver reacts with H₂S in the air): 2Ag + H₂S → Ag₂S + H₂. Copper turns green with a layer of basic copper carbonate (from CO₂ and moisture in the air). Corrosion damages bridges, ships, cars and iron railings every year.

• Rancidity is the oxidation of fats and oils, which changes the smell and taste of food (for example butter or chips left open)
• Prevention: add antioxidants (like BHA) to food, store it in airtight containers, refrigerate it, and fill chips packets with nitrogen gas (an inert gas that replaces oxygen, which is why they look puffed even when not full)

Tap an option to check it at once. Open Check answer for the reason.