CHEMISTRY-1.3

CloseWhen ferrous sulphate crystals (FeSO4.7H2O) are heated strongly, they first lose water of crystallisation and turn from green to white. On further heating, they decompose into reddish-brown ferric oxide (Fe2O3), sulphur dioxide (SO2), and sulphur trioxide (SO3), showing both a colour change and breakdown into simpler substances. —

CloseThis reaction involves a single reactant (ferrous sulphate) breaking down into multiple products (ferric oxide, sulphur dioxide, and sulphur trioxide) when heat is supplied. This is the defining characteristic of a decomposition reaction, where a compound splits into simpler substances. —

CloseThe gaseous products formed during the decomposition of ferrous sulphate are sulphur dioxide and sulphur trioxide, both of which have a sharp, choking smell similar to burning sulphur. This odour is a key observation in the experiment. —

CloseUpon strong heating, ferrous sulphate decomposes to produce ferric oxide as a solid residue and two acidic gases: sulphur dioxide (SO2) and sulphur trioxide (SO3). Both gases are responsible for the characteristic odour of burning sulphur. —

CloseWhen calcium carbonate (limestone) is heated strongly, it undergoes thermal decomposition to form calcium oxide (quicklime) and carbon dioxide gas. The solid residue left behind is calcium oxide, which is an important industrial material. —

CloseLimestone (calcium carbonate) is heated in kilns to produce quicklime (calcium oxide) and carbon dioxide. This reaction is the basis for manufacturing cement, as quicklime is then mixed with clay and other materials to produce cement clinker. —

CloseWhen a decomposition reaction requires heat energy to break down a compound into simpler products, it is specifically called thermal decomposition. Examples include the heating of ferrous sulphate, calcium carbonate, and lead nitrate. —

CloseLead nitrate decomposes on heating to produce lead oxide (yellow when hot, white when cold), oxygen gas, and nitrogen dioxide gas. Nitrogen dioxide is a reddish-brown gas with a pungent odour, and the brown fumes observed are due to this gas. —

CloseThe balanced chemical equation for the decomposition of lead nitrate is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. So, along with brown nitrogen dioxide gas, colourless oxygen gas is also produced, which supports combustion. —

CloseElectrolysis is the process in which an electric current is passed through a compound (usually in molten or dissolved state) to break it down into its constituent elements. The decomposition of water into hydrogen and oxygen using electricity is a classic example of electrolytic decomposition. —

ClosePure water is a poor conductor of electricity because it has very few ions. Adding a few drops of dilute sulphuric acid increases the concentration of H+ and SO42- ions, which makes the water conductive enough to allow electrolysis to occur efficiently. —

CloseWater (H2O) is made of hydrogen and oxygen atoms. During electrolysis, water decomposes into hydrogen gas at the cathode (negative electrode) and oxygen gas at the anode (positive electrode), in a volume ratio of approximately 2:1 (hydrogen:oxygen). —

CloseWhen sodium sulphate solution is mixed with barium chloride solution, an insoluble white solid called barium sulphate is formed instantly. This appearance of a white precipitate is the key observation, and no gas is evolved in this reaction. —

CloseThe reaction between sodium sulphate and barium chloride produces barium sulphate (BaSO4) and sodium chloride (NaCl). Barium sulphate is insoluble in water and appears as a white precipitate, while sodium chloride remains dissolved in the solution. —

CloseIn this reaction, the two reactants exchange their ions: the sulphate ion (SO42-) from sodium sulphate combines with barium ion (Ba2+) from barium chloride to form barium sulphate, while the chloride ions (Cl-) combine with sodium ions to form sodium chloride. This exchange of ions between two compounds makes it a double displacement reaction. —

CloseDouble displacement reactions involve two ionic compounds in which the positive ion (cation) of one compound pairs with the negative ion (anion) of the other compound, and vice versa. This mutual exchange of partners results in the formation of two new compounds. —

CloseWhen a chemical reaction produces an insoluble solid that separates from the solution, that solid is called a precipitate, and the reaction is known as a precipitation reaction. For example, the formation of barium sulphate in the reaction between sodium sulphate and barium chloride is a precipitation reaction. —

CloseWhen lead nitrate solution reacts with potassium iodide solution, a bright yellow precipitate of lead iodide (PbI2) is formed. This is a characteristic double displacement reaction often used to demonstrate precipitation reactions in the laboratory. —

CloseIn this reaction, barium sulphate is insoluble and forms a precipitate that settles down. Sodium chloride, however, is highly soluble in water and remains dissolved in the solution as ions (Na+ and Cl-). The clear liquid above the precipitate contains sodium chloride. —

CloseThe precipitate formed is barium sulphate (BaSO4), which is insoluble in water. It is produced when barium ions (Ba2+) from barium chloride combine with sulphate ions (SO42-) from sodium sulphate. The pairing of Na+ and Cl- forms soluble sodium chloride, which does not precipitate. —

CloseIn the chemical equation Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq), the notation (s) stands for solid. Barium sulphate is insoluble in water and forms a solid precipitate, so its state symbol is written as (s) to indicate it is a solid. —

CloseThe white precipitate is barium sulphate. Barium ions (Ba2+) coming from barium chloride and sulphate ions (SO42-) coming from sodium sulphate combine to form this insoluble compound. Sodium and chloride ions remain in solution and do not contribute to the precipitate. —

CloseDecomposition reactions can be classified based on the energy source used. When electricity is used to break down a compound, it is called electrolytic decomposition. The breakdown of water into hydrogen and oxygen using an electric current is a perfect example of this type. —

CloseThe cement industry relies heavily on the thermal decomposition of calcium carbonate (limestone) to produce calcium oxide (quicklime). This quicklime is then mixed with clay and other materials and heated to form cement clinker, which is ground to make cement. —

CloseFerrous sulphate heptahydrate (FeSO4·7H2O) is the green crystalline compound. Upon heating, it first loses its seven water molecules to become white anhydrous ferrous sulphate (FeSO4). Only on further strong heating does it decompose into ferric oxide, SO2, and SO3. —

ClosePure water is a very poor conductor of electricity due to a lack of free ions. Adding a few drops of dilute sulphuric acid introduces H+ and SO42- ions into the water, which significantly increases its electrical conductivity, allowing the electrolysis to proceed at a reasonable rate. —

CloseWhen lead nitrate is heated, it decomposes to produce lead oxide (PbO), nitrogen dioxide (NO2), and oxygen (O2). The solid residue left in the test tube is lead oxide, which appears yellow when hot and turns pale yellow or white upon cooling. —

CloseIn Activity 5.5 (heating of ferrous sulphate crystals), the key visual change is that the green crystals first turn white (loss of water) and finally become reddish-brown (formation of ferric oxide). This colour change, along with the evolution of gases, indicates that a decomposition reaction has taken place. —

CloseIn chemistry, when a reaction in solution produces an insoluble solid that separates from the liquid, that solid is called a precipitate. The term “precipitation reaction” specifically refers to such reactions where an insoluble product forms and settles down or remains suspended. —

CloseDilute sulphuric acid is corrosive and can cause burns or damage to clothes and surfaces. Therefore, in the electrolysis of water activity, the addition of sulphuric acid to water must be done carefully, preferably by the teacher, using proper safety precautions such as wearing gloves and goggles. —

CloseWater has the molecular formula H2O, meaning two hydrogen atoms for every one oxygen atom. During electrolysis, the volume of hydrogen gas collected at the cathode is exactly twice the volume of oxygen gas collected at the anode, giving a 2:1 volume ratio (hydrogen:oxygen). —

CloseIn Activity 5.2, lead nitrate solution is mixed with potassium iodide solution. They undergo a double displacement reaction to form a bright yellow precipitate of lead iodide (PbI2). The other product, potassium nitrate (KNO3), remains dissolved in the solution. —

CloseThe decomposition reaction of lead nitrate is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. The products are lead oxide (solid), nitrogen dioxide (brown gas), and oxygen (colourless gas). Nitrogen gas (N2) is not produced in this reaction; it is a different, unreactive gas. —

CloseIn the chemical equation for the decomposition of ferrous sulphate (2FeSO4 → Fe2O3 + SO2 + SO3), sulphur dioxide is produced as a gas. Its state symbol is (g), which stands for gas. It escapes from the solid reaction mixture and can be identified by its characteristic smell. —

CloseThe defining feature of a double displacement reaction is the exchange of ions between the two reacting compounds. In this case, Ba2+ and Na+ exchange their partner anions (SO42- and Cl-), forming BaSO4 and NaCl. This exchange of ions is the primary reason for the classification. —

CloseLead nitrate crystals are white. Upon heating, they do not simply change colour; they decompose with the evolution of brown fumes of nitrogen dioxide gas. The solid residue left is lead oxide, which is yellow when hot and pale yellow/white when cold. The emission of brown fumes is the most characteristic observation. —

CloseThe prefix “thermal” comes from the Greek word “therme,” meaning heat. In decomposition reactions, “thermal decomposition” specifically refers to reactions that require heat energy to break down a compound into simpler products. Examples include heating ferrous sulphate, lead nitrate, or calcium carbonate. —

CloseAll three are thermal decomposition reactions. In each case, a single solid compound is heated, and it breaks down into two or more simpler products (solids and gases). They are not all caused by electricity, nor do they all produce oxygen or a precipitate. —

CloseThe valency of an ion is equal to its charge. The sulphate ion carries a charge of 2- (SO42-), meaning it has gained two electrons. Therefore, its valency is 2. It can combine with two monovalent positive ions like H+, Na+, or K+ to form H2SO4, Na2SO4, or K2SO4. —

CloseHydrogen gas is highly combustible. When a burning candle or a glowing splint is brought near a sample of hydrogen gas, the hydrogen ignites with a characteristic “pop” sound. This is a standard test for the presence of hydrogen gas. —

CloseCalcium oxide (CaO) is commonly known as quicklime. It is produced by the thermal decomposition of limestone (calcium carbonate). When water is added to quicklime, it forms slaked lime (calcium hydroxide), which is used in whitewashing. —

CloseQuicklime is manufactured by heating limestone, which is primarily calcium carbonate (CaCO3), in a rotary kiln. The reaction is: CaCO3 + Heat → CaO + CO2. Marble chips are also a form of calcium carbonate and would produce quicklime on heating as well, but limestone is the industrial source. —

CloseBarium chloride (BaCl2) dissociates into Ba2+ and Cl- ions in solution. The sulphate ion (SO42-) carries a 2- charge. To form a neutral compound, it combines with the positively charged barium ion (Ba2+), producing insoluble barium sulphate (BaSO4). —

CloseHydrated ferrous sulphate crystals (FeSO4·7H2O) are pale green in colour. This green colour is due to the presence of water of crystallisation. Upon heating, they first lose this water and become white anhydrous ferrous sulphate before further decomposing. —

CloseIn a chemical equation, writing the word “Heat” above the forward arrow indicates that heat energy must be supplied to the reaction mixture for the reaction to occur. It is not an indication of whether the reaction is exothermic (gives out heat) or endothermic (absorbs heat), but rather a condition for the reaction. —

CloseThe thermal decomposition of calcium carbonate (limestone) to produce calcium oxide (quicklime) is a crucial step in cement manufacturing. The quicklime is then combined with clay, silica, and other materials and heated to form cement clinker, which is ground into cement. —

CloseDecomposition reactions are named based on the energy source used. When an electric current is passed through a compound (in molten or dissolved state) to break it down, the process is called electrolytic decomposition. The splitting of water into hydrogen and oxygen is a classic example. —

CloseWhile all precipitation reactions are double displacement reactions, not all double displacement reactions produce a precipitate (some produce a gas or water). When a double displacement reaction results in the formation of an insoluble solid called a precipitate, it is specifically known as a precipitation reaction. —

CloseFerrous sulphate crystals contain seven water molecules per formula unit (FeSO4·7H2O). Upon gentle heating, these water molecules are driven off as water vapour, leaving behind white anhydrous ferrous sulphate. Only at higher temperatures does the anhydrous FeSO4 decompose into Fe2O3, SO2, and SO3. —

CloseDecomposition reactions are the opposite of combination reactions. They demonstrate that a single compound, when supplied with sufficient energy (heat, electricity, or light), can be broken down into two or more simpler substances, which may be elements or simpler compounds. This is a fundamental principle of chemical reactivity.