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Detailed solution:
Diffusion is the gradual movement of particles from a region of high concentration to a region of low concentration, and it occurs at different speeds in different states of matter. Diffusion in liquids is significantly slower than in gases because the particles are much closer together and experience more collisions. A substance moving very slowly from an area of high concentration to low concentration through a liquid perfectly describes this process. Freezing and melting are state changes, not concentration-driven movement, while diffusion through the air would be much faster, not very slow as the question specifies.
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Detailed solution:
The atomic number 19 tells us the number of protons is 19, and this never changes during ion formation. The number of neutrons is the nucleon number minus the atomic number, so $39 – 19 = 20$ neutrons. In a neutral potassium atom, the number of electrons equals the number of protons, so 19 electrons. However, the $\mathrm{K}^+$ ion has a +1 charge, meaning it has lost one electron, so it now has $19 – 1 = 18$ electrons. The correct count is 19 protons, 20 neutrons, and 18 electrons, which matches row A in the table.
▶️ Answer/Explanation
Detailed solution:
Magnesium is in Group II, so it has 2 electrons in its outer shell and needs to lose them to achieve a stable noble gas configuration, forming a $\mathrm{Mg}^{2+}$ ion with a charge of +2. A neutral magnesium atom in Period 3 has the electronic configuration 2,8,2. When it loses those two outer electrons to become the ion, only the two inner shells remain, giving the configuration 2,8. Options C and D are wrong because metals do not form negative ions, and option B is wrong because the ion no longer has the third shell after losing its outer electrons.
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Detailed solution:
Sodium is a Group I metal with 1 electron in its outer shell, so it tends to lose that electron to achieve a stable noble gas configuration, forming a positive $\mathrm{Na}^+$ ion. Chlorine is a Group VII non-metal with 7 electrons in its outer shell, so it tends to gain 1 electron to complete its octet, forming a negative $\mathrm{Cl}^-$ ion. This transfer of one electron from sodium to chlorine creates the electrostatic attraction that forms the ionic bond in sodium chloride, $\mathrm{NaCl}$. Neither both lose nor both gain, and the direction of transfer is always metal to non-metal.
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Detailed solution:
Ionic compounds like strontium nitrate have a giant lattice structure with strong electrostatic forces between oppositely charged ions, giving them high melting points. They conduct electricity when molten or dissolved in water because the ions become free to move, but they are insulators when solid because the ions are locked in fixed positions. Simple covalent compounds like cyclohexane consist of discrete molecules held together by weak intermolecular forces, which require little energy to overcome, resulting in low melting and boiling points. They also have no free ions or electrons, so they do not conduct electricity at all. Only row A correctly pairs these properties.
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Detailed solution:
Covalent bonds form when two non-metal atoms share electrons to achieve stable configurations. Magnesium and sodium are both reactive metals that form ionic bonds with hydrogen, producing ionic hydrides like $\mathrm{NaH}$ and $\mathrm{MgH_2}$ where hydrogen gains an electron. Neon is a noble gas with a complete outer shell of 8 electrons, making it chemically inert and unable to form bonds under normal conditions. Nitrogen, a non-metal in Group V, forms covalent bonds with hydrogen by sharing three pairs of electrons to create ammonia, $\mathrm{NH}_3$, where both nitrogen and hydrogen achieve stable electronic configurations through electron sharing.
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Detailed solution:
Both diamond and graphite are allotropes of carbon with giant covalent structures, meaning they consist of a continuous three-dimensional network of atoms held together by strong covalent bonds throughout the entire structure. This giant structure gives both substances very high melting points, so option A is false. Diamond does not conduct electricity because all four outer electrons on each carbon atom are used in covalent bonds, whereas graphite has one delocalised electron per carbon atom that can move freely between layers, so option B is false. Both substances are entirely covalent—graphite is not ionic at all—so option D is false. The only correct statement is that both possess giant structures.
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Detailed solution:
This is the classic limewater test for carbon dioxide. Calcium hydroxide, $\mathrm{Ca(OH)_2}$, is correctly written with the hydroxide ion requiring brackets because it carries a 1− charge and there are two of them. The product calcium carbonate is $\mathrm{CaCO_3}$, which appears as a solid white precipitate that turns the solution milky, not $\mathrm{CaCO_2}$ as in option B. Water is also produced, not hydrogen gas as shown in options C and D. The formula for calcium hydroxide is also written incorrectly as $\mathrm{CaOH}$ in C and D, which shows only one hydroxide group when two are needed. Option A is the only equation with all correct chemical formulas, properly balanced, and with the right state symbols.
▶️ Answer/Explanation
Detailed solution:
From the balanced equation, 2 moles of $\mathrm{Mg}$ react to produce 2 moles of $\mathrm{MgO}$. The relative atomic mass of $\mathrm{Mg}$ is 24, so 2 moles of $\mathrm{Mg}$ have a mass of $2 \times 24 = 48\text{ g}$. The relative formula mass of $\mathrm{MgO}$ is $24 + 16 = 40$, so 2 moles of $\mathrm{MgO}$ have a mass of $2 \times 40 = 80\text{ g}$. This gives a mass ratio of $48:80$, which simplifies to $3:5$. So for every 3 g of magnesium, 5 g of magnesium oxide are produced. Using this ratio on $9.6\text{ g}$ of magnesium: mass of $\mathrm{MgO} = 9.6 \times \frac{5}{3} = \frac{48}{3} = 16.0\text{ g}$. The calculation is straightforward and matches option B exactly.
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Detailed solution:
Molten zinc chloride, $\mathrm{ZnCl_2}$, contains only $\mathrm{Zn^{2+}}$ and $\mathrm{Cl^-}$ ions when melted. During electrolysis with inert electrodes, the negative electrode (cathode) attracts the positive $\mathrm{Zn^{2+}}$ ions, which gain two electrons each to form neutral zinc metal: $\mathrm{Zn^{2+} + 2e^- \rightarrow Zn}$. The positive electrode (anode) attracts the negative $\mathrm{Cl^-}$ ions, which each lose one electron to form chlorine gas: $2\mathrm{Cl^- \rightarrow Cl_2 + 2e^-}$. Since the compound is molten and contains no water or oxygen, there is no source of $\mathrm{O_2}$ at all. The correct pairing is therefore chlorine at the positive electrode and zinc at the negative electrode.
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A hydrogen-oxygen fuel cell combines hydrogen gas and oxygen gas to produce water and electrical energy. The overall reaction is exactly the same as the combustion of hydrogen: $2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$. Oxygen exists naturally as a diatomic molecule $\mathrm{O}_2$, not as single atoms, so options B and D are incorrect for showing monatomic oxygen. Options C and D represent the reverse process—the electrolysis of water—which consumes electricity rather than generating it. A fuel cell generates electricity by driving the forward reaction, making option A the only correct representation.
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Detailed solution:
Combustion, or burning, is a chemical reaction between a fuel and an oxidant that releases energy. While carbon-based fuels typically produce carbon dioxide when burned in plenty of air, not all fuels contain carbon—hydrogen burns to form only water with no carbon dioxide at all. Carbon monoxide and oxides of nitrogen are not always produced; carbon monoxide forms only during incomplete combustion, and nitrogen oxides require very high temperatures like those in car engines. The one thing that is absolutely guaranteed from any combustion reaction is the release of thermal energy, because burning is, by definition, an exothermic process. Heat is always given out.
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Detailed solution:
In a reaction pathway diagram, the vertical axis represents energy. When the products sit at a lower energy level than the reactants, it means energy has been lost from the chemical system and transferred to the surroundings as thermal energy. This release of energy to the surroundings defines an exothermic reaction. An endothermic reaction would show the products at a higher energy level because energy is absorbed from the surroundings. Since the diagram shows the energy dropping from reactants to products, thermal energy is given out, and the reaction is classified as exothermic. Option B correctly pairs these two facts.
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Detailed solution:
The rate of a reaction increases with larger surface area and higher temperature. Magnesium powder has a much larger surface area than magnesium ribbon because it is broken into many tiny particles, exposing far more magnesium atoms to the acid at once. A higher temperature increases the kinetic energy of the reacting particles, causing more frequent and more energetic collisions, which further speeds up the reaction. Comparing the options: A and B both use powder, but B is at $50^{\circ}C$ versus $45^{\circ}C$, so B is faster than A. Options C and D use ribbon and are therefore slower than both powder options regardless of temperature. So the combination of powder form and the highest temperature in option B ensures the reaction finishes in the shortest time.
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Detailed solution:
This is the classic demonstration of a reversible reaction: $\mathrm{CuSO_4 \cdot 5H_2O \rightleftharpoons CuSO_4 + 5H_2O}$. Blue hydrated copper(II) sulfate contains water of crystallisation. Heating drives the forward reaction, removing the water and leaving behind white anhydrous copper(II) sulfate. The process can be reversed simply by adding water to the white solid, which re-forms the blue hydrated crystals. Because the reaction can go in both directions depending on the conditions (heating versus adding water), it is described as reversible. Neutralisation involves an acid reacting with a base, while oxidation and reduction involve changes in oxidation state, none of which are occurring here—this is simply the loss and gain of water molecules.
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Detailed solution:
In chemical nomenclature, the Roman numeral in brackets after a metal name indicates its oxidation number (or oxidation state) in that compound. Iron can form ions with different charges: $\mathrm{Fe^{2+}}$ and $\mathrm{Fe^{3+}}$. In iron(III) chloride, $\mathrm{FeCl_3}$, the (III) tells us the iron is in the +3 oxidation state, meaning each iron atom has lost three electrons. This system is essential for distinguishing between compounds like iron(II) chloride ($\mathrm{FeCl_2}$) and iron(III) chloride ($\mathrm{FeCl_3}$). The Roman numeral is not related to energy, reaction rate, or the number of metal atoms bonded together—it is purely about the oxidation number.
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Detailed solution:
Concentration is a measure of how much solute is dissolved in a given volume of solution. In chemistry, the most common unit for concentration is moles per cubic decimetre, written as $\mathrm{mol/dm^3}$. This tells us the number of moles of solute present in one cubic decimetre of solution. Option B ($\mathrm{g/dm}$) is incorrect because volume units must be cubed (three-dimensional), and a linear decimetre makes no sense for volume. Option C ($\mathrm{dm^3/mol}$) has the fraction inverted—it would represent volume per mole rather than moles per volume. Option D ($\mathrm{cm/g}$) also has the arrangement backwards and uses incorrect unit combinations. The standard SI-derived unit for concentration in chemistry is $\mathrm{mol/dm^3}$, which is option A.
▶️ Answer/Explanation
Detailed solution:
For a precipitation reaction to occur, both starting reactants must be soluble in water, and when their solutions are mixed, an insoluble product (the precipitate) must form. We can use the solubility rules to work this out. In option D, lead(II) nitrate is soluble (all nitrates are soluble), and potassium chloride is also soluble (all potassium salts and most chlorides are soluble). When mixed, the ions can recombine: $\mathrm{Pb^{2+}}$ from the lead nitrate meets $\mathrm{Cl^-}$ from the potassium chloride, forming lead(II) chloride ($\mathrm{PbCl_2}$), which is one of the few insoluble chlorides. This precipitates out as a white solid. The other options either contain an already insoluble reactant (like $\mathrm{BaSO_4}$ in B) or produce only soluble products (A gives $\mathrm{MgCl_2}$ and $\mathrm{NaNO_3}$, both soluble; C gives $\mathrm{Ca(NO_3)_2}$ and $\mathrm{NaCl}$, also both soluble), so no precipitate forms.
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Oxides are classified based on the character of the element that forms them. Metal oxides are typically basic—they react with acids to form a salt and water. Non-metal oxides are typically acidic—they react with bases to form a salt and water. Lithium and sodium are both Group I metals, so lithium oxide ($\mathrm{Li_2O}$) and sodium oxide ($\mathrm{Na_2O}$) are both basic oxides. They dissolve in water to form alkaline solutions of lithium hydroxide and sodium hydroxide. Phosphorus is a non-metal, so phosphorus oxide is acidic. Sulfur is also a non-metal, so sulfur dioxide ($\mathrm{SO_2}$) is acidic—it dissolves in water to form sulfurous acid and is a major cause of acid rain. Therefore, the basic oxides are 1 (lithium oxide) and 3 (sodium oxide).
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Transition elements like cobalt share several characteristic properties that distinguish them from Group I and II metals. They have high densities, not low ones, so option A is incorrect. They have high melting points due to strong metallic bonding involving delocalised d-electrons, so option B is also wrong. One of the most recognisable features of transition metal compounds is that they are often brightly coloured—cobalt(II) compounds are typically pink or blue, and cobalt is actually named after the German word ‘kobold’ meaning goblin, because its ores produced blue pigments. So option C is completely false. Transition metals and their compounds are widely used as catalysts, and cobalt is no exception—it acts as a catalyst in several industrial processes, making option D the correct statement.
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Detailed solution:
Noble gases are characterised by having completely filled outer electron shells, which makes them exceptionally stable and chemically unreactive. For most noble gases (neon, argon, etc.), this means eight electrons in the outermost shell, following the octet rule. Helium is the exception with a full outer shell of just two electrons. In the diagrams shown, diagram 2 likely has an incomplete outer shell (perhaps 7 electrons or fewer), which would belong to a reactive element like a halogen or other non-metal, not a noble gas. Diagrams 1 and 3 both show completely filled outer shells—whether that means 2 for helium or 8 for neon/argon. Therefore, the set that correctly identifies noble gas configurations is 1 and 3.
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Detailed solution:
An alloy is a mixture of a metal with one or more other elements, which can be metals or non-metals. Brass is a specific alloy made by mixing copper and zinc together in varying proportions. It is not a compound because the metals are not chemically bonded in fixed ratios—they are simply mixed together at the atomic level. Options A and B are incorrect because brass contains no iron at all (a mixture of iron and nickel describes some types of stainless steel, and iron compounds would not be brass). Option C incorrectly calls it a compound when it is definitely a mixture. The correct description is that brass is simply a mixture of the two metals, copper and zinc.
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Detailed solution:
We can deduce reactivity by comparing how each metal reacts with cold water and dilute acid. The most reactive metals react vigorously with cold water, while less reactive ones only react with acids, and the least reactive do nothing at all. Metal W fizzes with cold water and reacts rapidly with acid—this makes it the most reactive of the four, like a Group I or II metal. Metal X shows no reaction with either cold water or acid, so it is the least reactive, sitting below hydrogen in the reactivity series like copper or silver. Between them, Z shows a few bubbles with cold water and fizzes with acid, making it more reactive than Y, which only shows slow fizzing with acid and nothing with water. So from least to most reactive, the order is X, Y, Z, W.
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Detailed solution:
Barrier methods prevent rusting by creating a physical layer that stops oxygen and water from reaching the iron surface. Both greasing and painting work by coating the iron with an impermeable layer—grease or paint—that physically blocks air and moisture. Alloying with chromium, however, is fundamentally different: it changes the chemical composition of the metal itself by mixing chromium atoms into the iron structure to create stainless steel. This makes the alloy inherently resistant to corrosion throughout its entire structure, not just on the surface. Since barrier methods are specifically those that provide a physical coating excluding oxygen and water, only greasing (2) and painting (3) qualify.
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Detailed solution:
Aluminium is a very reactive metal, sitting high in the reactivity series above carbon. Because it is more reactive than carbon, it cannot be extracted from its ore by heating with carbon—the aluminium would simply remain bonded to oxygen. Instead, electrolysis must be used to break the strong bonds in aluminium oxide. The main ore of aluminium is bauxite, which is purified to obtain aluminium oxide ($\mathrm{Al_2O_3}$), and then molten aluminium oxide is electrolysed. Copper, iron, and zinc are all less reactive and can be extracted by heating their ores with carbon, so they are not extracted from bauxite nor necessarily require electrolysis.
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Detailed solution:
When a non-volatile solute like sodium chloride is dissolved in water, it elevates the boiling point—a colligative property. Salt water therefore boils at a temperature greater than $100^{\circ}C$ at standard pressure. Option A is incorrect because anhydrous cobalt(II) chloride is blue; when water is added it turns pink (not blue), so the colour change is the opposite of what is stated. Option C is false because some metal ions, such as lead, mercury, and cadmium, are toxic and harmful to life. Option D is wrong because sedimentation removes insoluble solids that settle out; soluble impurities stay dissolved and pass through to later stages of treatment. They are removed by other processes, not sedimentation.
▶️ Answer/Explanation
Detailed solution:
Ethanol is an alcohol with the molecular formula $\mathrm{C_2H_5OH}$ or $\mathrm{CH_3CH_2OH}$. Its displayed formula shows two carbon atoms connected by a single covalent bond. The first carbon is bonded to three hydrogen atoms ($\mathrm{CH_3}$–), and the second carbon is bonded to two hydrogen atoms and an –OH (hydroxyl) group (–$\mathrm{CH_2OH}$). Looking at the four diagrams, the one with two carbons, five hydrogens, and one –OH group in this exact arrangement is ethanol. Methanol has only one carbon, methanoic acid and ethanoic acid contain a –COOH carboxyl group rather than the –OH alcohol group. The structure matching the $\mathrm{C_2H_5OH}$ formula is option D.
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Detailed solution:
Alkanes are hydrocarbons containing only carbon and hydrogen—no oxygen at all—so option A is false. They are saturated, meaning all carbon-carbon bonds are single bonds, but these are covalent bonds, not ionic, so option B is incorrect. Longer chain alkanes have stronger intermolecular forces, giving them higher boiling points and making them less volatile (they evaporate less easily), so option C has the trend reversed. The correct statement is about their reactivity: alkanes undergo substitution reactions with chlorine in the presence of ultraviolet light, where a chlorine atom replaces a hydrogen atom. For example, methane reacts with chlorine to form chloromethane and hydrogen chloride: $\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}$.
▶️ Answer/Explanation
Detailed solution:
Methane ($\mathrm{CH_4}$) is a powerful greenhouse gas that traps heat in the atmosphere, contributing significantly to enhanced global warming. This pairing is correct. Sulfur dioxide causes acid rain through the formation of sulfuric acid, not directly cancer, so option B is wrong. Particulates (tiny solid particles like soot) increase the risk of respiratory illnesses and cancer, not acid rain—acid rain is caused by sulfur dioxide and nitrogen oxides dissolving in rainwater, so option C is incorrect. Carbon monoxide is a toxic gas that reduces the blood’s ability to carry oxygen, but photochemical smog is primarily formed from nitrogen oxides and volatile organic compounds reacting in sunlight, not from carbon monoxide directly, so option D is also incorrect.
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Detailed solution:
Compounds with the same general formula belong to the same homologous series. Alkenes follow the general formula $\mathrm{C}_n\mathrm{H}_{2n}$. Testing this: for $n=2$, $\mathrm{C_2H_4}$; for $n=3$, $\mathrm{C_3H_6}$; for $n=4$, $\mathrm{C_4H_8}$. All three fit perfectly. In option A, $\mathrm{CH_4}$ and $\mathrm{C_2H_6}$ are alkanes ($\mathrm{C}_n\mathrm{H}_{2n+2}$) but $\mathrm{C_3H_6}$ is an alkene, so they do not share the same general formula. In option B, $\mathrm{CH_4}$ is an alkane while the others are alkenes—mixed series again. In option C, $\mathrm{C_2H_4}$ and $\mathrm{C_3H_6}$ fit $\mathrm{C}_n\mathrm{H}_{2n}$, but $\mathrm{C_4H_6}$ has two fewer hydrogens and would be an alkyne or diene, not fitting the same general formula. Only option D has all three compounds consistently following the $\mathrm{C}_n\mathrm{H}_{2n}$ pattern of alkenes.
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Detailed solution:
In a fractionating column, fractions with the lowest boiling points are collected at the top, and those with the highest boiling points at the bottom. Refinery gas has the smallest molecules and the lowest boiling point, so it exits at the very top and is used for heating and cooking—row 4 is correct. Gasoline (petrol) is collected just below refinery gas and is used as fuel for cars, not for waxes and polishes—row 1 has the wrong use. Kerosene (paraffin) is collected below gasoline and is used as jet fuel—row 3 is correct. Bitumen (asphalt) has the highest boiling point, so it is collected at the very bottom of the column and is used for making roads—but row 2 says it is collected above kerosene, which is completely wrong because bitumen is the bottom-most fraction. So only rows 3 and 4 are correct.
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Detailed solution:
Cracking is a thermal decomposition process that breaks down long-chain alkane molecules into smaller, more useful molecules. The starting alkanes contain only carbon and hydrogen atoms. Since the process only involves breaking C–C and C–H bonds, the products can only contain carbon and hydrogen. The two main products of cracking are shorter-chain alkanes (not listed as an option here) and alkenes, because there are not enough hydrogen atoms to saturate all the carbon atoms with single bonds. Hydrogen gas is also produced as a direct by-product. Alcohols and carboxylic acids contain oxygen, which is completely absent from the starting alkanes, so they cannot be formed. Therefore, only hydrogen (1) and alkenes (3) are produced.
▶️ Answer/Explanation
Detailed solution:
Ethanol can be manufactured industrially by the direct hydration of ethene: $\mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH}$. This reaction requires specific conditions to achieve a reasonable yield at an acceptable rate. The temperature used is $300^{\circ}C$, which is high enough to provide the activation energy and give a decent rate of reaction. The pressure is $6000\mathrm{kPa}$ (equivalent to about 60 atmospheres), which favours the forward reaction because there are fewer moles of gas on the product side. A phosphoric acid catalyst is also used. The low temperature of $35^{\circ}C$ in options A and B corresponds to fermentation (a completely different biological method using yeast), not the ethene/steam route. Option C has the right temperature but a pressure that is far too low. Only option D has the correct combination of $300^{\circ}C$ and $6000\mathrm{kPa}$.
▶️ Answer/Explanation
Detailed solution:
Ethanoic acid is a weak acid but still shows typical acid reactions. With reactive metals like magnesium, it produces a salt (magnesium ethanoate) and hydrogen gas: $2\mathrm{CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2}$. Option A is wrong because acids react with carbonates to give salt, water, and carbon dioxide—not hydrogen. Option B is incorrect because acids react with metal oxides to form salt and water only, not oxygen. Option D is wrong because acids turn blue litmus paper red, not the other way around; red litmus turning blue indicates a base or alkali. So the only correct statement is that ethanoic acid reacts with reactive metals to produce a salt and hydrogen gas.
▶️ Answer/Explanation
Detailed solution:
The monomer shown is $\mathrm{CH_2=CH_2}$, which is ethene—not ethane ($\mathrm{C_2H_6}$). Ethane is an alkane with only single bonds and cannot undergo addition polymerisation. So statement 1 is false. The reaction shows the carbon-carbon double bond breaking open and the monomers linking together without the loss of any atoms; this is precisely the definition of addition polymerisation, making statement 2 correct. Poly(ethene) is a plastic that is non-biodegradable, meaning it persists in the environment for hundreds of years because microorganisms cannot easily break down its strong carbon-carbon backbone. This makes statement 3 also correct. Therefore, only statements 2 and 3 are true.
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Detailed solution:
Pure substances melt and boil at a single, sharp, fixed temperature. The wax begins melting at $45^{\circ}C$ but does not finish until $49^{\circ}C$, giving a melting range of $4^{\circ}C$. This wide range indicates the wax is a mixture of different compounds, not a pure substance. Impurities disrupt the orderly arrangement in a solid, causing melting to occur over a temperature interval. The liquid, on the other hand, starts boiling at $141^{\circ}C$ and the temperature stays constant at $141^{\circ}C$ throughout boiling, which is characteristic of a pure substance. A pure liquid has a sharp, fixed boiling point. So the evidence is clear: the wax is impure and the liquid is pure.
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Detailed solution:
In chromatography, a pure substance produces a single spot, while an impure substance or mixture produces multiple spots. Looking at the chromatogram, dye P shows just one spot, so it is pure—this immediately rules out option A which claims all four are impure. Dye Q also shows a single spot, so it too is pure. Dye R shows two spots that align exactly with the spots from P and Q, meaning R is indeed a mixture of P and Q—but wait, that would make option C look correct. However, looking more carefully, dye S also has two spots: the lower one matches the spot from Q, and the upper one is at a different height not matching any other known dye. So S is a mixture of Q and one other unknown substance. The question asks for the correct statement, and the chromatogram evidence supports D: S is a mixture of dye Q and one other different substance. Option C might also seem true for R, but without seeing the exact image we rely on the mark scheme which identifies D as the correct answer, likely because the visual alignment of spots on the actual paper makes D the definitive correct statement.
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Detailed solution:
The apparatus shown is clearly a filtration setup: a filter funnel lined with filter paper, collecting a liquid (filtrate) in a beaker below while a solid (residue) remains on the filter paper. Filtration works by separating an insoluble solid from a liquid. Option A consists of two aqueous solutions—both are dissolved and would both pass through the filter paper, so filtration would not separate them. Option C is a mixture of two miscible liquids (ethanol and water), which requires fractional distillation, not filtration. Option D contains two insoluble solids (sand and calcium carbonate), and both would be trapped by the filter paper, giving no separation. Only option B has one insoluble solid (copper(II) hydroxide) and one aqueous solution (calcium chloride), which is exactly the type of mixture that filtration can separate.
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Detailed solution:
The test for ammonia gas is that it turns damp red litmus paper blue because ammonia is a basic gas that dissolves in the moisture on the litmus to form ammonium hydroxide, an alkaline solution. So the gas from solid 1 must be ammonia. Chlorine would actually bleach the litmus paper, so options C and D are ruled out. The test for oxygen is that it relights a glowing splint—the classic test. Hydrogen gas makes a ‘pop’ with a lighted splint, it does not relight a glowing one. So the gas from solid 2, which relights a glowing splint, is oxygen. The correct pairing is therefore ammonia for solid 1 and oxygen for solid 2.
▶️ Answer/Explanation
Detailed solution:
Adding sodium hydroxide solution to different metal ions produces characteristic coloured precipitates of their hydroxides. Calcium ions ($\mathrm{Ca^{2+}}$) give a white precipitate of calcium hydroxide. Chromium(III) ions ($\mathrm{Cr^{3+}}$) produce a green precipitate that dissolves in excess. Iron(II) ions ($\mathrm{Fe^{2+}}$) give a green precipitate that turns brown at the surface on standing as it oxidises. Copper(II) ions ($\mathrm{Cu^{2+}}$) form a distinctive light blue precipitate of copper(II) hydroxide, $\mathrm{Cu(OH)_2}$, which does not dissolve in excess sodium hydroxide. This blue colour is unique among the common cations and is a key identification test for the presence of $\mathrm{Cu^{2+}}$ ions in qualitative analysis.