▶️ Answer/Explanation
Detailed solution:
Substance L melts at \(-7^{\circ}C\) and is a brown liquid at room temperature, which is usually around \(20^{\circ}C\) to \(25^{\circ}C\). This means the boiling point must be higher than room temperature, otherwise it would be a gas already. Looking at the options, \(59^{\circ}C\) is the only temperature that sits comfortably above room temperature and makes sense as a boiling point for a substance that is liquid at room temperature. The other options are either below or too close to the melting point to keep it liquid.
▶️ Answer/Explanation
Detailed solution:
When you heat a gas and keep the pressure steady, the particles move faster and spread apart, so the volume increases. On the flip side, if you squeeze a gas by increasing the pressure while keeping the temperature constant, the particles get forced closer together, making the volume decrease. Row C captures both relationships correctly: volume increases with temperature, and volume decreases with pressure.
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Detailed solution:
Diffusion means that particles of nitrogen dioxide and air will mix together because of their constant random motion. Even though nitrogen dioxide is denser than air, given a full 24 hours, the particles will eventually spread evenly throughout both gas jars. The final result is a uniform mixture that looks like a lighter, even brown colour across the entire space of both jars, which is exactly what diagram D shows.
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Detailed solution:
The question wants a mixture made specifically from one element and one compound. Aqueous glucose is glucose (a compound) dissolved in water (another compound), so that is two compounds. Clean dry air and stainless steel are both mixtures of many different elements and compounds. Oxygen dissolved in distilled water is simply oxygen molecules (an element) mixed with water molecules (a compound), fitting the description perfectly.
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Detailed solution:
The atomic number is the basic identity card of an element, and it is defined solely by the count of protons in the nucleus. Option D states this exactly. Option C says “number of particles in the nucleus,” but that would include neutrons too, which actually gives you the mass number, not the atomic number. The group number and the mass of an atom have nothing to do with defining atomic number, so D is the clear winner.
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Detailed solution:
Isotopes are versions of the same element that have an identical number of protons but a different number of neutrons. Since the nucleon number is the sum of protons plus neutrons, a change in the number of neutrons directly changes the nucleon number. The electron arrangement, including the number of shells and outer electrons, stays exactly the same because isotopes of an element have identical chemical properties governed by their matching electronic configurations.
▶️ Answer/Explanation
Detailed solution:
In the reaction between sodium and chlorine, sodium atoms from Group I want to lose one electron to get a full outer shell, and chlorine atoms from Group VII want to gain one electron to achieve the same stable configuration. So each chlorine atom gains one electron to become a $Cl^{-}$ ion. Gaining or losing a proton is not possible in a chemical reaction because that would change the element’s identity, so the word “proton” acts as a distractor here.
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Detailed solution:
We need to find the atoms that put every single one of their outer shell electrons into shared covalent bonds. Hydrogen has only one electron, and in all these molecules it shares that one electron, so it always uses its entire outer shell. Carbon has four outer electrons, and in $CH_4$ it forms four covalent bonds, using them all. Chlorine in HCl and oxygen in $H_2O$, however, have lone pairs of electrons that sit on the atom and are not used for bonding, so they do not use all their outer shell electrons. That makes C and H the only correct pair.
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Detailed solution:
A giant covalent structure is a huge network of atoms all connected by covalent bonds. Sodium chloride and magnesium chloride are ionic compounds with giant ionic lattices, not covalent networks. Ammonia is a simple molecular substance made of small $NH_3$ molecules with weak intermolecular forces. Graphite is a perfect example of a giant covalent structure, where each carbon atom is covalently bonded to three others, forming layers of interlocking hexagons that extend throughout the whole solid.
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Detailed solution:
By carefully counting the carbon and hydrogen atoms in each diagram, the first, third, and fourth molecules all have four carbon atoms and ten hydrogen atoms, giving the molecular formula $C_4H_{10}$. The second molecule stands out because it has five carbon atoms and twelve hydrogen atoms, making it $C_5H_{12}$. So three of the four molecules share the exact same molecular formula, while one is different, which is exactly what option D states.
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Detailed solution:
The balanced equation $2Mg + O_2 \rightarrow 2MgO$ tells us that 2 moles of magnesium atoms react with 1 mole of oxygen molecules. Using relative atomic masses, $A_r$ of Mg is 24 and $M_r$ of $O_2$ is 32, so 48g of magnesium reacts with exactly 32g of oxygen. This simplifies to a 3:2 mass ratio. So for 6g of magnesium, we set up the proportion: if 3 parts is 6g, then 2 parts is 4g. Hence, exactly 4g of oxygen is needed.
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Detailed solution:
Looking at the cell diagram, X is connected to the positive terminal of the power supply, which makes it the anode. Y is attached to the negative terminal, so that is the cathode. Z is the substance the electrodes dip into that conducts the current, which is the electrolyte. For electrolysis to work, the ions must be free to move around, so the salt needs to be dissolved in water (aqueous) or melted. Solid salt won’t work. So A has everything labelled perfectly.
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Detailed solution:
In a hydrogen-oxygen fuel cell, hydrogen and oxygen are fed in and react cleanly to produce a flow of electrons (electricity), with water being the only chemical product. Option A is wrong because it makes hydrogen peroxide, $H_2O_2$. Option B is actually the reverse process, the electrolysis of water, which consumes electricity rather than producing it. The correct and balanced equation is $2H_2 + O_2 \rightarrow 2H_2O$, which is simply the reaction of hydrogen burning in oxygen, but carried out in a controlled way inside the fuel cell.
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Detailed solution:
The more exothermic a reaction is, the more heat energy it dumps into the surroundings, raising the temperature. On a reaction pathway diagram, an exothermic reaction has the products sitting lower in energy than the reactants. The bigger the energy drop from reactants to products, the more heat is given out. Diagram C shows the largest vertical gap downwards, which represents the biggest overall transfer of thermal energy to the surroundings, so it will cause the largest temperature increase.
▶️ Answer/Explanation
Detailed solution:
A chemical change produces a brand-new substance, while a physical change just alters the form or state. Dissolving copper(II) sulfate, distilling ethanol, and freezing water are all reversible physical processes where the chemical identity of the substance stays the same. Neutralising copper(II) oxide, however, means reacting it with an acid to form a copper salt and water. This creates entirely new chemical compounds, making it the only chemical change in the list.
▶️ Answer/Explanation
Detailed solution:
On a graph of gas volume against time, the rate of reaction at any particular moment is shown by how steep the curve is at that point—the steeper the tangent, the faster the reaction. Point A sits on the steepest part of the upper curve, right at the start where the slope is sharp and the reaction is going full speed. The other points are on much flatter sections, meaning the reaction has slowed down significantly by the time it reaches those points.
▶️ Answer/Explanation
Detailed solution:
The symbol $\rightleftharpoons$ has two arrows pointing in opposite directions, one on top of the other. This means the reaction can go both forwards and backwards at the same time. In this case, sulfur dioxide and oxygen can combine to form sulfur trioxide, but simultaneously, sulfur trioxide can break down back into sulfur dioxide and oxygen. This two-way street behaviour is the very definition of a reversible reaction.
▶️ Answer/Explanation
Detailed solution:
Redox reactions involve changes in oxidation states, often visible as one substance gaining oxygen while another loses it. Incomplete combustion of propane involves oxygen adding to carbon and hydrogen atoms (oxidation), and rusting is iron metal being oxidised to iron(III) oxide. So reactions 1 and 2 are definitely redox. Reaction 3 is a straightforward precipitation of silver chloride with no oxidation state changes. Reaction 4 is just calcium carbonate breaking down into calcium oxide and carbon dioxide, which is a thermal decomposition but not a redox reaction. So only 1 and 2 fit the bill.
▶️ Answer/Explanation
Detailed solution:
Let’s go through each row and check if the description matches the pH. A pH of 1 is extremely acidic, not alkaline at all, so A is wrong. A pH of 5 is slightly acidic, nowhere near neutral (which is pH 7), so B fails. A pH of 9 is alkaline, but universal indicator turns blue or purple at that pH, not yellow—yellow is for around pH 6. D says a pH of 14 contains $OH^{-}$ ions, and this is absolutely correct: any alkaline solution with a high pH has an excess of hydroxide ions, and pH 14 is the maximum alkalinity on the scale.
▶️ Answer/Explanation
Detailed solution:
Basic oxides are formed exclusively by metals, and the more metallic the element, the more basic its oxide. Group I alkali metals are the most metallic elements on the whole table, so their oxides (like $Na_2O$ or $K_2O$) are strongly basic. Period 3 is a mix—it starts with metals like sodium and magnesium that form basic oxides, but then moves into non-metals like sulfur which form acidic oxides. Group VIII noble gases are almost completely unreactive and don’t form oxides at all under normal conditions. So only Group I guarantees that its elements form purely basic oxides.
▶️ Answer/Explanation
Detailed solution:
Elements in the same group have the same number of outer shell electrons, not the same full electronic configuration, so A is not correct. Metallic character actually decreases from left to right across a period, making C false. The Periodic Table is arranged by increasing proton number, not nucleon number, so D is also wrong. Option B is correct: Group VII elements have 7 outer electrons, and to get a full shell of 8 they need to gain one electron, forming an ion with a 1- charge. The maths is 7 minus 8, which equals -1.
▶️ Answer/Explanation
Detailed solution:
Going down Group I from lithium to sodium to potassium, the atoms get bigger and the single outer electron gets further from the nucleus, making it easier to lose. This means reactivity increases down the group. The melting points of alkali metals are fairly low and they drop as you go down because the metallic bonding gets weaker. Density, however, generally increases down the group as the atomic mass grows faster than the atomic size. So the correct trends are decreasing melting point, increasing density, and increasing reactivity, which matches row A perfectly.
▶️ Answer/Explanation
Detailed solution:
All halogens exist as diatomic molecules like $Cl_2$, $Br_2$, and $I_2$, so statement 1 is correct. However, they are not all gases—bromine is a liquid and iodine is a solid at room temperature, so statement 2 is false. Density definitely increases as you go down the group because the atoms get much heavier, so statement 3 is correct. Reactivity in Group VII actually decreases down the group as it gets harder for larger atoms to attract an extra electron, making statement 4 false. So only statements 1 and 3 are correct.
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Detailed solution:
Transition elements are metals with some very characteristic properties. They are known for having high densities and high melting points, so option A fits perfectly. Being metals, they conduct heat very well, so B is wrong. Transition metals are famous for forming coloured compounds, not just white ones, so C is false. And they have characteristically high melting points, not low ones, so D is also incorrect.
▶️ Answer/Explanation
Detailed solution:
The description fits noble gases from Group VIII. Argon and krypton both have a full outer shell of eight electrons and exist as single atoms. Helium is a noble gas that exists as single atoms and is unreactive, but it only has two electrons in its outer shell, not eight, because its first shell is complete with just two. Oxygen is not a noble gas at all—it is diatomic, reactive, and has six outer electrons. So only argon and krypton perfectly match all three parts of the description: monatomic, unreactive, and with eight outer electrons.
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Detailed solution:
Calcium is above magnesium in the reactivity series, so it reacts more vigorously with cold water, making statement 1 correct. Aluminium is more reactive than iron and is extracted by electrolysis, which is much harder and costlier than extracting iron in a blast furnace, so statement 2 is false. Copper sits below hydrogen in the reactivity series and cannot react with dilute acids at all, whereas iron does, so statement 3 is also false. All metals are good conductors of electricity, so both the highly reactive potassium and the unreactive silver conduct well, making statement 4 true. Only 1 and 4 are correct.
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Detailed solution:
In the reaction with dilute hydrochloric acid, bubbles are hydrogen gas, and the more bubbles produced, the faster and more vigorously the metal reacts. The faster the reaction, the more reactive the metal. So ‘lots of bubbles’ for P means it’s the most reactive, ‘few bubbles’ for Q puts it second, ‘very few bubbles’ for S means it’s reacting extremely slowly, and ‘no bubbles’ for R means it isn’t reacting at all and is the least reactive. So the order from most to least reactive is P, then Q, then S, then R, which matches option C exactly.
▶️ Answer/Explanation
Detailed solution:
For iron to rust, it needs both water (or moisture) and oxygen. Looking at the conditions: test-tube 1 has an iron nail exposed to air and water, so rusting will definitely happen. Test-tube 2 has water that has been boiled to remove dissolved air, and it’s covered with a layer of oil to prevent more air from getting in, so there’s no oxygen available and rusting won’t happen. Test-tube 3 has a drying agent that removes all moisture from the air, so there’s no water present and no rusting can occur. Only test-tube 1 provides both essential ingredients for rust formation.
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Detailed solution:
The main reason we mix metals into alloys is to make them stronger and more useful. Pure metals have a regular arrangement of atoms where layers can slide over each other easily, making them soft and easily bent out of shape. In an alloy, different-sized atoms are introduced which disrupt the neat layers and stop them from sliding. This makes the alloy harder, stronger, and better at holding its shape under stress. They are not necessarily cheaper or easier to extract, and they are definitely not easier to hammer into shape than the soft pure metals.
▶️ Answer/Explanation
Detailed solution:
When heated, copper reacts with the oxygen in the air to form solid copper(II) oxide, removing the oxygen from the gas mixture. Clean, dry air is about 21% oxygen and about 79% nitrogen plus other inert gases that do not react with copper. So starting with 100cm³ of air, the 21cm³ of oxygen gets used up and locked into the solid oxide, leaving behind the 79cm³ of unreactive gases. When all the remaining gas is pushed into the syringe, it reads 79cm³ exactly.
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Detailed solution:
Acid rain forms when certain gases dissolve in rainwater to produce acids. Sulfur dioxide dissolves and oxidises to form sulfuric acid, while oxides of nitrogen (like NO and $NO_2$) dissolve to form nitric acid. Carbon monoxide is a toxic gas, but it is neutral and does not form an acid when dissolved in water, so it does not contribute to acid rain at all. Therefore, only sulfur dioxide and oxides of nitrogen are the culprits here.
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Detailed solution:
Members of the same homologous series share the same general formula. Compounds 1 and 2 have formulas $C_2H_4$ and $C_4H_8$, which both fit the general formula for alkenes, $C_nH_{2n}$. They both contain a carbon-carbon double bond and belong to the alkene series. Compound 3 is $C_4H_{10}$, which fits the alkane general formula $C_nH_{2n+2}$ and has only single bonds. Since it belongs to a different homologous series from the other two, only 1 and 2 are in the same series.
▶️ Answer/Explanation
Detailed solution:
Gasoline, also known as petrol, is a fraction obtained from the fractional distillation of petroleum. It consists of hydrocarbon molecules with chain lengths typically in the range of about 5 to 10 carbon atoms. Looking at the options presented, the fuel with molecules in the appropriate chain length range matching the gasoline fraction is option A.
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Detailed solution:
The bromine water test distinguishes between alkanes and alkenes. The first two hydrocarbons shown are alkanes—methane and ethane—which are saturated and do not react with bromine water in the dark, so the orange colour stays unchanged. The third hydrocarbon is ethene, an alkene with a carbon-carbon double bond. It is unsaturated and undergoes an addition reaction with bromine, instantly turning the orange bromine water colourless. So the correct sequence is: stays unchanged, stays unchanged, changes from orange to colourless.
▶️ Answer/Explanation
Detailed solution:
Ethene is an alkene with a carbon-carbon double bond. When it reacts with steam ($H_2O$) in the presence of a phosphoric acid catalyst, a water molecule adds across the double bond. This is an addition reaction that produces ethanol ($C_2H_5OH$). It is one of the two main industrial methods for manufacturing ethanol, the other being fermentation. The reaction specifically gives ethanol, not ethane, ethanoic acid, or a polymer.
▶️ Answer/Explanation
Detailed solution:
Ethanoic acid is a weak acid, so its pH is around 3 to 4, not 10 which would be an alkaline pH. Statement 1 is false. As an acid, it does react with metal carbonates in a classic fizzing reaction that produces carbon dioxide gas, a salt, and water, so statement 2 is true. It also reacts with reactive metals like magnesium to give off hydrogen gas, just like any other acid, so statement 3 is also true. Therefore, only statements 2 and 3 are correct.
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Detailed solution:
Poly(ethene) is a very stable, unreactive addition polymer made up of long hydrocarbon chains. This stability means it is not biodegradable—microorganisms cannot break it down, and water does not decompose it. So when plastic bags end up in the ocean, they just sit there for potentially hundreds of years, accumulating over time because they do not decompose. Whether it is saturated or made from hydrocarbon monomers is not the direct reason for its persistence in the environment; it is the lack of decomposition that is the core problem.
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Detailed solution:
In paper chromatography, the paper must be set up so the baseline where the sample spots are placed is above the level of the solvent. If the solvent reaches the spots directly, the samples will simply dissolve straight into the bulk liquid instead of travelling up the paper. Diagram B correctly shows the solvent level sitting below the pencil baseline and the sample spots, which is the proper setup to allow the solvent to rise up the paper by capillary action and carry the components with it.
▶️ Answer/Explanation
Detailed solution:
To separate a mixture of liquids that have different boiling points, you need a technique that exploits those boiling point differences. Fractional distillation does exactly that—it heats the mixture so the liquid with the lower boiling point vaporises first, the vapours rise up a fractionating column, and they condense and are collected separately. Simple distillation might work for two liquids with very different boiling points, but fractional distillation is the specific technique for mixtures where the boiling points could be relatively close together. Dissolving, crystallisation, and filtration are all for separating solids from liquids, not liquid-liquid mixtures.
▶️ Answer/Explanation
Detailed solution:
Sodium chloride solution contains $Na^{+}$ and $Cl^{-}$ ions. When aqueous sodium hydroxide is added, the $Na^{+}$ ions do not form a precipitate with $OH^{-}$ ions because sodium hydroxide is itself soluble, so no visible reaction occurs. However, when acidified silver nitrate is added to the sodium chloride solution, the $Ag^{+}$ ions from the silver nitrate react with the $Cl^{-}$ ions to form a white precipitate of silver chloride, $AgCl$. This is the classic confirmatory test for chloride ions. Therefore, row A shows exactly the right results.