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When a liquid freezes into a solid, the particles lose kinetic energy and the attractive forces between them pull them into a regular, fixed arrangement. They can no longer move past each other freely—they can only vibrate in fixed positions. So, their positions become fixed. The particles actually move closer together (not further apart), move slower (not faster), and the attractions become stronger (not weaker).
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A mixture of an element and a compound must show particles of a single element (like diatomic molecules or single atoms of one type) mixed with particles of a compound (two or more different atoms chemically bonded). In option D, you see one type of atom (element) alongside molecules made of two different atoms bonded together (compound), and they are not chemically combined with each other, which fits the description of a mixture perfectly.
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Sulfur has the atomic number 16, so its electronic configuration is 2,8,6. This means it has three occupied electron shells, placing it in the third period, and it has six electrons in its outer shell, which matches its group (Group VI). Aluminium is in period 3, not period 2. Helium is in Group VIII but has only 2 outer electrons (its first shell is full). Lithium has two occupied shells (2,1), not one.
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In the notation ${}^{167}_{68}\text{Er}$, the bottom number (68) is the atomic number, which tells us the number of protons. A neutral atom has the same number of electrons as protons, so electrons are also 68. The top number (167) is the mass number, which is protons + neutrons. So, neutrons = mass number − atomic number = 167 − 68 = 99. This matches row A perfectly.
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Rubidium (Rb) is in Group I, so it has one electron in its outer shell. To achieve a stable noble gas configuration, it loses that one electron easily. Losing an electron forms a positive ion with a 1+ charge, written as $\text{Rb}^+$. So, the electron change is “electron lost” and the formula is $\text{Rb}^+$, as shown in row C.
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Nitrogen has five outer electrons, and each hydrogen has one. In ammonia ($\text{NH}_3$), nitrogen shares three of its electrons with three hydrogen atoms to form three single covalent bonds. This leaves one lone pair (two electrons) on the nitrogen. So we look for a diagram where the central nitrogen atom has three bonding pairs (each a shared pair with H) and one lone pair. That configuration is shown in option C.
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In graphite, each carbon atom is bonded to only three others (not four), forming hexagonal layers. Statement 1 is wrong. Graphite conducts electricity because it has delocalised electrons between the layers, not because it contains free ions. So statement 2 is also wrong. However, the layers are held together by weak forces and can easily slide over each other, making graphite a great lubricant. Only statement 3 is correct.
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Magnesium reacts with hydrochloric acid to form magnesium chloride (which is soluble, so it’s aqueous) and hydrogen gas. The correct formula for magnesium chloride is $\text{MgCl}_2$ because Mg has a 2+ charge and Cl has a 1– charge. Hydrochloric acid is an aqueous solution, so its state symbol is (aq), not (l). Option A shows the correct balanced equation with the right state symbols: $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$.
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The formula $\text{CaCO}_3$ contains one carbon atom. The relative atomic mass of carbon is 12. Since the $M_r$ of $\text{CaCO}_3$ is 100, carbon makes up $\frac{12}{100}$ of the total mass. So, in $100\text{g}$ of calcium carbonate, the mass of carbon is $\frac{12}{100} \times 100 = 12\text{g}$. Simple proportion gives us 12g.
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Relative atomic mass is the *average* mass of the isotopes of an element, taking into account their relative abundances, compared to $\frac{1}{12}$th the mass of a carbon-12 atom. It must mention “isotopes” and “average”, which option A does. The other options incorrectly say “total mass” or “average mass of an element” without specifying isotopes.
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Molten potassium bromide ($\text{KBr}$) contains potassium ions ($\text{K}^+$) and bromide ions ($\text{Br}^-$). During electrolysis, the positive potassium ions move to the negative cathode and gain electrons to form potassium metal. The negative bromide ions move to the positive anode and lose electrons to form bromine gas. So, bromine at the anode and potassium at the cathode is correct.
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A hydrogen-oxygen fuel cell works by reacting hydrogen and oxygen together electrochemically to produce electricity. The great thing about it is that the only chemical product is water ($\text{H}_2\text{O}$). There are no carbon emissions like carbon dioxide, and the original hydrogen and oxygen are reactants, not products.
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A temperature decrease means heat is absorbed from the surroundings, so the process is endothermic. Since the ammonium nitrate is simply recovered by evaporating water, it has dissolved, not undergone a chemical reaction. If it had reacted, you wouldn’t get the original substance back by just removing water. So, it’s a dissolving process that is endothermic.
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The fastest rate means the largest volume of hydrogen is produced per second. Let’s check the volume changes: 0–30s: 32 cm³ produced (average ~1.07 cm³/s). 30–60s: 59 − 32 = 27 cm³. 60–90s: 68 − 59 = 9 cm³. 90–120s: 74 − 68 = 6 cm³. The largest change is in the first 30 seconds (32 cm³), so the rate is fastest in the 0–30 second period.
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Copper(II) sulfate ($\text{CuSO}_4$) contains the transition metal copper, and its compounds are characteristically blue in solution. Cobalt(II) chloride can be blue when anhydrous but forms a pink solution in water. Thymolphthalein and universal indicator are acid-base indicators and don’t give a simple blue solution with just water.
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Reduction can be defined as the loss of oxygen. Here, water ($\text{H}_2\text{O}$) loses its oxygen to carbon, turning into hydrogen ($\text{H}_2$). So, water is reduced. Carbon gains oxygen (becomes CO), so it is oxidised. We look for the substance that loses oxygen, which is $\text{H}_2\text{O}$.
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When a base like sodium hydroxide is heated with an ammonium salt, ammonia gas is produced. Both ammonium chloride and ammonium nitrate contain the ammonium ion ($\text{NH}_4^+$), so they release ammonia. Aluminium nitrate does not contain ammonium ions, so it won’t produce ammonia in this reaction. That means compounds 2 and 3 produce ammonia.
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This equation shows hydrogen ions (from an acid) reacting with hydroxide ions (from an alkali) to form water. That’s the classic definition of a neutralisation reaction. Hydrogen ions represent an acid, not an alkali. Adding OH⁻ ions neutralises the acid, causing the pH to rise, not fall. It’s not a reduction because no electrons are transferred in this specific neutralisation.
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Acidic oxides are usually oxides of non-metals. Carbon dioxide ($\text{CO}_2$) is a non-metal oxide and dissolves in water to form carbonic acid, so it’s acidic. Barium oxide, copper(II) oxide, and magnesium oxide are all metal oxides, and metal oxides are typically basic, reacting with acids rather than bases.
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A hydrated salt is a solid that contains water of crystallisation—water molecules that are chemically bound within its crystal structure. This is different from being dissolved in water (an aqueous solution) or being just a dry powder. Anhydrous salts contain no water. So, a hydrated salt is a solid chemically combined with water.
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As you move from left to right across a period, elements change from metallic to non-metallic. This means metallic character decreases. On the left, you have reactive metals; on the right, you find non-metals. For example, in period 3, sodium (metal) is on the left, chlorine (non-metal) is on the right.
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All Group I metals react with water to produce hydrogen gas and a metal hydroxide (statement 1 is correct). Down the group, the melting point actually decreases (not increases), and the density generally increases (not decreases). So statements 2 and 3 are wrong. Reactivity does increase down Group I (statement 4 is correct). That leaves 1 and 4 as the correct pair.
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No reaction means the halogen added is less reactive than the halide it’s mixed with. X with Y ions → no reaction, so X is less reactive than Y. Y with Z ions → no reaction, so Y is less reactive than Z. Reactivity order (least to most) is X < Y < Z. So X is iodine (least reactive), Y is bromine, and Z is chlorine. Bromine (Y) is indeed a red-brown liquid at room temperature and pressure, making C correct.
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Transition elements are typical metals with high melting points and high densities. They also form coloured compounds. A high melting point like 1085°C and a high density fit perfectly. The oxide being a red solid indicates it is coloured, another characteristic property of transition elements. Options A, B, and C have either very low melting points, low densities, or white (colourless) compounds.
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Noble gases are monatomic gases, so statement 3 is correct. However, helium only has 2 outer electrons, not 8—so statement 1 is incorrect. Their full outer shells make them very unreactive; they do not easily react with sodium to form ionic compounds, so statement 2 is incorrect. Only statement 3 is correct.
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Potassium is a metal, so it has high thermal conductivity and is malleable. Sulfur is a non-metal, so it has poor thermal conductivity and is brittle (low malleability). So, potassium has the higher thermal conductivity, and sulfur has the lower malleability. That matches row C perfectly.
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For overhead cables, low density is the key advantage of aluminium—it makes the cables lighter, so they can be strung over longer distances without sagging. Although copper has higher conductivity, aluminium’s lower density is the main reason it is preferred for this specific use. Reactivity doesn’t matter much here because aluminium is protected by its oxide layer. So, only statement 1 explains the choice.
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L is definitely the most reactive because it reacts with cold water (a very vigorous reaction for metals). J reacts with steam but not cold water, putting it next. K only reacts with acid, so it’s less reactive than J. M doesn’t react with acid at all, making it the least reactive. So, the order from most to least reactive is L → J → K → M, which matches option A.
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Rust is specifically hydrated iron(III) oxide. It requires both oxygen and water to form, so it includes water molecules in its structure (hydrated). The iron is in the +3 oxidation state (iron(III)), not iron(II). The terms “anhydrous” (without water) or “iron(II)” are therefore incorrect for rust.
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Dissolved oxygen is actually beneficial—aquatic life needs it to survive. Nitrates and phosphates, however, are harmful. They come from fertilisers and detergents, and they cause excessive plant growth. When these plants die and decompose, the process uses up oxygen, leading to deoxygenation and damage to aquatic ecosystems. So only 2 and 3 are harmful.
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Clean, dry air is approximately 78% nitrogen. That’s well over 30%. Oxygen makes up about 21%, which is less than 30%. Argon is about 0.9%, and carbon dioxide is only about 0.04%. So nitrogen is the only gas listed that exceeds 30%.
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Fertilisers are labelled as NPK, which stands for the three essential elements they supply: Nitrogen (N), Phosphorus (P), and Potassium (K). Plants need these in relatively large amounts, and they are often depleted from soil by crops. So, the elements replaced are N, P, and K.
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The reaction shown is the photosynthesis equation: carbon dioxide and water react, using sunlight energy and chlorophyll, to form glucose and oxygen. This is how plants make their food. Combustion uses oxygen to burn a fuel, decomposition breaks things down, and displacement involves replacing an element in a compound.
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Members of a homologous series are organic compounds that have the same functional group and similar chemical properties, but they are not elements or atoms. Their physical properties, like boiling point, show a trend but are not the same. The key characteristic is the presence of the same functional group.
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The general formula for a carboxylic acid is $\text{C}_n\text{H}_{2n+1}\text{COOH}$, which works out to $\text{C}_n\text{H}_{2n}\text{O}_2$. Y has the formula $\text{C}_4\text{H}_8\text{O}_2$, which fits this pattern (with $n=4$). W has oxygen but doesn’t fit the alcohol general formula $\text{C}_n\text{H}_{2n+1}\text{OH}$; it’s not an alcohol. W contains oxygen, so it’s not a hydrocarbon. X could be propene, not ethene (ethene is $\text{C}_2\text{H}_4$). So only Y can be a carboxylic acid.
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As you go down the fractionating column, the fractions get heavier, with longer chain molecules, higher boiling points, lower volatility, and higher viscosity. Fuel oil is collected lower down than diesel oil, so it has a higher viscosity. Refinery gas is at the top, so it’s more volatile than gasoline. Gasoline is higher than naphtha, so its molecules are shorter. Naphtha is higher than kerosene, so it has a lower boiling point.
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Alkanes are saturated hydrocarbons with only single bonds, so statement 3 is false. Because they have no double bonds, they do not decolourise bromine water (statement 4 false). They have the general formula $\text{C}_n\text{H}_{2n+2}$ (statement 2 true) and are generally unreactive except for combustion and substitution reactions (statement 1 true). So, 1 and 2 are correct.
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Cracking breaks a large alkane into a smaller alkane and an alkene. Hexane ($\text{C}_6\text{H}_{14}$) has 6 carbons. Option A gives butane ($\text{C}_4\text{H}_{10}$, 4C alkane) and ethene ($\text{C}_2\text{H}_4$, 2C alkene), total 6C — perfect. Option B has more carbons than the original molecule, which is impossible. Option C shows a fragment, not stable molecules. Option D gives two alkanes, but cracking must produce at least one alkene. So only A works.
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From the chromatogram, P shows more than one spot, so it’s a mixture, not pure. A spot from P lines up at the same height as a spot from Q, meaning they share a colour that travels the same distance in the solvent. The top line is the solvent front, not the baseline. R has some spots not present in Q, so D is false. Therefore, B is the correct statement.
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A yellow flame test indicates the presence of sodium ions ($\text{Na}^+$). Effervescence (fizzing) when dilute nitric acid is added means a carbonate is present, as carbon dioxide gas is produced. So the compound must contain both sodium and carbonate ions — that’s sodium carbonate ($\text{Na}_2\text{CO}_3$). Chlorides wouldn’t fizz with acid, and calcium gives a different flame colour.