Physical and Chemical Changes, Solution

21. Which one among the following salts causes hardness to the water?

Which one among the following salts causes hardness to the water?

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Calcium bicarbonate
Calcium carbonate
Sodium sulphate
Potassium chloride
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UPSC CAPF – 2012
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Water hardness is caused by dissolved multivalent cations, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are usually present in water as salts like bicarbonates, sulfates, or chlorides.
Calcium bicarbonate (Ca(HCO3)₂) is a common salt found dissolved in water that causes temporary hardness. When heated, calcium bicarbonate decomposes to form calcium carbonate, which precipitates out, reducing the hardness. Calcium carbonate (CaCO₃) itself is largely insoluble in water under normal conditions and does not dissolve significantly to cause hardness.
Sodium salts (like sodium sulphate) and potassium salts (like potassium chloride) do not cause water hardness, as sodium (Na⁺) and potassium (K⁺) are monovalent ions. The term “hardness” specifically refers to the presence of divalent or trivalent metallic cations.

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22. When an iron nail is dipped in a solution of copper sulphate, the iron

When an iron nail is dipped in a solution of copper sulphate, the iron nail becomes brownish in colour and the blue colour of copper sulphate solution fades after some time. The reaction is an example of chemical

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combination
decomposition
displacement
double displacement
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UPSC CAPF – 2012
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When an iron nail is dipped in a solution of copper sulphate (CuSO4), a chemical reaction occurs. Iron (Fe) is more reactive than copper (Cu), so it displaces copper from the copper sulphate solution. The reaction is: Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s). This results in a brownish deposit of copper on the iron nail and the blue colour of the copper sulphate solution fading as iron sulphate (FeSO4), which is light green, is formed.
This type of reaction, where a more reactive element displaces a less reactive element from its compound, is called a displacement reaction.
Combination reactions involve two or more substances combining to form a single product. Decomposition reactions involve a single compound breaking down into two or more simpler substances. Double displacement reactions involve the exchange of ions between two compounds.

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23. Which one among the following statements is correct ?

Which one among the following statements is correct ?

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Melting of ice and burning of candle represent the same kind of chemical change
Melting of ice is a physical change and burning of candle is a chemical change
Melting of ice is a chemical change and burning of candle is a physical change
Melting of ice and burning of candle represent the same kind of physical change
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UPSC CAPF – 2011
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The correct option is B. Melting of ice is a physical change because water changes its state from solid (ice) to liquid (water), but its chemical composition (H₂O) remains the same. Burning of a candle is a chemical change because the wax (a complex hydrocarbon) reacts with oxygen in the air to produce new substances like carbon dioxide, water vapor, soot, heat, and light.
The question differentiates between physical changes (alteration in form, not composition) and chemical changes (formation of new substances).
Other examples of physical changes include boiling, freezing, condensation, sublimation, breaking glass, and dissolving salt in water (where the salt can be recovered). Examples of chemical changes include rusting of iron, cooking food, digestion, photosynthesis, and explosion of fireworks.

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24. Which one of the following statements regarding matter is *not* correc

Which one of the following statements regarding matter is *not* correct ?

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Matter is anything that occupies space
At high temperatures matter becomes plasma
All matter is composed of large particles such as atoms and molecules
Colloids are made up of larger particles of matter
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UPSC CAPF – 2010
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The statement that is *not* correct regarding matter is “All matter is composed of large particles such as atoms and molecules”.
While atoms and molecules are fundamental building blocks for common states of matter (solid, liquid, gas), not *all* matter is composed *only* of atoms and molecules, especially described as “large particles”. Matter can exist as plasma (ionized gas, consisting of ions and free electrons), which is not composed of intact atoms or molecules. Fundamental particles (like quarks, electrons, neutrinos) are also considered matter. The term “large particles” is also relative and misleading in a fundamental description of matter composition. Statements A, B, and D are correct descriptions of aspects of matter: A) Matter occupies space and has mass. B) Plasma is a high-temperature state of matter. D) Colloids are indeed dispersions of particles larger than those in true solutions.
The four common states of matter are solid, liquid, gas, and plasma. Bose-Einstein condensates are another state of matter at extremely low temperatures. Matter is fundamentally composed of elementary particles, which can combine to form composite particles like protons and neutrons, which in turn form atomic nuclei. Atoms consist of a nucleus and electrons, and atoms combine to form molecules or ionic lattices.

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25. Which one of the following is a compound ?

Which one of the following is a compound ?

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Milk
Sugar
Tea
Coffee
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UPSC CAPF – 2010
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Among the given options, Sugar is a compound.
A compound is a substance formed when two or more different chemical elements are chemically bonded together in a fixed proportion. Milk, Tea, and Coffee are all mixtures. They consist of various substances physically mixed together, not chemically bonded in fixed ratios. “Sugar” typically refers to sucrose (C₁₂H₂₂O₁₁), which is a molecule composed of atoms of carbon, hydrogen, and oxygen chemically bonded in a fixed ratio, making it a compound.
A mixture is a substance consisting of two or more components not chemically combined. The components of a mixture retain their individual properties and can often be separated by physical means. Examples include air (mixture of gases), salt water (salt dissolved in water), milk, tea, and coffee.

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26. Match List-I with List-II and select the correct answer using the code

Match List-I with List-II and select the correct answer using the code given below :

List-I (Process) List-II (Contaminant)
A. Aeration followed by filtration 1. Removal of colour
B. Reverse Osmosis 2. Removal of microorganism
C. Filtration through activated carbon 3. Removal of iron
D. Treatment with UV light 4. Removal of excess salts

Code:

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3 1 4 2
4 3 2 1
3 4 1 2
2 4 1 3
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UPSC CAPF – 2009
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The correct match between the water treatment processes and the contaminants removed is A-3, B-4, C-1, D-2.
– A. Aeration followed by filtration: Aeration oxidizes dissolved iron and manganese, forming precipitates that are then removed by filtration. Thus, it’s used for the removal of iron (3).
– B. Reverse Osmosis: This process uses a semipermeable membrane to remove dissolved salts and other impurities from water, effectively used for desalination or removal of excess salts (4).
– C. Filtration through activated carbon: Activated carbon is highly effective at adsorbing organic compounds, chlorine, and other substances that cause color, odor, and taste in water. Thus, it’s used for the removal of colour (1).
– D. Treatment with UV light: Ultraviolet (UV) light treatment is a disinfection method that inactivates microorganisms like bacteria and viruses by damaging their DNA. Thus, it’s used for the removal of microorganisms (2).
Matching the processes to the contaminants:
A. Aeration followed by filtration – 3. Removal of iron
B. Reverse Osmosis – 4. Removal of excess salts
C. Filtration through activated carbon – 1. Removal of colour
D. Treatment with UV light – 2. Removal of microorganism
This corresponds to the code 3 4 1 2, which is option C.

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27. Which one among the following is an example of chemical change ?

Which one among the following is an example of chemical change ?

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Sublimation of camphor
Melting of ice
Heating of an iron rod
Mixing of NaOH and HCl
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A chemical change results in the formation of new substances with different chemical properties. Mixing sodium hydroxide (NaOH, a strong base) and hydrochloric acid (HCl, a strong acid) causes a neutralization reaction: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). Sodium chloride (NaCl) and water (H₂O) are produced, which have entirely different chemical properties compared to the reactants. Sublimation, melting, and simple heating of a solid are physical changes, altering the state or temperature but not the chemical identity of the substance.
Chemical changes involve the breaking and forming of chemical bonds, resulting in new substances. Physical changes involve alterations in form, state, or appearance without changing chemical composition.
Neutralization reactions are a common type of chemical change where an acid and a base react to form a salt and water. These reactions are often exothermic, releasing heat.

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28. Which one among the following is an example of oxidation reaction ?

Which one among the following is an example of oxidation reaction ?

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Freezing of water
Dissolving sugar in water
Rusting of iron
Boiling of petrol
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Oxidation is a chemical reaction where a substance loses electrons or its oxidation state increases. Rusting of iron is the process where iron reacts with oxygen and water to form hydrated iron(III) oxides, commonly known as rust. This is a chemical change where iron is oxidized from its elemental state (oxidation state 0) to an oxidation state of +3 in the iron oxides. Freezing of water, dissolving sugar, and boiling of petrol are physical changes or processes, not chemical reactions involving oxidation in the typical sense tested in such questions.
An oxidation reaction involves the loss of electrons or an increase in oxidation state. Chemical changes result in the formation of new substances. Physical changes alter the form or appearance but not the chemical composition.
Rusting is a specific type of corrosion, which is an electrochemical process. The chemical equation for the formation of rust is complex, but the overall process involves the oxidation of iron: Fe → Fe³⁺ + 3e⁻, and the reduction of oxygen.

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29. The chemical reaction: 2AgCl (s) → 2Ag (s) + Cl₂ (g) takes place

The chemical reaction:
2AgCl (s) → 2Ag (s) + Cl₂ (g)
takes place

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in dark
in sunlight
on heating
under high pressure
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The correct answer is B) in sunlight.
The chemical reaction shown, 2AgCl (s) → 2Ag (s) + Cl₂ (g), is the decomposition of silver chloride into silver and chlorine gas. Silver halides like silver chloride (AgCl) and silver bromide (AgBr) are known to be sensitive to light. This decomposition reaction is catalyzed by light, particularly sunlight or bright artificial light. This property is fundamental to traditional black and white photography where silver halides in photographic film undergo decomposition upon exposure to light, forming a latent image.
This type of decomposition reaction, which occurs in the presence of light, is called a photolytic decomposition reaction. While some decomposition reactions occur on heating (thermolysis), the decomposition of silver chloride is prominently triggered by light energy. Dark conditions prevent this reaction. High pressure would likely favour the solid reactant over the gaseous product, not cause decomposition.

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30. The intermixing of particles of two different types of matter on their

The intermixing of particles of two different types of matter on their own is called

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Osmosis
Brownian motion
Diffusion
Conductance
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UPSC NDA-2 – 2024
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The correct answer is C) Diffusion.
Diffusion is the net movement of anything generally from a region of higher concentration to a region of lower concentration. This movement is driven by the random motion of particles (molecules or atoms) leading to their spontaneous intermixing until the concentration is uniform throughout. The intermixing of particles of two different types of matter on their own is the definition of diffusion.
Osmosis is a specific type of diffusion involving the movement of solvent molecules (usually water) across a semipermeable membrane from a region of high solvent concentration to a region of low solvent concentration. Brownian motion is the random movement of microscopic particles suspended in a liquid or gas, caused by collisions with the molecules of the surrounding medium; it is the underlying cause of diffusion but not the intermixing process itself. Conductance refers to the ability of a material to conduct electricity.

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