Physical and Chemical Changes, Solution Archives

41. Which one of the following substances is not a mixture ?

Which one of the following substances is not a mixture ?

Tin

Sea water

Soil

Air

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UPSC NDA-2 – 2019

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Matter can be classified into pure substances (elements and compounds) and mixtures. A mixture is formed when two or more substances are combined physically but not chemically. The components of a mixture retain their individual properties and can often be separated by physical means.
A) Tin (Sn) is a chemical element, found on the periodic table. Elements are pure substances, consisting only of one type of atom. Therefore, tin is not a mixture.
B) Sea water is a solution of various salts (like sodium chloride, magnesium chloride, etc.), dissolved gases, and other substances in water. It is a homogeneous mixture (a solution).
C) Soil is a complex combination of inorganic mineral particles, organic matter, water, air, and living organisms. It is a heterogeneous mixture.
D) Air is a mixture of gases, primarily nitrogen ($\text{N}_2$), oxygen ($\text{O}_2$), argon (Ar), carbon dioxide ($\text{CO}_2$), and trace gases. It is a homogeneous mixture (a solution of gases).
Since Tin is an element and not a combination of different substances physically mixed, it is not a mixture.

– Pure substances include elements and compounds.
– Mixtures are physical combinations of two or more substances.
– Elements cannot be broken down into simpler substances by chemical means.

Elements are the basic building blocks of matter. Compounds are formed by chemical reactions between elements in fixed proportions. Mixtures can have variable compositions. Identifying whether something is a mixture involves checking if it’s a pure element or compound, or a physical blend of multiple substances.

42. Which one of the following statements about the law of conservation of

Which one of the following statements about the law of conservation of mass is correct?

A given compound always contains exactly same proportion of elements.

When gases combine in a reaction, they do so in a simple ratio by volume provided all gases are at room temperature.

Matter can neither be created nor destroyed.

Equal volumes of all gases at same temperature and pressure contain equal number of molecules.

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UPSC NDA-2 – 2018

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The Law of Conservation of Mass, first stated by Antoine Lavoisier, posits that mass is conserved in a chemical reaction or physical change. This means that the total mass of the reactants before a chemical reaction is equal to the total mass of the products after the reaction. The statement “Matter can neither be created nor destroyed” is a fundamental principle expressing this law.

The Law of Conservation of Mass is a cornerstone of classical chemistry, stating that the total quantity of matter remains constant in an isolated system. It is often expressed as matter cannot be created or destroyed, though it can change form.

Option A describes the Law of Definite Proportions (Proust). Option B describes Gay-Lussac’s Law of Combining Volumes. Option D describes Avogadro’s Law. These are different fundamental laws of chemistry. While related to how matter behaves in chemical reactions, they are distinct from the Law of Conservation of Mass.

43. Which one of the following is an oxidation-reduction reaction?

Which one of the following is an oxidation-reduction reaction?

NaOH + HCl → NaCl + H₂O

CaO + H₂O → Ca(OH)₂

2Mg + O₂ → 2MgO

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

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UPSC NDA-2 – 2018

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An oxidation-reduction (redox) reaction is a chemical reaction in which the oxidation states of atoms are changed. This involves the transfer of electrons.
Let’s check the oxidation states of elements in each reaction:
A) NaOH + HCl → NaCl + H₂O: Na (+1→+1), O (-2→-2), H (+1→+1), Cl (-1→-1). No change in oxidation states. This is an acid-base neutralization reaction.
B) CaO + H₂O → Ca(OH)₂: Ca (+2→+2), O (-2→-2), H (+1→+1). No change in oxidation states. This is a combination reaction.
C) 2Mg + O₂ → 2MgO: Mg (0→+2), O (0→-2). Magnesium’s oxidation state increases from 0 to +2 (oxidation). Oxygen’s oxidation state decreases from 0 to -2 (reduction). This is a redox reaction.
D) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl: Na (+1→+1), S (+6→+6), O (-2→-2), Ba (+2→+2), Cl (-1→-1). No change in oxidation states. This is a double displacement (precipitation) reaction.

Redox reactions are characterized by changes in the oxidation states of the participating atoms. Oxidation is the loss of electrons (increase in oxidation state), and reduction is the gain of electrons (decrease in oxidation state). Elemental substances (like Mg and O₂) have an oxidation state of 0.

Many common reactions are redox reactions, including combustion, corrosion, and cellular respiration. Acid-base neutralization and simple precipitation reactions typically do not involve changes in oxidation states and are therefore not redox reactions.

44. Which one of the following is a heterogeneous mixture?

Which one of the following is a heterogeneous mixture?

Hydrochloric acid

Vinegar

Milk

Soda water

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UPSC NDA-2 – 2018

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Milk is a heterogeneous mixture (specifically, a colloid/emulsion).

– A heterogeneous mixture has a non-uniform composition where different components can often be seen or separated.
– A homogeneous mixture (solution) has a uniform composition throughout.
– Hydrochloric acid (HCl in water), vinegar (acetic acid in water), and soda water (CO₂ in water) are all solutions, meaning the solute is dissolved at a molecular level, forming homogeneous mixtures.
– Milk is an emulsion of fat globules and proteins dispersed in water. While it may appear homogeneous, these particles are large enough to scatter light (Tyndall effect) and are not dissolved at a molecular level, making it heterogeneous.

Colloids like milk are considered heterogeneous mixtures because the dispersed phase (e.g., fat particles) is not uniformly distributed throughout the dispersion medium at the molecular level and can be observed or separated under certain conditions (e.g., centrifuging milk to separate cream).

45. Consider the following reaction: CH₄ + 2O₂ → CO₂ + 2H₂O Which of the f

Consider the following reaction: CH₄ + 2O₂ → CO₂ + 2H₂O
Which of the following about the reaction given above is/are correct ?

1. Carbon is oxidized.
2. Hydrogen is oxidized.
3. Hydrogen is reduced.
4. Carbon is reduced.

Select the correct answer using the code given below :

1 only

1 and 2 only

2 and 3 only

2 and 4 only

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UPSC NDA-2 – 2017

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Statement 1 is correct: Carbon is oxidized. Statements 2, 3, and 4 are incorrect.

Oxidation is the process where an atom or molecule loses electrons or increases its oxidation state. Reduction is the process where an atom or molecule gains electrons or decreases its oxidation state. In the given reaction (combustion of methane), let’s look at the oxidation states:
– In CH₄: Hydrogen is +1, so Carbon is -4 (since CH₄ is neutral).
– In O₂: Oxygen is 0 (elemental form).
– In CO₂: Oxygen is -2, so Carbon is +4 (since CO₂ is neutral).
– In H₂O: Hydrogen is +1, so Oxygen is -2 (since H₂O is neutral).
Carbon’s oxidation state changes from -4 to +4 (increase), so Carbon is oxidized.
Hydrogen’s oxidation state remains +1 (no change), so Hydrogen is neither oxidized nor reduced.
Oxygen’s oxidation state changes from 0 to -2 (decrease), so Oxygen is reduced.

This reaction is a classic example of a redox (reduction-oxidation) reaction. Methane (CH₄) acts as the reducing agent (gets oxidized), and Oxygen (O₂) acts as the oxidizing agent (gets reduced). Combustion reactions are typically redox reactions where a fuel is oxidized rapidly, usually releasing heat and light.

46. Which one of the following is a chemical change ?

Which one of the following is a chemical change ?

Cutting of hair

Graying of hair naturally

Swelling of resin in water

Cutting of fruit

This question was previously asked in

UPSC NDA-2 – 2017

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A chemical change results in the formation of new substances with different chemical properties.
A) Cutting of hair is a physical change; the hair material itself (keratin) remains chemically unchanged, only its physical shape and size are altered.
B) Graying of hair naturally involves chemical changes in the hair follicle where the production of melanin (the pigment that gives hair its color) decreases or stops. This is a biological process that alters the chemical composition of the hair shaft over time, leading to a change in color. It is an irreversible change at the level of pigment production.
C) Swelling of resin in water is a physical change; the resin absorbs water but its chemical composition does not fundamentally change. This is often a process of hydration or dissolution/dispersion.
D) Cutting of fruit is primarily a physical change; the fruit’s chemical composition remains the same, although the exposed surface might subsequently undergo chemical reactions like oxidation (browning), but the act of cutting is physical.
Therefore, graying of hair is a chemical change.

– Chemical change involves the formation of new substances.
– Physical change alters form or appearance without changing chemical composition.
– Graying of hair involves a change in pigment (melanin) production, a chemical/biological process.

Distinguishing between physical and chemical changes is a fundamental concept in chemistry. Physical changes are often easily reversible (e.g., melting ice), while chemical changes are typically irreversible under normal conditions (e.g., burning wood). Clues for chemical change include color change (not due to mixing), gas production, heat absorption or release, and precipitate formation.

47. The proposition ‘equal volumes of different gases contain equal number

The proposition ‘equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure’ is known as

Avogadro's hypothesis

Gay-Lussac's hypothesis

Planck's hypothesis

Kirchhoff's theory

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UPSC NDA-2 – 2017

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The proposition that ‘equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure’ is a fundamental principle in chemistry and physics of gases.

This statement is known as Avogadro’s hypothesis (or Avogadro’s Law). It was first proposed by Amedeo Avogadro in 1811. It directly relates the volume of a gas to the number of particles (molecules or atoms) it contains, under constant temperature and pressure conditions.

Gay-Lussac’s Law of Combining Volumes states that when gases react, the volumes of the reactants and products, measured at the same temperature and pressure, bear a simple whole-number ratio to one another. Planck’s hypothesis (specifically, Planck’s quantum hypothesis) states that energy can be emitted or absorbed only in discrete packets called quanta. Kirchhoff’s laws are important in electrical circuit analysis (Kirchhoff’s circuit laws) and thermal radiation (Kirchhoff’s law of thermal radiation).

48. When pure water boils vigorously, the bubbles that rise to the surface

When pure water boils vigorously, the bubbles that rise to the surface are composed primarily of

air

hydrogen

hydrogen and oxygen

water vapour

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UPSC NDA-2 – 2017

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When pure water boils vigorously, the process involves the phase transition of liquid water into gaseous water.

Boiling occurs when the vapour pressure of the liquid equals the surrounding atmospheric pressure, and the liquid turns into gas within the bulk of the liquid as well as at the surface. The bubbles formed during boiling are filled with the gaseous form of the liquid, which in this case is water vapour.

Air might be present as dissolved gas in water, and tiny air bubbles might be released upon heating before boiling, but the large, vigorous bubbles during boiling are predominantly water vapour. Water molecules (H₂O) decompose into hydrogen and oxygen only under extreme conditions (like electrolysis), not during simple boiling.

49. Emulsion is known as a

Emulsion is known as a

colloidal solution of substances having different physical states

true solution

distillation mixture for making alcohols

colloidal solution of two liquids

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UPSC NDA-2 – 2016

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The correct answer is D) colloidal solution of two liquids. An emulsion is a specific type of colloid where one liquid is dispersed throughout another liquid in the form of tiny droplets. The two liquids are typically immiscible or poorly miscible.

– Emulsions are colloidal systems.
– The dispersed phase and the dispersion medium are both liquids.
– Examples include milk (fat droplets dispersed in water) and mayonnaise (oil dispersed in vinegar).

– Colloidal solutions can involve different physical states (e.g., solid in gas for smoke, liquid in gas for fog), but an emulsion is specifically liquid in liquid.
– A true solution is a homogeneous mixture where the solute is dissolved at the molecular or ionic level, unlike a colloid which has larger dispersed particles.
– Distillation is a separation technique, not a type of solution or mixture in this context.

50. 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 the Lists :

List I (Process)	List II (Type of change)
A. Heating Camphor	1. Chemical
B. Cooling of water vapour up to room temperature	2. Evaporation
C. Cooking an egg	3. Condensation
D. Formation of water vapour at room temperature	4. Sublimation

Code :

A-4, B-3, C-1, D-2

A-4, B-1, C-3, D-2

A-2, B-1, C-3, D-4

A-2, B-3, C-1, D-4

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UPSC NDA-2 – 2016

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Matching the process to the type of change:
– A. Heating Camphor: Camphor changes directly from solid to gas upon heating, a process called sublimation. This is a physical change. (A-4)
– B. Cooling of water vapour up to room temperature: Water vapour (gas) changes into liquid water upon cooling, a process called condensation. This is a physical change. (B-3)
– C. Cooking an egg: Cooking an egg involves the denaturation of proteins, which is an irreversible chemical change where new substances with different properties are formed. (C-1)
– D. Formation of water vapour at room temperature: Liquid water changes into water vapour (gas) at room temperature through evaporation. This is a physical change. (D-2)
The correct match is A-4, B-3, C-1, D-2.

– Physical changes alter the form or appearance of a substance but not its chemical composition (e.g., melting, boiling, freezing, condensation, sublimation, dissolution).
– Chemical changes result in the formation of new substances with different chemical properties (e.g., cooking, burning, rusting, digestion).

Sublimation is a less common phase transition where a substance goes directly from solid to gas without passing through the liquid phase. Deposition is the reverse process (gas to solid).