Physical and Chemical Changes, Solution

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

[amp_mcq option1=”Calcium bicarbonate” option2=”Calcium carbonate” option3=”Sodium sulphate” option4=”Potassium chloride” correct=”option1″]

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.

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

[amp_mcq option1=”combination” option2=”decomposition” option3=”displacement” option4=”double displacement” correct=”option3″]

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.

Which one among the following statements is correct ?

[amp_mcq option1=”Melting of ice and burning of candle represent the same kind of chemical change” option2=”Melting of ice is a physical change and burning of candle is a chemical change” option3=”Melting of ice is a chemical change and burning of candle is a physical change” option4=”Melting of ice and burning of candle represent the same kind of physical change” correct=”option2″]

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.

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

[amp_mcq option1=”Matter is anything that occupies space” option2=”At high temperatures matter becomes plasma” option3=”All matter is composed of large particles such as atoms and molecules” option4=”Colloids are made up of larger particles of matter” correct=”option3″]

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.

Which one of the following is a compound ?

[amp_mcq option1=”Milk” option2=”Sugar” option3=”Tea” option4=”Coffee” correct=”option2″]

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.

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:

[amp_mcq option1=”3 1 4 2″ option2=”4 3 2 1″ option3=”3 4 1 2″ option4=”2 4 1 3″ correct=”option3″]

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.