Which of the following processes involves both a chemical and a physical change?

Here are the solutions to the questions:

15. Which of the following processes involves both a chemical and a physical change?

• A chemical change alters the chemical composition of a substance, while a physical change alters its form but not its composition.
• Burning a candle involves the physical change of wax melting and vaporizing, and the chemical change of wax reacting with oxygen to produce carbon dioxide and water.

The correct option is B.

16. When silver nitrate solution is added to a solution containing a halide ion, a white precipitate is formed. Identify the precipitate.

• Silver nitrate ($\text{AgNO}_3$) reacts with halide ions ($\text{X}^-$) to form silver halides ($\text{AgX}$).
• Silver chloride ($\text{AgCl}$) is a white precipitate.
• Silver bromide ($\text{AgBr}$) is a cream precipitate.
• Silver iodide ($\text{AgI}$) is a yellow precipitate.
• Silver fluoride ($\text{AgF}$) is soluble.
• Since a white precipitate is formed, it must be silver chloride.

The correct option is B.

17. Which of the following is a saturated hydrocarbon?

• Saturated hydrocarbons are alkanes, which have the general formula $\text{C}_n\text{H}_{2n+2}$.
• A. $\text{C}_2\text{H}_4$ (Ethene) is an alkene ($n=2$, $2n=4$), which is unsaturated.
• B. $\text{C}_3\text{H}_6$ (Propene) is an alkene ($n=3$, $2n=6$), which is unsaturated.
• C. $\text{C}_4\text{H}_8$ (Butene) is an alkene ($n=4$, $2n=8$), which is unsaturated.
• D. $\text{C}_2\text{H}_6$ (Ethane) is an alkane ($n=2$, $2n+2=6$), which is saturated.

The correct option is D.

18. The pH value of a solution is 4.6. The solution is?

• The pH scale indicates the acidity or alkalinity of a solution.
• A pH value less than 7 indicates an acidic solution.
• A pH value equal to 7 indicates a neutral solution.
• A pH value greater than 7 indicates a basic (alkaline) solution.
• Since 4.6 is less than 7, the solution is acidic.

The correct option is C.

19. According to Boyle's law which of the following statement is CORRECT?

• Boyle's Law states that for a fixed amount of gas at constant temperature, the pressure and volume are inversely proportional.
• Therefore, a key condition for Boyle's Law to hold true is that the temperature must be constant.

The correct option is A.

20. Identify the method that can be used to separate a solid mixture of ammonium chloride and sodium chloride.

• Ammonium chloride ($\text{NH}_4\text{Cl}$) is a substance that sublimes, meaning it changes directly from a solid to a gas upon heating without passing through a liquid phase.
• Sodium chloride ($\text{NaCl}$) does not sublime under normal heating conditions.
• Therefore, heating the mixture will cause the ammonium chloride to sublime and separate from the sodium chloride.

The correct option is C.