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Correct answer: 4)

Concept:

• All matter has physical and chemical properties.
• Physical properties are characteristics that scientists can measure without changing the composition of the sample under study, such as mass, color, and volume (the amount of space occupied by a sample).
• Chemical properties describe the characteristic ability of a substance to react to form new substances; they include its flammability and susceptibility to corrosion.

Explanation:

• Density can be a very useful parameter for identifying an element.
• Density = mass/volume.
• Weight = mass x acceleration (of gravity).
• If two substances have the same density and the same mass then their volumes must also be the same otherwise they would not have the same density.

Conclusion:

Thus, density is most useful for determining whether two samples of metal are the same or not.

Correct answer: 1)

Concept:

• As gases have large intermolecular space present between them, these molecules have a greater amount of freedom.
• This is due to the fact that gases have the weakest intermolecular force present between molecules

Explanation:

• Intermolecular forces among molecules of chlorine are the weakest and those in the iodine are the strongest.
• Because At ordinary temperatures and pressure, among halogens, chlorine is a gas, bromine is a liquid and iodine is a solid.
• Water is a liquid so the intermolecular force of attraction is higher than chlorine gas.
• In a gas lower the intermolecular force, molecules are farthest apart from each other.

Conclusion:

Thus. at room temperature and 1 atm pressure, the molecules are farthest apart in chlorine.

Concept:

• The melting point of a substance is the temperature at which its solid and liquid phases are in balance.
• The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure.

Explanation:

• Phosphorous and gallium are solid at 25°C and 1 atm pressure.
• Gallium is one of the metals that can be liquid near room temperature.
• Mercury is liquid at room temperature and noble gases are gases at room temperature.

Conclusion:

Thus, both Phosphorous and gallium are solids at 25°C and 1 atm pressure.

Correct answer: 4)

Concept:

• On heating, the liquid starts changing into its vapor state at the place where it is being heated.
• The liquid converts to gas which appears in the form of bubbles.

Explanation:

• On heating when water starts boiling, bubbles are formed which are basically air bubbles.
• These are bubbles formed from the dissolved gases in water.
• The solubility of a gas in water decreases when the temperature increases, and that is why the dissolved air bubbles come out from the water.
• Then, as the boiling point of water is reached, water vapor starts to form in the form of bubbles.
• The molecules gain enough energy to transition from the liquid phase to the gaseous phase. These bubbles are water vapor.

Conclusion:

Thus, during the process of boiling pure water in a beaker, the bubbles that initially rise to the surface are composed primarily of water vapors.

Correct answer: 4)

Concept:

• When molecules of water vapor transform from liquid phase to gaseous phase, the amount of energy required is termed enthalpy of vaporization.
• In order to reverse this process i.e., transforming molecules from gaseous phase to liquid phase, the amount of energy released is equal to the enthalpy of vaporization.

Explanation:

• Melting is an endothermic process because when heat absorbs then the physical state will change from solid to liquid.
• Boiling is endothermic, which means that the reaction absorbs energy. This is because it requires energy to turn a liquid into a gas.
• Ammonium thiocyanate is mixed with barium hydroxide. The mixture is stirred. Water is one of the products of the reaction. The reaction is endothermic.
• For condensation, the molecules are giving up their heat energy. When molecules give up heat energy, it is called exothermic. Condensation would be exothermic.

Conclusion:

Thus, the Condensation of water vapor is an exothermic process.

Correct answer: 4)

Concept:

• A chemical change is a type of permanent change and there is a change in both the physical and chemical properties of the compound.
• Burning of coal, digestion of food, and rusting are also some examples of chemical changes.
• But if the molecules are rearranged even though the composition remains the same then the change is physical and not chemical.

Explanation:

• Chemical changes are detected by noticing any change in color, odor, temperature or energy, precipitate formation, gases evolved, etc.
• Therefore, if a substance reacts and forms a new substance, is considered a chemical property.

Conclusion:

Thus, it reacts with hydrogen to form a gas is a chemical property.

Correct answer: 4)

Concept:

• Atomicity is the total number of atoms present in a molecule.
• Epsom salt, or magnesium sulfate heptahydrate, is a hydrous magnesium sulfate mineral with the formula MgSO₄·7H₂O.

Explanation:

• Molecules that are composed of more than four atoms are known as polyatomic molecules.
• **The total number of atoms represented by the compound, MgSO4.7 H2O are - =1+ 1+ (4 x 1) + (7 x 3)  **= 27 atoms

Conclusion:

Thus, the total number of atoms (atomicity) represented in a compound having formula MgSO4.7 H2O are 27.

CONCEPT

Average velocity:

The average of all the velocities of a body in a particular path is called the average velocity of the body. 

If the velocity at point A and point B is V and V' then the average velocity over the path AB

→Vavg = (V+V')/2 ---- (1)

Also, the average velocity can be defined as the total distance divided by the total time. 

→Vavg = Total distance/Total time taken. 

EXPLANATION:

In the question given above, 

The final velocity is V' = 16 ms-1.

The average velocity = Total distance/Total time = 60/6 = 10 ms-1

From equation (1) we get, 

⇒ Vavg = (V+V')/2

⇒ 10 = (V+16)/2

⇒ 20 = V+16

⇒ V = 20 - 16 = 4 ms-1

Hence the correct answer is option (1).

CONCEPT

The distance traveled by any body moving in a straight line

→S = ut ± ½gt² 

If the body was moving opposite to the ground then the sign would be negative 

Otherwise positive. 

Here, u = initial velocity of the body, t = time taken, g = acceleration due to gravity 

EXPLANATION 

Here, u = 4 m/s and t = 1 sec

∴ Height h = 4×1 + ½g(1)²

⇒ h = 4 + ½ × 10 = 9 m

Hence the correct answer is option (3).

CONCEPT

For a body moving in a straight line by the influence of gravity

Distance travelled,

→S = ut ± ½gt2 

Final velocity travelled,

→v = u + gt 

and v2 = u2 - 2gs

EXPLANATION 

In the first case, the ball starts from the rest 

∴ v = 0 + g×4 = 4g --- (1)

now third-fourth of the velocity is lost. So, the remaining is 1 - (3/4) = 1/4

So, the 1/4^th of the velocity = (1/4) × 10×4 = 10 m/s

For the next case, i.e. when the ball is falling then the initial velocity is = 10 m/s

Now again, the distance travelled S = ut + ½gt2  = 10 × 2 + ½ × 10 × (2)2

 = 20+ 20

 = 40

Hence the correct answer is option (2).