HOTS Questions: Is Matter Around Us Pure - Class 9 MCQ

 Starch solution

Explanation: A colloidal solution consists of tiny particles suspended in a continuous medium. The particle size in a colloid is between that of a true solution and a suspension, meaning the particles do not settle out upon standing.

• Starch solution is a colloid because the starch particles are small enough to remain suspended in water without settling.
• Copper sulphate solution is a true solution, not a colloid.
• Chalk powder in water is a suspension, as the chalk particles settle down when left undisturbed.
• Kerosene oil and water form an immiscible liquid mixture, not a colloid.

Thus, a starch solution is the correct example of a colloidal solution.

Physical Changes:
A physical change is a change in the state or form of matter without any change in the chemical composition or identity of the substance. In other words, the substance remains the same before and after the change.
Examples of Physical Changes:
A. Burning of a Natural Gas:
- This is a clear example of a combustion reaction. Natural gas comprehends methane gas. When methane reacts with oxygen in the atmosphere it produces water and carbon dioxide.
B. Rusting of iron:
- This is a chemical change, not a physical change. Rusting occurs when iron reacts with oxygen in the presence of moisture, forming iron (III) oxide (rust).
C. Freezing of water:
- This is a physical change. When water is cooled below its freezing point, it undergoes a phase change from a liquid to a solid, forming ice. The chemical composition of water (H2O) remains the same.
D. Digestion of food:
- This is a chemical change, not a physical change. Digestion involves the breakdown of complex food molecules into simpler substances through various chemical reactions in the digestive system.
Conclusion:
The freezing of water is the only example mentioned that represents a physical change. The other options involve chemical changes.

• Homogeneous mixtures (also called solutions) have a uniform composition throughout, but the proportions of their components can vary. For example, a sugar-water solution can have different amounts of sugar dissolved in water.

• Heterogeneous mixtures do not have a uniform composition, and their components are visibly distinct (e.g., a mixture of sand and iron filings).

• A salt solution is a homogeneous mixture, not a heterogeneous one.

• Homogeneous mixtures do not always have a fixed composition, as their components can be mixed in varying proportions.

Thus, option a) is correct.

• A solution's classification depends primarily on the relative proportion of solute to solvent.
• When there is a small amount of solute compared to the solvent, the solution is termed dilute.
• If the solute amount is significant, the solution is referred to as concentrated.
• A saturated solution occurs when no more solute can dissolve in the solvent at a specific temperature.
• Different solutes have varying solubilities, affecting how they are classified in a solution.

Robert Boyle was the first scientist to use the term element in 1661. However, it was Antoine Lavoisier, a French chemist, who provided a clear definition of an element. He described an element as a basic form of matter that cannot be broken down into simpler substances through chemical reactions.

Elements can be classified into three main categories:

• Metals
• Non-metals
• Metalloids

Mass of common salt (solute) = 40g
Mass of water (soluted) = 320g
Mass of solution = 320 + 40 = 360g
Concentration of solution = M(mass of solute) / mass of solution = 40/360 x 100 = 11.11%

The particles of suspension:
- Can be seen with the naked eye. Suspensions are heterogeneous mixtures in which solid particles are dispersed in a liquid or gas. These particles are usually large enough to be visible to the naked eye.
- Cannot be seen with a powerful microscope. While the particles in a suspension may not be visible under a powerful microscope, they can still be seen without the aid of any magnification.
- Cannot be seen with an electron microscope. Electron microscopes are capable of magnifying objects to a much higher degree than light microscopes, but even they may not be able to visualize the particles in a suspension due to their size and distribution.
- Are larger than the particles in a solution. In contrast to solutions, where the solute particles are evenly distributed and too small to be seen, the particles in a suspension are larger and can settle over time due to gravity.
- Can be separated by filtration or settling. Since the particles in a suspension are larger and more visible, they can be separated from the liquid or gas phase through processes such as filtration or allowing them to settle and then decanting the liquid above.

Statement: Compound is homogeneous and have fixed composition.

Definition of compound:
- A compound is a substance that is composed of two or more elements chemically combined in a fixed ratio.
- It has its own unique set of properties that are different from the properties of its constituent elements.
Explanation:
- A homogeneous mixture is a mixture that has a uniform composition throughout, meaning that the components are evenly distributed.
- A compound is a type of homogeneous mixture because it has a fixed composition.
- The elements in a compound are combined in a specific ratio, which means that the relative amounts of the elements are always the same.
- This fixed composition gives compounds their distinct properties and makes them different from mixtures.
Key points:
- A compound is homogeneous, meaning it has a uniform composition throughout.
- The composition of a compound is fixed, with elements combined in a specific ratio.
- Compounds have distinct properties that are different from the properties of their constituent elements.
Conclusion:
The correct statement is option A: Compound is homogeneous and has a fixed composition.

• Centrifugation is a process used to separate particles of a colloidal solution.
• In this method, the mixture is spun at high speeds, which causes the denser particles to move outward and separate from the rest of the solution.
• Filtration is not effective because colloidal particles are too small to be captured by ordinary filter paper.
• Evaporation and distillation are not suitable methods for separating colloidal particles.

Explanation:
When a substance undergoes a physical change, its composition remains the same. This means that the atoms or molecules that make up the substance do not rearrange or combine with other atoms or molecules to form a new substance. Instead, the physical properties of the substance may change, such as its shape, size, density, state (solid, liquid, gas), or phase (melting, freezing, boiling, condensation).
Key points:
- Physical change refers to a change in the physical properties of a substance without any change in its chemical composition.
- In a physical change, the substance may change its state (e.g., from solid to liquid or gas) or undergo a change in physical properties (e.g., color, shape, size), but the composition of the substance remains the same.
- The atoms or molecules that make up the substance do not rearrange or combine with other atoms or molecules to form a new substance.
- Examples of physical changes include melting ice, boiling water, dissolving salt in water, and crushing a can.