Test: Is Matter Around Us Pure - Class 9 MCQ

A mixture does not have a fixed melting point, boiling point etc. On the other hand, compounds have a a fixed melting point, boiling point.

- The size range of particles in a colloidal solution is between 1 nm and 100 nm.

- Colloids are mixtures where one substance is evenly dispersed throughout another.

- The particles are larger than those in true solutions (less than 1 nm) but smaller than those in suspensions (greater than 1000 nm).

- This size allows colloidal particles to remain suspended, providing stability without settling.

- Examples of colloids include:

• Milk
• Fog
• Gelatin

- In these examples, particles are visible under an ultramicroscope but do not settle out.

Answer: C. Milk

Explanation: Milk is an example of a colloidal solution where fat globules are evenly dispersed in water. Here are some key points about colloidal solutions:

• Uniform Distribution: The particles are spread throughout the solution, making it appear homogeneous.
• Particle Size: Colloidal particles are smaller than those in suspensions but larger than those in true solutions.
• Visibility: Colloidal particles cannot be seen with the naked eye.
• Tyndall Effect: These particles can scatter light, which is observable as the Tyndall effect.

Answer: A. The scattering of light by particles in a colloid.

Explanation: The Tyndall effect occurs when light is scattered by particles in a colloidal solution. Here are some key points:

• The effect is visible when light passes through a mixture containing tiny particles.
• Common examples include sunlight filtering through mist in a forest.
• Colloids are stable mixtures where particles do not settle over time.
• These particles are too small to be seen individually but can scatter light, making the path of the light visible.

Answer: C. Water

Explanation: Water (H₂O) is a pure substance made up entirely of water molecules. Here are some key points:

• Definition: A pure substance contains only one type of particle.
• Composition: Water consists solely of H₂O molecules.
• Characteristics: It has consistent properties throughout, such as boiling and freezing points.

Elements:

• Elements are the simplest form of matter and cannot be broken down into simpler substances by any chemical means.
• They are homogeneous and can exist in different physical states.

Compounds:

• Compounds are formed by a chemical combination of two or more elements and can be broken down into their constituent elements.
• Compounds are also homogeneous but are not the simplest substances.

Thus, the statement "They are the simplest substances and cannot be broken down further" applies only to elements.

The process where a solid changes directly into a gas without becoming a liquid first is called sublimation.

A compound is a substance formed when two or more elements are chemically joined. Water, salt, and sugar are examples of compounds. When the elements are joined, the atoms lose their individual properties and have different properties from the elements they are composed of.

Tap water is not a pure substance due to the following reasons:

• It contains various chemicals added during the purification process.
• If sourced from the ground, it includes naturally occurring minerals.
• These additional components make it a mixture rather than a pure substance.