Assertion and Reason Questions for Class 9 Science Chapter 1 Matter in Our Surroundings

Gases are highly compressible because the particles are far apart. When pressure is applied, these large inter-particle spaces decrease, bringing the particles closer together.

Gases exert pressure due to the random and continuous bombardment of their high-speed particles against the container walls. However, the intermolecular force of attraction in gases is very weak, not strong.

It is easier to cook at sea level, but the reason is false. At higher altitudes, atmospheric pressure is lower, which causes the boiling point of water to decrease (not increase). This means water boils at a lower temperature, taking longer to cook food.

During a phase change (like melting), the temperature does not rise because the supplied heat energy (latent heat of fusion) is completely utilized in breaking the intermolecular forces of attraction, rather than increasing the kinetic energy of particles.

Solids do not generally diffuse into air because their particles are tightly packed and held by strong intermolecular forces, preventing them from moving freely. The reason states particles are loosely packed, which is false.

The normal boiling point of water at sea level is indeed 100°C. However, the reason is false because atmospheric pressure drops at higher altitudes, causing the boiling point to decrease, not increase.

Both statements are factually correct. However, naphthalene subliming (leaving no residue) is an example of sublimation, not the underlying explanation for the definition of sublimation itself.

Ice floats on water because it has a unique cage-like structure that makes it less dense than liquid water (an anomaly). The reason statement (liquids generally have lower density than solids) is true for most substances, but it contradicts the case of water and therefore cannot be the explanation for why ice floats.

Camphor undergoes sublimation, meaning it converts directly from a solid state to a gaseous state. Because of this, it leaves behind no residue when it burns. Thus, the assertion is false.

Evaporation definitely increases with temperature. However, the reason is false because an increase in temperature increases the kinetic energy of the particles, allowing more of them to overcome intermolecular forces and escape as vapor.

Gases diffuse much faster and more easily than liquids because gas particles have higher kinetic energy and weaker intermolecular forces. The assertion is false, but the reason stating that forces are greater in liquids is a true statement.

Even though both exist at 100°C, steam contains additional hidden heat energy called the latent heat of vaporization. This extra energy makes steam far more effective for heating and causes more severe burns than boiling water.

Both statements are factually correct independently. Gases diffuse easily, and solids do have the greatest intermolecular forces. However, the forces in solids have nothing to do with explaining the diffusion rates between gases and liquids.

During a change of state (like melting or boiling), the temperature remains constant despite continuous heating. The heat is absorbed as latent heat to break bonds, making the reason true but the assertion false.

A mass of sugar or salt crystals takes the shape of the container, so the assertion is true. However, sugar is a solid, not a liquid. Individual sugar crystals maintain a fixed shape.