On Line Test For Class 7 Chemistry

Gas is the state of matter that can be compressed easily. Unlike solids and liquids, gases have particles that are widely spaced and move freely. This allows them to be easily compressed because the particles can be pushed closer together. In contrast, solids have tightly packed particles that are difficult to compress, while liquids have particles that are close together but still have some freedom of movement. Therefore, the correct answer is gas.

Tearing of paper is a physical and irreversible change because once the paper is torn, it cannot be restored to its original form. The tearing of paper involves the breaking of chemical bonds within the paper fibers, resulting in a permanent change in its physical structure. Unlike reversible changes, such as melting or freezing, where the substance can return to its original state, tearing is irreversible as the torn pieces cannot be reassembled to form the original paper.

Liquefaction is the process in which a gas or vapor is converted into a liquid state. This is the other name for condensation, as condensation refers to the change of state from a gas to a liquid. During condensation, the molecules of a substance slow down and come together to form a liquid. Therefore, liquefaction accurately describes the process of condensation.

Mercury has a higher density as a liquid compared to iron. Density is a measure of how much mass is packed into a given volume. Since mercury is a dense liquid, it means that a greater amount of mass is packed into a smaller volume compared to iron. This is why the density of mercury as a liquid is higher than that of iron.

When lead dioxide is heated, it undergoes a chemical reaction called thermal decomposition. This reaction breaks down lead dioxide into its constituent elements, which are lead and oxygen. Therefore, the gas released when we heat lead dioxide is oxygen.

The sublimation of solid ammonium chloride (NH4Cl) constitutes a physical change. In this process, the solid directly transforms into a vapor state upon heating, bypassing the liquid phase. When the heat is withdrawn, the vapors condense back into a solid form, exemplifying the reversible nature of this physical transformation.

During a chemical change, the composition of the original substance is not altered. This means that the arrangement and types of atoms in the substance remain the same, even though the substance may undergo a physical or chemical transformation. Chemical changes involve the breaking and forming of chemical bonds, but the overall composition of the substance remains unchanged.

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. Ammonium chloride is a classic example of a substance that undergoes sublimation. When heated, it readily transforms into ammonia and hydrogen chloride gases, bypassing the liquid state.

When milk is boiled, the heat causes the molecules in the milk to gain energy and move faster. As a result, these molecules spread out and move from an area of higher concentration (inside the pot) to an area of lower concentration (outside the pot). This movement of molecules from an area of higher concentration to an area of lower concentration is known as diffusion. Therefore, the aroma of the boiling milk can be felt from outside due to diffusion.

When separating naphthalene from a mixture of naphthalene and sodium chloride, a physical change occurs. This is because physical changes involve altering the physical properties of a substance without changing its chemical composition. In this case, the separation process involves utilizing the difference in solubility between naphthalene and sodium chloride. By dissolving the mixture in a suitable solvent, such as water, the naphthalene can be selectively separated through processes like filtration or evaporation, leaving behind the sodium chloride. This separation does not involve any chemical reactions or changes in the molecular structure of the substances, making it a physical change.