CBSE Class 9 Science Exemplar Chapter 2: Is Matter Around Us Pure - NCERT Solutions

Solution: The correct statements about pure substances are (i) and (iii).

A pure substance is defined as a substance that consists of only one type of particle. This means that all the constituent particles (atoms or molecules) are identical. Furthermore, pure substances exhibit a uniform composition throughout their mass. For example, pure water (H₂O) always consists of only water molecules and has the same properties everywhere. Elements like iron or oxygen, and compounds like sodium chloride (NaCl) or water (H₂O), are pure substances. Mixtures, on the other hand, contain two or more pure substances that are not chemically combined and can have variable compositions. Therefore, statement (ii) is incorrect because pure substances can be elements or compounds, but not mixtures. Statement (iv) is also incorrect because while elements are pure substances, the exclusion of nickel is arbitrary and incorrect; nickel itself is a pure substance.

Answer: (i) and (iii) are true statements about pure substances.

Solution: Rusting of iron is a process of corrosion, and it is fundamentally a chemical change.

Corrosion is the process where a metal reacts with substances in its environment, such as oxygen and moisture, to form new chemical compounds. Rusting specifically refers to the corrosion of iron and its alloys. The reddish-brown substance formed, known as rust, is primarily hydrated iron(III) oxide ($Fe\_2O\_3\; \backslash cdot\; nH\_2O$). This is a new chemical compound with properties different from elemental iron. Therefore, rusting is a chemical change because the identity of the substance is altered, and it cannot be easily reversed to obtain the original iron. A physical change, in contrast, alters the form or appearance but not the chemical identity of the substance (e.g., melting ice). Dissolution is the process of a solute dissolving in a solvent, which can be physical or chemical depending on the substances involved, but rusting is specifically classified as corrosion.

Answer: Corrosion and it is a chemical change.

Solution: A mixture of sulphur and carbon disulphide, when properly prepared, often forms a colloidal solution, which is heterogeneous and exhibits the Tyndall effect.

When sulphur is mixed with carbon disulphide, it dissolves to some extent. If the concentration is appropriate, or if the mixture is allowed to stand, it can form a colloidal dispersion. Colloidal solutions consist of particles that are larger than those in a true solution but small enough to remain suspended. These particles are large enough to scatter light, a phenomenon known as the Tyndall effect. Although colloids may appear homogeneous to the naked eye, they are fundamentally heterogeneous because the dispersed particles are distinct from the dispersion medium. True solutions, which are homogeneous, do not show the Tyndall effect because their solute particles are too small to scatter light.

Answer: Heterogeneous and shows Tyndall effect.

Solution: Tincture of iodine is prepared by dissolving iodine in alcohol.

Tincture of iodine is a common antiseptic solution used for disinfecting wounds. It is typically prepared by dissolving a small percentage (usually 2-7%) of elemental iodine ($I\_2$) in a solvent. While potassium iodide ($KI$) is often added to help solubilize the iodine in the solvent, the primary solvent used is alcohol (ethanol or a mixture of ethanol and water). Iodine has limited solubility in water, making water an unsuitable solvent for preparing an effective tincture. Vaseline is a petroleum jelly and not a solvent for iodine in this context. Therefore, alcohol is the correct solvent for preparing tincture of iodine.

Answer: Iodine in alcohol.

Solution: Among the given options, ice and air are homogeneous in nature.

A homogeneous substance has a uniform composition and appearance throughout. Ice, being the solid form of pure water, consists solely of water molecules arranged in a crystalline structure, making it homogeneous. Air is a mixture of gases (primarily nitrogen and oxygen), but these gases are uniformly distributed, resulting in a homogeneous mixture. Wood, on the other hand, is a heterogeneous material; it contains different components like cellulose, lignin, and various other organic compounds, often visible as grains or knots. Soil is also heterogeneous, composed of mineral particles, organic matter, water, and air, with a non-uniform distribution of components.

Answer: (i) Ice and (iv) Air are homogeneous.