Substance & Its Nature - Free MCQ Practice Test with solutions, UPSC Lucent

The smallest unit of an element that retains its chemical properties is an atom. Atoms are the building blocks of matter and cannot be further divided without losing their chemical properties.

An example of a pure substance is (c) Salt. A pure substance consists of only one type of substance and has a definite composition. Salt, in this case, refers to a specific compound, sodium chloride (NaCl).

The process of converting a solid directly into a gas is called (b) Sublimation. Sublimation occurs when a solid undergoes a phase change and directly transitions into a gaseous state without passing through the liquid phase.

An example of a chemical change is (c) Burning of wood. Chemical changes involve the formation or breaking of chemical bonds, resulting in the creation of new substances with different properties. Burning wood involves a combustion reaction that releases energy and transforms the wood into ash, gases, and smoke.

The chemical symbol for gold is (b) Au. Chemical symbols are used to represent elements, and "Au" is the symbol for gold on the periodic table.

An example of a physical change is (c) Melting of ice. Physical changes involve a change in the physical state or appearance of a substance without altering its chemical composition. Melting of ice refers to the solid ice converting into liquid water without any chemical transformation.

The SI unit of mass is (a) Kilogram. The kilogram is the base unit of mass in the International System of Units (SI).

An example of a heterogeneous mixture is (d) Sand and water. Heterogeneous mixtures are composed of two or more substances that are visibly distinguishable and not uniformly distributed. In the case of sand and water, the sand particles are not evenly dispersed throughout the water.

The process of separation of a mixture through the action of a solvent is called (b) Filtration. Filtration involves passing a mixture through a filter to separate the components based on their size or physical properties. The solvent is used to facilitate the separation.

A physical property of a substance is (c) Density. Physical properties describe the characteristics of a substance that can be observed or measured without altering its chemical composition. Density refers to the mass per unit volume of a substance.

The pH value of a neutral substance is (a) 7. The pH scale measures the acidity or alkalinity of a substance. A pH of 7 indicates a neutral substance, meaning it is neither acidic nor alkaline.

An example of a non-metal is (c) Oxygen. Non-metals are elements that lack metallic properties such as conductivity and luster. Oxygen is a non-metal and is a vital component of many compounds and the Earth's atmosphere.

The chemical formula for water is (a) H₂O. The chemical formula represents the composition of a compound, indicating the types and number of atoms present. H₂O signifies that water consists of two hydrogen atoms bonded to one oxygen atom.

The state of matter with a definite shape and volume is (a) Solid. In the solid state, particles are closely packed together, maintaining a fixed shape and volume.

Answer: b) Combustion

• Exothermic reactions release heat energy.
• Combustion is an exothermic reaction, as it releases energy in the form of heat and light.
• Photosynthesis is an endothermic reaction because it absorbs sunlight to produce food in plants.
• Evaporation of water and melting of ice are endothermic processes as they both absorb heat.

The process of converting a gas directly into a solid is called (d) Deposition. Deposition occurs when a gas bypasses the liquid state and directly transforms into a solid. This process is commonly observed in sub-freezing temperatures when water vapor changes into ice crystals.

The atomic number of carbon is (a) 6. The atomic number represents the number of protons found in the nucleus of an atom. Carbon, with an atomic number of 6, has six protons.

A physical property is (c) Boiling point. Physical properties describe the characteristics of a substance that can be observed or measured without altering its chemical composition. The boiling point refers to the temperature at which a substance changes from a liquid to a gas under standard atmospheric pressure.

The chemical symbol for sodium is (b) Na. Sodium is represented by the chemical symbol "Na" on the periodic table.

The process of converting a liquid into a gas is called (c) Vaporization. Vaporization, also known as evaporation, occurs when a liquid changes into a gaseous state, usually due to heating or an increase in temperature.