Here are the answers to the questions: 23. A. Measuring cylinder A measuring cylinder is specifically designed for accurate measurement of liquid volumes in a laboratory. Beakers and test tubes are primarily for holding or mixing, and a hygrometer measures humidity. 24. B. Shells of the atom Electrons are negatively charged particles that orbit the nucleus of an atom in specific energy levels or shells. The nucleus contains protons and neutrons. 25. C. H₂O Ionic compounds are typically formed between a metal and a non-metal, involving the transfer of electrons. KCl (Potassium Chloride) is ionic (K is a metal, Cl is a non-metal). NaCl (Sodium Chloride) is ionic (Na is a metal, Cl is a non-metal). CaCl₂ (Calcium Chloride) is ionic (Ca is a metal, Cl is a non-metal). H₂O (Water) is a covalent compound, formed by the sharing of electrons between two non-metals (Hydrogen and Oxygen). 26. B. Observe safety guidelines and instructions The most fundamental rule in laboratory safety is to always follow established safety guidelines and instructions to prevent accidents and ensure a safe working environment. 27. B. True Sodium chloride (NaCl) is an ionic compound. When molten, its ions ($Na^+$ and $Cl^-$) are free to move, allowing it to conduct electricity. It is also a soluble salt. 28. C. Pascal Mole, Second, and Kelvin are all base SI units* (for amount of substance, time, and temperature, respectively). Pascal (Pa) is a derived SI unit* for pressure, defined as Newtons per square meter ($N/m^2$), where Newton itself is a derived unit ($kg \cdot m/s^2$). 29. B. Sulphuric acid Sulphuric acid ($H_2SO_4$) is a strong acid and is highly corrosive, meaning it can cause severe damage to living tissue and many materials. Vinegar (acetic acid) is a weak acid and mildly corrosive. Water and sugar solutions are not corrosive.