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Answer:
18. option c (red)
Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5–8.3 at 25 °C (77 °F). Neutral litmus paper is purple. Litmus can also be prepared as an aqueous solution that functions similarly.
19. option (b) sulphuric acid
Sulfuric acid is sometimes referred to as the “king of chemicals” because it is produced... Due to its affinity for water, pure anhydrous sulfuric acid does not exist in nature.
20. option (b) convection
Convection is like an invisible conveyor belt that can transfer heat through fluids (liquids and gases). When part of a fluid is heated up, it expands. This makes it lighter and less dense than the fluid around it, so it rises upwards. As it rises, it moves away from the source of heat.
18. c. red
19. b. sulphuric acid
20. b. convection of heat