Mercury is used in thermometers because of its unique physical and chemical properties that make it ideal for accurately measuring temperature changes. Key reasons include its wide temperature range where it remains liquid (from -38.83°C to about 357°C), high thermal conductivity for quick response to temperature changes, uniform expansion ensuring precise measurement, shiny metallic appearance for clear visibility, non-wetting nature allowing smooth movement in the glass tube, and chemical stability for long-lasting accuracy. These factors combined have historically made mercury the preferred substance for thermometers despite recent concerns about its toxicity.