1)Why does metal feel cold 

 You must have noticed that anything made of metal feels cold to the touch Because metal is a good conductor of heat and heat flows through easily .Your fingers are warm from your body heat and when you touch a metal surface the heat from your fingers flows to the surface of the metal.Because metal is a good  conductor of heat the heat moves on through the metal,so that it stays cooler than your skin and heat continues to flow  from your fingertips to the metal.When the skin on your fingers loses heat it feels cold and as a result the metal feels cold too.

2) Flammable

       Hydrogen is the first element on the periodic table.it is highly flammable and is one of the most common element found in the Universe

3)How did modern chemistry begin 

              Modern chemistry began to emerge when Robert Boyle made a clear distinction between chemistry and alchemy in 1660.German chemists Johann becher and Georg ernst Stahl laid the foundation for the modern theory of combustion .The English chemist Joseph priestly discovered in 1774 that oxygen is essential to the burning process .Henry Cavendish identified hydrogen and in1783 lavoisier  developed the theory of conservation of mass.Chemicals are spirit up in to organic chemicals and inorganic chemicals.It was earlier believed that organic chemicals were found only in living things.In 1828 Friedrich wohler a German scientist accidentally synthesized urea an organic compound know to occur in living things from an inorganic substance ammonium cyanate.This opened a new era of research in chemistry. 

4)Why was the discovery of radioactivity said to be accidental 

        Henri becquerel was using naturally fluorescent minerals to study the properties of X-ray which had been discovered in 1895 by Wilhelm roentgen.He exposed potassium uranylsulfate to sunlight and then placed it on photographic plates wrapped in black paper thinking that the uranium absorbed the sun's energy and then emitted it as X-rays.His experiment failed because the sky was overcast and there was no sunlight.For some reason becquerel decided to develop his photographic plates anyway.To his surprise the images were strong and clear proving that the uranium emitted rays spontaneously without the help of any external source like the sun.Becquerel had discovered radioactivity by accident and now an element that spontaneously emits radiation is said to be radioactive.Radioactive substance such as urnium produce three type alpha,beta and gamma rays.Its High useful tre