1What is physical change?
2What do you mean by chemical change?
3What are the differences between physical and chemical changes?
4What type of change is the rusting of iron, forming of curd from milk, cooking rice, burning of wood to form ash, combustion of a matchstick and photosynthesis?
5Discuss the type of change in each of these: a dry cell giving electricity, burning of firewood, melting of ice, digestion of food, rusting of iron nails, boiling of water and photosynthesis.
6What do you mean by reactants and products?
7What do you mean by a chemical reaction? Explain with an example.
8Which substance is X in the reaction \( \mathrm{HCl} + \mathrm{X} \rightarrow \mathrm{NaCl} + \mathrm{H_2O} \)?
9What is a chemical equation? What are the rules for writing one?
10What is the difference between a subscript and a coefficient? How many atoms of each element are in \( 3\mathrm{H_2SO_4} \)?
11No colour change is seen when sodium hydroxide is added to dilute hydrochloric acid. Why is it still called a chemical reaction?
12Why must each element have the same number of atoms on both sides of a chemical equation? Check this for \( 2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO} \).
13Write any four signs that show a chemical reaction has taken place.