CBSE Class 10 Science

## Chemical change and its signs In a chemical reaction the substances present at the start, the **reactants**, are changed into different substances, the **products**, with different properties. The atoms are the same before and after; they are rearranged into new combinations. Some observations show that a reaction has happened: - a change of state, as when a gas is given off - a change of colour - a change of temperature - the formation of a precipitate, an insoluble solid that appears in a solution Burning magnesium ribbon in air gives a dazzling white flame and leaves a white powder, magnesium oxide. Dilute sulphuric acid added to zinc granules gives bubbles of hydrogen gas and the zinc slowly disappears. Iron nails left in copper sulphate solution turn the solution from blue towards green, and a brown coating of copper forms on the nails. Each of these signs is evidence, and none is proof by itself, because a change of state such as boiling water also gives off a gas without forming a new substance. Two preparations are worth doing before a reaction: the magnesium ribbon should be cleaned with sandpaper to remove the layer of magnesium oxide on it, so that it is pure magnesium that burns, and it should be held with tongs, well away from the eyes, because the flame is bright enough to hurt them. ## Word equations and chemical equations The shortest way to record a reaction is a **word equation**, with an arrow from reactants to products: $$\text{magnesium} +…