Title 3. How things change & react with one another.pdf ✅

HOW THINGS CHANGE & REACT WITH ONE ANOTHER CONTENTS ➢ Physicals and chemical changes ➢ Change ➢ Rusting and its prevention ➢ Crystals and Crystallisation ➢ PHYSICAL AND CHEMICAL CHANGES There are two possible definitions for physical and chemical change :  A physical change is reversible, a chemical change is not always. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change and the burning cannot be withdrawn once having burnt.  A physical change is a change in which no new substance is formed, a chemical change results in the formation of one or more new substances. Again, consider the previous examples: freezing water into ice just results in water molecules, which are stuck together it is still H2O. Whereas burning wood results ash, carbon dioxide etc, all new substances, which were not, there when you started burning. A physical change in a substance does not change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed. For example, if a piece of paper is cut up into small pieces, it still is paper. This would be a physical change in the shape and size of the paper. If the same piece of paper is burned, it is broken up into different substances that are not paper. Physical changes can be reversed; chemical changes cannot be reversed with the substance changed back without extraordinary means, if at all. For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated. If one decided to mix sugar into water to make sugar water, this would be a physical change as the water could be left out to evaporate and the sugar crystals would remain. However, if one made a recipe for a cake with flour, after, sugar and other ingredients and baked them together, it would take extraordinary means to separate the various ingredients out to their original form. When heat is given off in a chemical change or reaction, it is called an exothermic reaction. When heat is absorbed in a chemical change or reaction, it is called an endothermic reaction. The speed at which chemical reactions take place depend on the temperature pressure and how concentrated the substances involved in the chemical reaction are. Sometimes substances called catalysts are used to speed up or help along a chemical reaction. ➢ CHANGE There are two types of changes:  Physical Change In a physical change, there is only a change of state. The new substance has the same properties as the old one. No new substance(s) are produced. Ice- water-steam (They are all still water !) For example: ice melting to water or water boiling. In all of these changes, you can get the original materials back! A physical change may also involve changing the shape of the substance. Paper cut into pieces is still paper, ploughing a field but the field still remains as soil, cutting wood into pieces is still wood, and moulding a sculpture is still cement or marble Physical changes are reversible changes. The following are the examples of physical change, which is also a temporary change:- 3 CHAPTER