3.3 Physical and Chemical Changes

3.3 Physical Changes In physical changes, the appearance of a substance changes, but the chemical bonds holding the substance together do not change. Examples: melting, freezing, boiling, dissolving Ex. When salt and sugar dissolve in water it is a physical change. The chemical makeup of the salt and sugar are not changed.

Chemical Changes In chemical changes, new substances are produced in the process of breaking chemical bonds and forming new ones. You start with the substances that are going to react. These substances are called reactants. The new substances produced are called products.

What are the reactants and what is the product?

How do you know a chemical reaction has taken place? Think about: Gas is produced and makes bubbles. the colour The colour has changed gas The protein in the egg becomes firmer when it is cooked. the egg

How do you know which are the reactants? When do you use them? Think about: They react with each other to make something new They are used at the beginning of a chemical reaction What happens to them?

How would you write an equation for making a cake? Discuss with your partner. You will need to use all these words and symbols. + butter cake + + eggs flour sugar

Which are the reactants and products? Spend a few minutes underlining the reactants and circling the products on your sheet When you are ready, compare your answers with your partner and discuss what you have written Phrases you could use: It must be the reactant / product because… I think it’s the reactant/product because… It’s the reactant/product because….

Which are the reactants? Mike took methane and burnt it in oxygen. This produced carbon dioxide and water. Iron sulphate is formed when iron is added to sulphuric acid. Hydrogen gas is also given off. Pupils found that sodium chloride and water were formed when sodium hydroxide was added to hydrochloric acid. 4. If you add nitric acid to sodium carbonate they will react together and produce sodium nitrate, carbon dioxide and water.

Which are the reactants? Mike took methane and burnt it in oxygen. This produced carbon dioxide and water. Iron sulphate is formed when iron is added to sulphuric acid. Hydrogen gas is also given off. Pupils found that sodium chloride and water were formed when sodium hydroxide was added to hydrochloric acid. 4. If you add nitric acid to sodium carbonate they will react together and produce sodium nitrate, carbon dioxide and water.

Which are the products? Mike took methane and burnt it in oxygen. This produced carbon dioxide and water. Iron sulphate is formed when iron is added to sulphuric acid. Hydrogen gas is also given off. Pupils found that sodium chloride and water were formed when sodium hydroxide was added to hydrochloric acid. 4. If you add nitric acid to sodium carbonate they will react together and produce sodium nitrate, carbon dioxide and water.

Which are the products? Mike took methane and burnt it in oxygen. This produced carbon dioxide and water. Iron sulphate is formed when iron is added to sulphuric acid. Hydrogen gas is also given off. Pupils found that sodium chloride and water were formed when sodium hydroxide was added to hydrochloric acid. 4. If you add nitric acid to sodium carbonate they will react together and produce sodium nitrate, carbon dioxide and water.

The chemical equations (grade 10) methane + oxygen  carbon dioxide + water 2. iron + sulphuric acid  iron sulphate + hydrogen 3. sodium hydroxide + hydrochloric acid  sodium chloride + water 4. nitric acid + sodium carbonate  sodium nitrate + carbon dioxide + water.

Chemical Changes Evidence of chemical change: Colour change Heat, light, sound produced or consumed Bubbles of gas form Formation of a precipitate (solid formed from 2 liquids) The change is difficult to reverse

Energy Changes In both physical and chemical changes, energy changes take place. This energy change can mean releasing energy to or absorbing energy from the environment. See page 99

Energy Changes Exothermic reactions involve the overall release of energy in the form of heat and light. Memory Tip: Exo – starts with the letters “ex” similar to exit Endothermic reactions involve the overall absorption of energy. Endo – starts with the letters “en” similar to enter

Applications of Chemical Changes Some chemical changes present problems, while others provide opportunities and advantages. Corrosion is a major problem for steel structures. By protecting steel surfaces, the chemical reaction of iron with oxygen can be prevented.

Applications of Physical and Chemical Changes First Nations people of the Pacific Coast have used drying and smoking as a means of preserving food. Drying is a physical change that removes water from the cells of the fish and bacteria which prevent the bacteria from attacking the meat. Smoke causes chemical changes in meat that kill bacteria.