Science chapter 11

• Chemical changes is the formation of new substances due to the rearrangement of atoms which means mass remains constant
• Chemical changes is usually irreversible
• During chemical changes, some observations are visible: colour change, production of gases, heat or light energy
• Change of state is a physical change
• Examples of chemical changes include: combustion, thermal decomposition, oxidation, photosynthesis, electrolysis
• Combustion is a chemical reaction where a substance is heated in the presence of oxygen to form new substances
• Decomposition is the breaking down of a substance into two or more simpler substances
• Acids have a sour taste and a pH of below 7
• Alkali have a bitter taste, soapy feel and a pH of above 7
• A hydroxide is a alkali
• Acid + alkali -> water + salt (neutralisation)
• Acid + carbonate -> water + salt + carbin dioxide
• Acid + metal -> hydrogen + salt
• To test for carbon dioxide, effervescence the solution into calcium hydroxide. If white precipitate forms, there is a presence of carbon dioxide
• To test for hydrogen, place a lighted splint near the solution. If a "pop" sound is heard and the splint extinguishes, hydrogen is present
• Acid turns blue litmus paper red
• Alkali turns red litmus paper blue
• Neutral solutions do not change the colour of the litmus paper
• Neutralisation can be seen in examples like toothpaste, antacids and soil fertilisers