chemical changes (wa2) Flashcards
physical changes
no new susbtances provided
usually reversible
chemical changes
new substances produced, usually irreversible
total mass of atoms
no atoms are created or destroyed during chemical reaction, hence the total mass of reactants is equal to the total mass of products in a chemical reaction
thermal decomposition
condition: heat
single compound breaks down into two or more simpler susbtances when heated
oxidation
condition: oxygen
combustion
reacts with oxygen in presence of heat
condition: heat, oxygen
cellular respiration
glucose + oxygen → carbon dioxide + water
rusting
iron + water + oxygen → iron(III) oxide
electrolysis
condition: electricty
single compound decomposes into simpler substances
photosynthesis
carbon dioxide + water → (light energy) glucose + oxygen
condition: light
observations during chemical changes
- change in colour
- formation of insoluble solids (precipitate)
- formation of a gas (fumes or bubbles, efferevescenes)
- change in temperature
- emission of light
beneficial effects of chemical changes
- cooking
- cellular respiration
- combustion of fuels
- production of medicines
- manufacture of daily products
harmful effects of chemical changes
- rusting
- air pollution
- decay of food
- global warming
acids
need to be dissolved in water to show their acidic properties
- sour taste
- conduct electricity in aqueous state as there are free-moving ions that can act as mobile charge carriers
- dilute acids are irritants while concentrated acids are corrosive
- change the colour of pH indicator (blue litmus paper red)
- reacts with bases, metals and carbonates
acid-alkali
- produces salt and water
- neutralisation reaction
acid + base → salt + water
acid + alkali → salt + water
salt: acid second word + base/alkali first word
rla of acid alkali
- to treat acidic soil
- to neutralise acid produced by bacteria
- to neturalise excess acid produced by the stomach using antacids
acid-metal
- produces salt and hydrogen
- less reactive metals such as copper, silver and gold do not react with acids
acid + metal → salt + hydrogen
salt: acid second word, metal first word
observation and test for hydrogen gas
- effervescence of a colourless and odourless gas is observed
- insert a lighted splint into the test tube
- hydrogen extinguishes the lighted splint with a ‘pop’ sound
acid-carbonate
- produce salt, carbon dioxide and water
acid + carbonate → salt + carbon dioxide + water
observation and test for co2
- effervescence of a colourless and odourless gas is observed
- bubble the gas produced through limewater (aqueous calcium hydroxide)
- a white precipitate is produced
alkali
- alkalis feels soapy and have a bitter taste
- dilute alkalis are irritants while concentrated alkalis are corrosive
- alkalis change the colour of pH indicators (red litmus paper blue)
- conduct electricity in aqueous state as they dissolve in water and ionise to produce free-moving ions that can act as mobile charge carriers to conduct electricty
- react with acids and ammonium compounds
pH
- measures acidity or alkalinity
acidic solutions have a pH below 7
neutral solutions have a pH of 7
alkaline solutions have a pH above 7
indicators
pH, universal indicator, red cabbage solution, litmus paper
