chemical sciences revision

• what is a variable?


  a factor which may change or be changed
• what is an independent variable?


  the variable which is changed to determine its effect on the dependent variable
• what is a dependent variable?


  the variable which is affected and therefore measured or calculated
• what is a controlled variable?

  these variables must not be allowed to change as they could affect the results
• a hypothesis is a single testable statement which may describe a possible relationship between the independent and dependent variable
• what is a relationship/pattern/trend


  the effect of one variable on another is called a relationship/pattern or trend
• validity is the degree to which a test measures what it is supposed to measure (one thing is being changed at a time, not several things)
• validity is so that meaningful conclusions can be drawn about the effect of each variable in turn
• reliability is how similar/consistent the data is for a series of trials
• to improve reliability of an experiment, trials can be repeated
• an outlier is a value that doesn't appear to follow the trend in a reliable data set
• an independent variable is the variable that is being changed in an experiment to affect the dependent variable
• a dependent variable is the variable that is measured by the experimenter and is the outcome of the experiment
• a controlled variable is a variable that is kept constant throughout the experiment, so that it can be measured and controlled
• chemical change is a change in the composition of a substance, but not in its physical state
• physical change is a change that can be seen with the naked eye, such as melting or boiling and does not create a new substance
• combustion is the process of burning a substance in oxygen to produce heat and light
• burning is the process of combustion, the reaction of a substance with oxygen to produce heat and light
• smouldering is is the slow, low-temperature, flameless form of combustion
• the fire triangle is the three elements needed for combustion to occur: fuel, oxygen and heat
• fire is a rapid, persistent chemical change that releases heat and light and is accompanied by flame.