practicals

• How do we calculate rate of reaction in an experiment measuring how much something changes over time?

  Amount it has changed by / Time taken
• How can we investigate reaction times?

  1) The person being tested should sit with their arm resting against a table (prevents them from moving around)

  2) Hold a ruler vertically between their thumb and forefinger. Zero end should be in line with thumb and forefinger. Then, let go without warning.

  3) The person being tested should try to catch the ruler as quickly as they can

  4) The reaction time is measure by the number on the ruler where it is caught (from the top of the thumb). The higher the number, the slower the reaction time.

  5) Repeat the test the calculate the mean distance that the ruler fell.
• What are the control variables for the test used to investigate reaction times?

  -Same person to catch the ruler each time

  -Ruler should always be dropped from the same height

  -Read the ruler from the same place e.g. above the thumb

  -Make sure that the person being tested can't be influenced by anything that may alter their reaction time e.g caffeine
• What is the independent variable for the test used to investigate reaction times?


  Effect that something may have on reaction times e.g caffeine
• What is the dependent variable for the test used to investigate reaction times?

  -Reaction times
• How can computers be used to measure reaction times?

  1) Use a simple computer test e.g click the mouse as soon as a stimulus is seen on the screen

  2) More precise reaction time (removes human error)

  3) Can record to milliseconds and give a more accurate measurement

  4) Using a computer also removes the possibility that the person can predict when to respond
• Describe how you can investigate the effect of light on the growth of cress seeds:

  1) Put 10 cress seeds into 3 different petri dishes, each lined with moist filter paper. (Remember to label them!)

  2) Shine a light onto one of the dishes from above and the other two from different directions

  3) Leave for 1 week until you can observe their responses --> will find that the seedlings grow towards the light

  4) If all variables are controlled, you know that the growth response is due to light only
• What variables should be controlled when investigating the effect of light on the growth of cress seeds? How?

  1) Number of seeds - use the same number of seeds in each dish

  2) Type of seed - use seeds that all come from the same packet

  3) Temperature - keep petri dishes in a place with a stable temperature

  4) Water - Use a measuring cylinder to add the same amount of water

  5) Light intensity - Keep distance between the bulb and the dish the same
• How can you investigate the effect of gravity on plant growth?

  1) Place 4 seedlings on damp cotton wool in a petri dish, each with their roots pointing in a different direction

  2) Store the petri dish vertically for a few days in the dark and you should notice that the roots of each seedling grows downwards
• How can you investigate the effect of temperature on the rate of decay?

  IV-Temperature (e.g. 20, 30, 40, 50, 60 degrees)

  DV- Time taken to decompose milk/colour change from pink to colourless.

  CV-Volume of milk, Volume of sodium carbonate solution, volume of phenolthalein, Volume and concentration of lipase.

  
  

  Add 5ml of milk, 7ml of sodium carbonate solution, 5 drops of phenolthalein indicator into a test tube.

  Place into water bath at desired temperature and wait to reach equilibrium. Add 5ml of lipase.

  Record the time it takes to decompose the milk. The colour change will be from pink to colourless.

  Repeat with other temperatures.

  Calculate rate using Rate = 1000/time
• Why does the phenolphthalein indicator change colour as the lipase breaks down the milk.

  pH changes from alkaline (pH 10) to acidic (pH 2). This is because the fat is broken down into fatty acids and glycerol.
• How could you control the temperature of the experiment?

  Place in a water bath or use a beaker of water as a heat sink/ to absorb the heat from the lamp.
• How can you ensure data is reliable?

  Repeat the experiment. Remove outliers and calculate a mean.