Chemistry Techniques PDF

Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting) 1 minute = 60 seconds **Temperature** - Temperature is...

Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting) 1 minute = 60 seconds **Temperature** - Temperature is measured with a thermometer or digital probe - Laboratory thermometers usually have a precision of a half or one degree - Digital temperature probes are available which are more **precise** than traditional thermometers and can often read to 0.1 ^o^C - Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature; digital temperature probes can be just as, if not, more **accurate** than traditional thermometers - The units of temperature are **degrees Celsius (ºC)** **Mass** - Mass is measured using a digital balance which normally gives readings to two decimal places - Balances must be tared (set to zero) before use - The standard unit of mass in kilograms (kg) but in chemistry grams (g) are most often used - 1 kilogram = 1000 grams **Volume-liquids** - The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed - For approximate volumes where high accuracy is not an important factor, measuring ( or graduated) cylinders are used - These are graduated (have a scale so can be used to measure) and are available typically in a range of sizes from 10 cm^3 ^to 1 litre (1 dm^3^) - Volumetric pipettes are the most accurate way of measuring a **fixed** volume of liquid, usually 10 cm^3^ or 25 cm^3^ - They have a scratch mark on the neck which is matched to the bottom of the meniscus to make the measurement - Burettes are the most accurate way of measuring a **variable** volume of liquid between 0 cm^3^ and 50 cm^3^ (e.g. in a titration) - The tricky thing with burettes is to remember to read the scale from top to bottom as 0.00 cm^3^ is at the top of the column - Whichever apparatus you use, you may see markings in ml(millilitre) which is the same as a cm^3^ Equipment used for measuring liquids, IGCSE & GCSE Chemistry revision notes***Diagram of a burette with conical flask and pipette with filler***   **Volume-gases** - The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus - A **gas** **syringe** is usually the apparatus used - A graduated cylinder inverted in water may also be used, provided the gas isn\'t water-soluble - If the gas happens to be heavier than air and is coloured, the cylinder can be used upright ***Diagram of the set-up for an experiment involving gas collection*** **Examiner Tip** Be careful when recording time not to mix up seconds and minutes in the same table. If a table heading shows Time/mins and you record a stop watch display of 1.30, meaning 1 minute and 30 seconds, that is wrong as it should be 1.5 mins. To avoid any confusion, if the time intervals are less than a minute, its best to change the recorded units to seconds. That same stopwatch display would therefore be recorded as 90 seconds. Advantages & Disadvantages of Methods & Apparatus - In the lab we often have choices of different apparatus to do the same job - Evaluating what is the best one to use is part of good experimental planning and design - This means appreciating some of the advantages and disadvantages of laboratory apparatus **Table showing examples of Advantages and Disadvantages of Lab Apparatus** 12-1-1-advantages\--disadvantages-of-lab-apparatus  ***Five pieces of apparatus that can be used to measure the volume of a liquid. They all have their pros and cons*** **Planning your method** - Good experimental design includes the answers to questions like - Have I chosen a suitable apparatus for what I need to measure? - Is it going to give me results in an appropriate time frame? - Is it going to give me enough results to process, analyse and make conclusions? - Does it allow for repetitions to check how** reliable** my results are? - Does my plan give a suitable** range** of results? - How can I be sure my results are **accurate**? - Have I chosen an appropriate scale of quantities without being wasteful or unsafe? - You may be asked about experimental methods in exam questions and your experience and knowledge of practical techniques in chemistry should help you to spot mistakes and suggest improvements