Measuring apparatus

1. Every quantitative experiment depends on choosing apparatus that measures the right quantity with enough precision for the job.
2. Time: a stop-watch measures time, usually recorded to the nearest second.
   1. A digital stop-watch displays hundredths of a second, but your reaction time in starting and stopping it is far longer than this, so recording to the nearest second is sensible.
3. Temperature: a thermometer measures temperature in ${}^{\circ }C$.
   1. A typical laboratory thermometer reads from $-10{}^{\circ }C$ to $110{}^{\circ }C$ with $1{}^{\circ }C$ divisions, so you can read it to the nearest $0.5{}^{\circ }C$.
   2. Wait until the reading stops changing, then read the scale with your eye level with the top of the liquid in the thermometer.
4. Mass: a balance measures mass in grams, often to $0.1g$ or $0.01g$.
   1. Place the empty container on the pan and press the tare button to set the display to zero, so that only the mass of the substance is recorded.
5. Volume of a liquid: three pieces of glassware are used, each suited to a different job.
   1. A measuring cylinder measures a volume of liquid quickly, and common sizes are $10$, $25$, $50$ and $100{cm}^3$, but it is the least precise of the three.
   2. A burette delivers a variable volume through a tap, and its scale is graduated every $0.1{cm}^3$, so it measures volumes far more precisely than a measuring cylinder.
   3. A volumetric pipette transfers one fixed volume, such as $25.0{cm}^3$, very accurately, using a pipette filler to draw the liquid up to a single line on its neck.
6. Read every liquid volume at the bottom of the meniscus, the curved surface of the liquid, with your eye level with it.
7. Volume of a gas: a gas syringe collects a gas and measures its volume directly, because the gas pushes the plunger out along a scale.
   1. A typical gas syringe holds up to $100{cm}^3$ and is graduated every $1{cm}^3$.
8. Choose apparatus whose range covers the quantity you expect, because a $10{cm}^3$ measuring cylinder cannot measure $40{cm}^3$ in one reading.

Advantages and disadvantages of methods

1. When you compare methods, judge each one against what the experiment needs: precision, speed, ease of use and safety.
2. Burette or volumetric pipette versus measuring cylinder: the burette and pipette measure volumes much more precisely, but they take longer to fill and use and are made of fragile glass.
   1. A measuring cylinder is the better choice when the exact volume does not matter, such as when an acid is added in excess.
3. Gas syringe versus collecting over water: a gas can also be collected in an upturned measuring cylinder that starts full of water.
   1. A gas syringe gives a direct reading and works for every gas, including gases that dissolve in water, such as ammonia and carbon dioxide.
   2. Collecting over water uses cheap, sturdy apparatus, but a gas that dissolves in water gives a volume reading that is too low.
   3. In both methods, some gas escapes in the moment before the bung is pushed into the flask, so the measured volume is slightly too small.
4. Polystyrene cup versus glass beaker: for an experiment that measures a temperature change, a polystyrene cup is better, because it is a good thermal insulator and reduces heat loss to the surroundings.
   1. Covering the cup with a lid reduces heat loss even further.
5. Timing by eye: a stop-watch is simple to use, but deciding exactly when a change happens, such as a cross disappearing from view, varies from one reading to the next.
6. Repeating measurements: repeating an experiment and calculating a mean lets you spot an anomalous result and reduces the effect of random errors, but it takes more time and uses more chemicals.

Solutions

1. When sugar dissolves in a cup of tea, the sugar is the solute, the water is the solvent and the sweet liquid is a solution.
2. A solution is a mixture, so the solute can be separated from the solvent again, for example by evaporating the solvent.
3. Water is the most common solvent, and a solution in water is called an aqueous solution, shown by the state symbol $(aq)$.
4. Other solvents, such as ethanol and propanone, dissolve some substances that do not dissolve in water.
5. A solution can contain more than one solute, as seawater contains sodium chloride, magnesium chloride and other dissolved salts.
6. If you keep adding a solid solute to a solvent at a constant temperature, you reach a point where no more dissolves and the extra solid stays at the bottom.
   1. The solution now holds the maximum concentration of solute that can dissolve at that temperature, so it is a saturated solution.
   2. For most solids, more solute dissolves at a higher temperature, so a solution that is saturated at $20{}^{\circ }C$ is no longer saturated at $60{}^{\circ }C$.
7. This is why a saturated solution is always described at a specified temperature.

Residue and filtrate

1. Separation processes leave some material behind and let other material pass on, and the words residue and filtrate name these two parts.
2. In filtration, the solid trapped on the filter paper is the residue and the liquid that runs through into the flask is the filtrate.
   1. When muddy water is filtered, the mud is the residue and the clearer water is the filtrate.
3. In evaporation, the solid left in the evaporating basin once the water has gone is also a residue.
4. In distillation, the substance left in the flask after the liquid has boiled away is a residue too.
5. A filtrate can be a pure liquid or a solution, because dissolved solutes pass straight through filter paper.
   1. Filtering a mixture of sand and salt water gives sand as the residue and salt water as the filtrate.