Weighing Solid and liquid masses are measured using balances. Three types of balances: Top-loading Triple-beam Analytical Weighing General rules for balance use: NEVER place chemicals directly on a balance pan. Place chemicals in a pre-weighed container or piece of weighing paper Use tongs or a clean piece of lab tissue to handle the container or weighing paper holding the sample being weighed. Weighing Do not place hot objects on a balance pan. The material being weighed must be at the same temperature as the balance to ensure accurate mass measurement. Inform your teacher if the balance is not level or at the zero mark when empty. The balance should read zero BEFORE and AFTER weighing. Weighing Immediately clean up any chemicals spilled on or near a balance, following directions from your teacher. Top-Loading Balances Top-loading balances are used for rapid determination of masses to the nearest grams. Using the balance: Turn the balance on using the power button and wait for the display to read g. Using tongs or tissue, place the empty sample container on the center of the pan. Top-Loading Balances The balance should display the mass of the sample container. If you need the mass of the container as part of your data, write it down in you lab notebook. Press the TARE (zero) button to return the display to zero. The TARE is the mass of the empty container, which is retained on the balances memory and is automatically subtracted from the total mass of the container and its contents. Top-Loading Balances Remove the empty container from the balance. Place the substance or object to be weighed in the container and return the container and its contents to the balance. Read and record the samples mass, using the number of figures to the right of the decimal point specified. Triple-Beam Balance A triple-beam balance has three beams with sliding masses, have about a 610 gram capacity and are used to weigh to the nearest 0.1 gram. Similar to a scale you would see in a doctors office Analytical Balance Analytical balances are used for very accurate, quantitative mass measurements to the nearest g. They are much more delicate than either top-loading or triple-beam balances. The General Rules must be followed to avoid damaging the balance. Analytical Balance Analytical balances look similar to a top- loading balance with a glass-sided box surrounding the pan. This box, or weighing chamber, acts as a draft shield, so that very accurate weighings will not be affected by any air currents in the lab. The weighing chamber has doors that allow access to the pan from the sides and top. Transferring Solids Take a labeled, wide-mouth container, such as a beaker, to the reagent shelf. Hold the reagent bottle cover in your fingers, or lay it on the benchtop with the top side down to prevent contamination. Pour solids into your container by gently tipping the reagent bottle while slowly rotating it back and forth. Use a spatula (Clean and Dry) only to transfer solids from your container DO NOT put your spatula into a reagent bottle Transferring Solids Replace the original cover on the reagent bottle. DO NOT return unused solids to their reagent bottles; this can also cause contamination. Separations - Decanting A solid formed in a solution during a reaction is called a precipitate. The precipitate can be separated from the solution by decantation. To decant a liquid, first allow the solid to completely settle to the bottom of the container. The remaining liquid is should be poured off without disturbing or transferring any of the solid. Separations - Filtering A precipitate can also be filtered from a solution when the solid has settled to the bottom of the reaction container. Two types of filtration: Gravity Filtration (already discussed) Stabilize a conical funnel by putting it into a support ring clamped to a ring stand. Select appropriate filter paper and fold to create a funnel. Place a clean beaker under the stem of the funnel to receive the filtered solution called the filtrate Separations Suction Filtration using a Buchner Funnel Buchner Funnels are made of porcelain, glass or plastic and have tiny perforations in the top of the funnel Filter paper is placed on top of the perforations to prevent any of the precipitate from draining though The funnels stem is inserted into a one-hold rubber stopper that fits a clean side-arm filter flask. Heavy-wall rubber tubing prevents the tubing from collapse during suction. A trap prevents accidental backflow from the aspirator into the filter flask. Separations Once the paper is moisten, suction can be created using either a water aspirator or a vaccum pump. The filtration process from this point is very similar to gravity filtration. Separations - Centrifuge A centrifuge is a device that spins solid- liquid mixtures at high speeds in order to separate the mixture components by their mass differences into two distinct layers. Samples are held in small test tubes called cuvets.