1. master the dissolution, filtration, evaporation and other experimental skills.

2. Understand the chemical principles of filtration to separate mixtures.

3. To appreciate the principle of filtration in life and production and other practical applications in society.

Second, the experimental apparatus and drugs

Drugs: crude salt, water

Apparatus: tray balance, measuring cylinder, beaker, glass rod, medicine spoon, funnel, iron stand (with iron ring), evaporation dish, alcohol lamp, crucible tongs, burette,

Filter paper, scissors, matches, pieces of paper

Third, the principle of the experiment

Crude salt contains Insoluble impurities such as silt and sand, and soluble impurities such as: and so on. Insoluble impurities can be removed by dissolving and filtering

and then evaporate the water to get the purer refined salt.

Four, experimental operations

1. Dissolution

Weigh 5 grams of crude salt (accurate to 0.1 grams) with a tray balance. Measure 10 ml of water with a measuring cylinder and pour it into a beaker. Take a spoonful of coarse salt with a medicine spoon and add it to the water and observe what happens. Stir with a glass rod and observe what happens (what role does the stirring of the glass rod play in dissolving the crude salt?). The Then add the crude salt, stirring with the glass rod as you do so, until the crude salt is no longer dissolved. Observe whether the solution is cloudy.

Weigh the remaining crude salt on a balance and calculate approximately how many grams of crude salt have been dissolved in 10 milliliters of water.

2. Filtration

Filter the solution as described in Basic Chemistry 6. Carefully observe the residue on the filter paper and the color of the filtrate. If the filtrate is still cloudy, it should be filtered again.

If the filtrate is still cloudy after two filtrations, check the experimental setup and analyze the reasons, e.g., the filter paper is damaged, the liquid level in the funnel is higher than the edge of the filter paper during filtration, the apparatus is not

clean. After finding out the reason, to re-open the operation.

3. Evaporation

Pour the clarified filtrate into the evaporating dish. The evaporating dish is placed on the iron ring of the iron frame table and heated with an alcohol lamp (Figure 20). Stir the filtrate continuously with a glass rod.

Wait until a large amount of solid appears in the evaporating dish, stop heating. Use the residual heat of the evaporating dish to vaporize the filtrate.

4. Use the glass rod to transfer the solid to the paper, weigh it, and recycle it into the container designated by the teacher. Compare the state of the salt before and after purification and calculate the yield of refined salt.

V. Experimental summary

Filtering operation in the problem analysis

Filtration is the most commonly used experimental operations to separate liquids and solids. The current junior high school chemistry textbooks only a cursory introduction to the operation of filtration points, and the actual operation process, often encounter many detailed problems. The author combined with the actual teaching of the filtering operation often encountered problems to talk about their own solutions, for reference only.

(a), how to choose the funnel and filter paper?

The size of the funnel depends mainly on the amount of precipitate to be filtered or the amount of precipitated solids, rather than the volume of liquid. If the amount of precipitate or the amount of solids is large, then the funnel chosen is large, and vice versa. The cone angle of the funnel should be 60 °. The diameter of the pipe is appropriate, too thick to maintain the water column, too fine water flow rate is slow, filtration takes too long. The end of the pipe diameter should be slightly inclined. The choice of filter paper is based on the experiment to determine. Filter paper is divided into qualitative filter paper and quantitative filter paper. Qualitative filter paper in the filtration operation is mainly used to study the physical and chemical properties of substances; quantitative filter paper is mainly used for quantitative analysis of substances. In secondary school experiments, filtration is often used in qualitative experiments, so most of them use qualitative filter paper. Selected filter paper into the funnel, the edge of the paper should be lower than the edge of the funnel about 5 millimeters appropriate.

(ii), how to assemble the filter?

First of all, the selected filter paper will be folded twice, the second fold with the first fold of the fold is not completely overlap. When such a filter paper into the funnel (top angle of 60 °), the sharp corners of the funnel wall with a certain gap, but the upper part of the funnel can be intact on the wall. The filter thus formed filters faster than a filter in which all surfaces are attached to the funnel. When folding, do not flatten the folds at the top corners of the filter paper, so as not to weaken the strength of the tip. Then cut off the small corners of two of the three layers of paper, so that when wet, the upper part of the filter paper can be tightly adhered to the funnel wall.

Secondly, put the folded filter paper into a suitable funnel, wet the filter paper with water from the bottle, and gently press the upper 1/3 of the filter paper against the funnel wall with your fingers. When filling the funnel with water, the funnel neck should be filled with water, or block the end of the funnel neck with your finger, so that it is filled with water to the top corner of the funnel slightly above. Keeping a continuous column of water in the funnel neck creates a downward gravitational force that accelerates the filtration process.

(c), how to filter correctly?

In filtration, the glass rod and the beaker containing filtrate mouth opposite; glass rod end and the funnel in the filter paper in the three-layer part of the proximity of the filter paper, but can not touch the filter paper; to maintain vertical (I believe that the glass rod inclined easy to lead to the spillage of the filtrate); the funnel's neck tip is close to the beaker receiving filtrate mouth part of the inner wall. Each transfer of liquid must not exceed two-thirds of the height of the filter paper, to prevent the filtrate does not pass through the filter paper and outflow from the wall. For the residual liquid and solid material in the beaker should be solvent or distilled water in small quantities in accordance with the principle of rinsing, will be transferred to the funnel for filtration of all the washings.

(iv), how to wash the precipitate correctly?

If you need to wash the precipitate, it should be washed immediately, otherwise the precipitate will be placed on the filter paper for a long time will be cracked or clumped, it is not easy to rinse. The original solvent, distilled water or other appropriate detergent can be used to rinse. Change a clean empty beaker to replace the original beaker to receive the filtrate, which can be avoided because the precipitate penetrates the filter paper and have to re-filter a large volume of liquid. Use as much detergent as necessary to submerge the collected precipitate. When washing, use a small amount of wash liquid to carefully rinse along the perimeter from top to bottom to wash the precipitate to the bottom of the funnel, do not make the liquid flow rate is too strong, otherwise the precipitate will be washed out of the filter. Do not use a glass rod to stir the material in the funnel, so as not to cut the filter paper, the previous work is lost. General wash 2 to 3 times or so, can be basically washed

(E), how to test whether the precipitate is washed?

Can be detected according to the impurities on the precipitate category, in the last wash out of the liquid add appropriate reagents to test the degree of washing. Such as filtration of Na2SO4, BaCl2 two solutions just after the complete reaction of the mixture, to test whether the precipitate is washed, should choose AgNO3 solution. If you add AgNO3 solution in the last washout no precipitate (AgCl) is produced, then the precipitate has been washed.

(F), filtration, the filtrate is too much and beyond the edge of the filter paper or the filter paper is scratched how to do?

A small amount of the original solvent can be used to rinse the funnel and the filter paper 2 to 3 times, the original filtrate together with the wash solution to re-filtration.

(vii) Is filtration necessary to separate precipitates and liquids?

No. When the amount of separated precipitate is very small, can be contained in a centrifuge test tube, with a centrifuge for separation of precipitation at room temperature. The supernatant of the precipitate is sucked up with a pipette. Wash and centrifuge as needed. Only when the amount of precipitation is large, it is suitable for separation by filtration

(viii), filtration operation is there any other way?

Yes. To make filtration fast and easy to wash, the use of Brinell's funnel for depressurized filtration, which makes filtration and washing less time-consuming, and easy to wash; when filtration needs to be carried out at a certain temperature should be used for filtration of the insulated funnel.