How to accurately measure the volume of water

When measuring the density of brine, the most common mistakes made by students are: first measure the mass of beaker m 1 with a balance, then measure the mass m2 of beaker and brine, pour the brine in beaker into a measuring cylinder, and measure the volume v of brine, so as to get the density ρ = m/v = (m2-m1)/v.

The result of the above mistakes is that the quality and volume of brine do not correspond. This is because when the salt water in the beaker is poured into the measuring cylinder, the salt water will more or less adhere to the beaker wall due to the seepage phenomenon caused by molecular attraction, resulting in the quality of the salt water entering the measuring cylinder being less than the measured quality (m2-M 1), and then the density of the salt water ρ = m/V = (m2-M 1)/V is less than the actual salt water density.

In order to avoid the errors caused by the defects of the method itself, we usually measure the mass of beaker and brine m 1 first, then pour a proper amount of brine in beaker into a measuring cylinder, and measure the mass m2 of beaker and residual brine and the volume v of brine in the measuring cylinder, so that the density ρ = m/V = (M 1-m2)/V can be obtained.

When students operate, they often pour salt water on the wall of the measuring cylinder carefully, so that some salt water will adhere to the wall of the measuring cylinder, which will still cause the mismatch between the quality and volume of salt water; In addition, proper amount of salt water is poured into the measuring cylinder to make the salt water level flush with the whole scale line of the measuring cylinder, which is convenient for calculation and can improve the measurement accuracy. But how can we be sure? If you pour too much, pour it into the beaker. Isn't salt water attached to the measuring cylinder wall again? How to properly solve these problems? In the process of experiment and thinking, we guide students to do it in two steps: when pouring the salt water from the beaker into the measuring cylinder, it should be poured along the middle of the measuring cylinder as far as possible, where there is no chemical reaction problem and can be distinguished from chemical experiments; In order to avoid pouring too much salt water, pour less salt water first, and then add it with a dropper until the water surface is flush with the whole scale line. After making it clear, let everyone try again, and the students all showed satisfied smiles.

Second, the measurement of irregular solid density

1. Measurement of common irregular stone density

For measuring the density of ordinary irregular stones, we always measure the mass m of stones with a balance first, and then measure the volume v of stones with drainage method; That is, after measuring the volume of water in the measuring cylinder V 1, put the stones in the balance plate into the measuring cylinder and measure the volume of water and stones in the measuring cylinder and V2, then the increase of V2 relative to V 1 is caused by the drainage of stones, that is, the volume of stones V = V2-V 1. Therefore, the density of stone can be easily solved, that is, ρ = m/v = m/(V2-V 1).

2. Measure the density of volcanic rocks and water-absorbing stones.

Rockery bonsai with its beautiful shape, tall and straight, majestic posture, gives people a kind of motivation, full of vitality. Both in the park and in the living room are deeply loved by people. We should pay attention to the fact that the density measurement method of ordinary irregular stone can not be applied to the density measurement of volcanic rocks and water-absorbing stone samples for making rockery bonsai. The reason is that this kind of stone is loose and has strong water absorption capacity, and its volume is almost unchanged after water absorption. If the above method is used to measure again, after the stone is put into the measuring cylinder, the water surface will drop because of water absorption, and the longer the time, the more obvious the drop will be. If V = V2-V 1 is still used as the stone volume, it will be much smaller than the real value of the stone volume, and the measured density will be significantly larger. Therefore, the correct measurement method is: firstly, measure the mass m of the water-absorbing stone with a balance, put it into a beaker filled with water to absorb water, and measure the volume of water in the measuring cylinder, then throw the water-absorbing stone into the measuring cylinder, and then measure the sum of the volumes of water and stone in the measuring cylinder, and use the density formula ρ = m/v = m/(V2-v65438+).

3. Measurement of soybean density

How to measure the density of soybean? For this problem, children in the city, because they have no life experience, often mistakenly think that the density of rocks such as volcanic rocks and water-absorbing stones should be measured, so that after measuring the quality of soybeans, they put them into water and pour them into a measuring cylinder to measure the volume of soybean boiling water V = V2-V 1. By using the density formula ρ = m/V = m/(V2-. The most taboo here is making, because the volume of soybeans will increase after soaking in water, and the density of water soybeans is usually smaller than that of dried soybeans. When we measure the density of soybean, we should not only soak it, but also move it as quickly as possible to improve the measurement accuracy. Of course, if bubbles are generated when the soybeans are poured into the measuring cylinder, the air should also be shaken evenly to avoid the measurement result becoming smaller due to the increase in volume.

4. Measurement of bulk solid density

If the solid volume is large, after measuring the mass m, it cannot be put into the measuring cylinder, but the beaker can be filled with water. Slowly put the object into the beaker, and the measuring cylinder should be docked with the overflowing water in the beaker. At this time, the volume of water in the measuring cylinder is the volume v of the object, and the density of the object can be obtained by using the density formula ρ = m/v.

5. Measurement of the density of irregular solids whose density is less than that of water.

For the density measurement of irregular solids with density less than water, after measuring its mass m, we can use toothpick "pressing method" or high-density solid "binding method" to measure the volume of water displaced by it as the volume V of the object, and then use the density formula ρ= m/V to get the density of the object.