Contents

Spectroscopy is an important technique used in Chemistry Topics to analyze the interaction of matter with electromagnetic radiation.

## Derive the Percentage of Water Absorbed by Soil and How does Soil Allows Water to Percolate

Though soil usually contains some water (or moisture), it can still absorb (or soak up) a lot of water. Absorption is called ‘sokhna’ in Hindi. The absorption of water by soil will become clear from the following activity.

**Activity 5**

Let us take two test-tubes and fill equal amounts of water ¡n them. Pour water from one test-tube on the cemented floor in the house. We will find that the water poured on the cemented floor flows down, it is not absorbed by the cemented floor. Let us now pour water from the second test-tube on the soil kept in a flower pot. We will see that the soil absorbs the water quite rapidly.

Now, cemented floor does not absorb water because it is not porous. The soil absorbs water because it is porous (having tiny pores in it). All types of soils, however, do not absorb water to the same extent. Some soils absorb more water whereas other soils absorb less water.

### To Find the Percentage of Water Absorbed by a Soil

We will now describe an activity to find out the percentage of water absorbed by a given sample of soil. By saying the percentage of water absorbed by a soil we mean the mass of water absorbed by 100 grams of a soil. Let us describe the activity now.

(i) Take a filter paper, fold it properly and fix it in a plastic funnel. Keep the funnel (having filter paper fixed in it) in a beaker as shown in Figure.

(ii) Take some dry powdered soil and weigh it on a balance. This will give us the mass of soil taken. Pour this weighed soil into the filter paper fixed in the funnel (see Figure).

(iii) Fill a measuring cylinder with water and note the initial volume of water taken in the measuring cylinder. Take out water from the measuring cylinder with the help of a dropper and pour it drop by drop on the soil kept in the funnel. All the water should not fall at one spot on the soil.

(iv) Keep pouring water in the soil kept on filter paper till it just starts dripping from the lower end of the funnel (see Figure). Then stop pouring more water on soil. The dripping of water from funnel tells us that the soil taken on the filter paper has absorbed the maximum amount of water. Note the final volume of water left unused in the measuring cylinder.

(v) Subtract the volume of water left in the measuring cylinder from the initial volume of water taken in the measuring cylinder. This difference will give us the volume of water absorbed by the soil taken on filter paper in the funnel.

By using the above observations, the percentage of water absorbed by the soil can be calculated as follows.

Suppose : Mass of soil taken = m grams

Initial volume of water (taken in measuring cylinder) = v_{1} mL

Final volume of water (left in measuring cylinder) = v_{2} mL

So, Volume of water absorbed (by soil) = (v_{1} – v_{2}) mL

Now, it is known that 1 mL volume of water has a mass equal to 1 gram. So, we can also say that

Mass of water absorbed (by soil) = (v_{1} – v_{2}) grams

Percentage of water absorbed = \(\frac{\text { Mass of water absorbed }}{\text { Mass of soil taken }}\) × 100

Percentage of water absorbed = \(\frac{\left(v_1-v_2\right)}{m}\) × 100

where v_{1} = Initial volume of water (taken in measuring cylinder)

v_{2} = Final volume of water (left in measuring cylinder)

and m = mass of soil taken

The calculation of percentage of water absorbed by a given sample of soil will become clear from the following example.

**Example Problem.**

100 mL of water was taken in a measuring cylinder. This water was added dropwise to 50 g of dry soil kept on filter paper in a funnel. When the water just started dripping from the soil in the funnel, the amount of water left in the measuring cylinder was found to be 80 mL. Calculate the percentage of yrater absorbed by this soil.

**Solution:**

Here, Initial volume of water v_{1} =100 mL

Final volume of water, v_{2} = 80 mL

And, Mass of soil taken, m = 50 g

Now, we know the formula :

Percentage of water absorbed = \(\frac{\left(v_1-v_2\right)}{m}\) × 100

= \(\frac{(100-80)}{50}\) × 100

= \(\frac{20}{50}\) × 100

= 40 per cent

### Soil Allows Water To Percolate

Soil is a porous solid substance (having tiny pores in it). When water is poured over soil, then some of it gets absorbed in the soil and the rest passes down the soil. The process in which water passes down slowly through the soil is called percolation of water.

Percolation is called ‘risna’ in Hindi. Though water can percolate through all types of soils but the rate of percolation of water through different types of soils is different. The rate of percolation of water tells us how fast water passes down the soil. Please note that whether we call ‘rate of percolation of water’ or ‘percolation rate of water’, it means the same thing.

### To Measure the Percolation Rate of Water in Soil

The percolation rate of water in a soil is the volume of water in millilitres which passes down the soil per minute. The percolation rate of water in a soil can be measured as follows.

**Activity 7**

(i) Take a 20 cm long PVC pipe having a diameter of about 5 cm. At the place where percolation rate of water is to be measured, dig the soil to a depth of about 2 cm in the ground. Place one end of the pipe in the dug up ground and hold it vertically (see Figure). Fill a measuring cylinder with water. Note the volume of water taken in the measuring cylinder.

(ii) Pour the water taken in the measuring cylinder in the pipe slowly from the top end (as shown in Figure). Note the time when you just start pouring water in the pipe.

(iii) After some time, all the water poured in the pipe will percolate down through the soil, leaving the pipe empty. Note the time again when all the water has percolated down through the soil and the pipe becomes empty.

(iv) The difference in the two ‘time readings’ will give us the time taken by the water taken, in the measuring cylinder to percolate into the soil.

The percolation rate of water in the soil can be calculated by using the formula :

Percolation rate of water in soil = \(\frac{\text { Volume of water percolated in soil }}{\text { Time taken for percolation }}\)

The volume of water percolated in soil is measured in ‘millilitres’ (mL) and the time taken is measured in minutes (min). So, the percolation rate of water is expressed in the unit of ‘millilitres per minute’ which is written in short form as mL/min. The calculation of percolation rate of water in soil will become clear from the following example.

**Example Problem.**

200 mL of water takes 40 minutes to percolate completely in a particular soil. Calculate the percolation rate of water in this soil.

**Solution:**

Here, Volume of water percolated = 200 mL

And, Time taken for percolation = 40 min

Now, we know the formula :

Percolation rate = \(\frac{\text { Volume of water percolated in soil }}{\text { Time taken for percolation }}\)

Percolation rate = \(\frac{200 \mathrm{~mL}}{40 \mathrm{~min}}\)

= 5 mL/min

Thus, the percolation rate of water in this soil is 5 millilitres per minute. By saying that the percolation rate of water in this soil is 5 millilitres per minute, we mean that 5 millilitres of water passes down this soil every one minute.

Sandy soil is quite loose, so the percolation rate of water is highest in sandy soil. On the other hand, clayey soil is very compact. So, the percolation rate of water is lowest in the clayey soil. It has been found that 8 to 10 days after the rains, the level of water in a well rises.

The sandy soil (having the highest percolation rate) allows the rainwater to reach a well faster and in greater amount. Also, since sandy soil has the highest percolation rate, it retains the least rainwater in it. On the other hand, the clayey soil (having the lowest percolation rate) retains the highest amount of rainwater in it.

In rainwater harvesting, more rainwater is made to percolate into the soil by digging ‘percolation pits’. We have already studied rainwater harvesting in Class VI. Paddy (rice crop) is planted in standing water in the fields. Paddy also requires a lot of irrigation water afterwards. The soil with a low percolation rate of water would be the most suitable for growing paddy (rice crop) because it will allow the water to remain in the fields for a much longer time.