"Comparing soil samples" Return to list of pracs

Aim: This activity is used to investigate how to observe different soil types.

Method:

Visual investigation:

Equipment: The equipment includes...  

• 2 beakers;
• soil samples A and B;
• 2 large pieces of paper;
• hand lens.

Make observations of two types of soil according to the work sheet below:

• Soil sampling sheet.

Tip a beaker full of sample A onto one piece of paper and a beaker full of sample B onto another piece of paper. Use a handlens to observe different colours, particle size, animal and plant matter (eg. twigs, insects, leaves etc) and finally feel if the soil is damp, dry, clay or sandy.

Acidity of soil:

Equipment: The equipment includes...  

• universal indicator paper;
• clamp stand;
• 2 corks;
• spatula;
• alkaline solution;
• labels A and B;
• 5 test tubes (or containers);
• filter funnel;
• acid solution;
• test tube rack.

Put 1 spatula full of soil A in a test tube, add water and label it A. Do the same for soil B. Cork the top and shake both. Fold the filter paper in quarters and open it out to form a cone to place into the filter funnel. Clamp the filter funnel above a clean test tube and pour the contents of test tube A through the funnel. Add a piece of universal indicator paper to the new test tube sample to test for acidity. Do the same for soil sample B.

Background knowledge: The pH value should be the same for 2 soil samples from the same location that seem to be identical in every other way. Traces of soil are often found at the scene of a crime. The soil from 2 different places is often different in colours, particle size, animal and plant matter, damp, dry, clay, sandy or pH. Soil is made up of dirt, silt, small bits of rock, dead or rotting plant or animal matter. Often the rocks are made of different chemicals that affect the colour of the soil and the pH. The pH scale is ...

pH

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Colour

red

red

red

pink

orange

yellow

green

blue

blue

blue

violet

violet

violet

violet

Acidity

acid

acid

acid

acid

acid

acid

neutral

base

base

base

base

base

base

base

Some parts of a soil sample may dissolve in water to make the water acidic. Some may dissolve basic. The universal indicator measures this value. If the pH is less than 7 it is an acid; equal to 7 it is neutral; or greater than 7 it is basic.

Heating soil:

Equipment: The equipment includes...  

• 2 evaporative dishes;
• Bunsen burner;
• gauze;
• balance;
• sand bath;
• heatproof mat;
• tongs;
• tripod;
• soil samples A and B.

Record the colour of the soil. Weigh the evaporative dish; add 10 grams of soil sample A and record this in a table. Put the dish on the sand bath over a low heat for 20 minutes (see left). Do not let the soil burn. Using tongs, remove the dish and place it on a heatproof mat. Leave it there to cool and record the colour of the soil in your table.

Weigh the dish now and record the mass (weight) in your table. Repeat the experiment for sample B.

What did you see coming from the soil while it was being heated?

Make observations of two types of soil according to the work sheet below:

• Soil sampling sheet.

Background knowledge: The difference between the two measurements of mass for the same sample is the amount of water the sample holds. The percentage of water in the sample can be calculated by (difference in weight x 100 / weight of soil before heating).

Burning soil:

Equipment: The equipment includes...  

• 1 evaporative dish;
• Bunsen burner;
• gauze;
• balance;
• heatproof mat;
• tongs;
• tripod;
• soil samples A and B.

Record the colour of the soil. Weigh the evaporative dish; add 10 grams of soil sample A and record this in a table. Put the dish on the gauze mat over a Bunsen burner until the soil burns (see left). Using tongs, remove the dish and place it on a heatproof mat. Leave it there to cool and record the colour of the soil in your table.

Weigh the dish now and record the mass (weight) in your table. Repeat the experiment for sample B.

Make observations of two types of soil according to the work sheet below:

• Soil sampling sheet.

Background knowledge: The difference between the two measurements of mass for the same sample is the amount of water the sample holds. The percentage of water in the sample can be calculated by (difference in weight x 100 / weight of soil before heating). If the particles of rock in the soil are large then rainwater runs straight through the soil. Even after the heavy rain, the soil will be dry. We call this type of soil, sandy soil. If the particles of rock are small, then the water cannot get through so easily. The soil is wet and sticky except in very dry weather. We call this type of soil, clay soil. Usually sandy soil feels dry and clay soil feels damp.

A black soil has rotted animal and plant matter in it. Almost all soil has some of this organic matter in it and when completely rotten it is called humus. If we heat it strongly, this organic matter is burned away and we can then see if the particles are silica based or iron based. If the two samples result in the same colour then they more than likely come from the same place.