Science textbook solutions for Class 7 science Chapter 15 Materials we Use


Important points to remember :
  1. In the period prior to 500 BC, in countries such as  China, Greece, Rome, toothpaste  was made  by mixing  the powder of bones and shells. 
  2. The first commercial toothpaste was made by the Colgate company in New York city  in 1873. 
  3. A  detergent is ‘a substance that cleans or wipes away  dirt.’ Soap nut (ritha), soap pod (shikekai), soap, washing soda, washing powder, liquid soap, shampoo are all detergents. 
  4. The property of a substance of spreading on a surface is called  surface activity  and the substance is said to be a  surfactant. Detergents are surface active.  
  5. One effect  of surface activity is lather formation.


Question 1: Fill appropriate terms in the blanks .
( white cement , soap, detergent , wearing of bones, tooth decay, hard, soft, Portland, fatty acid )

a) The substance that helps water to remove dirt from the surface of materials is called detergent.

b) Fluoride is used in toothpaste to prevent tooth decay.

c) Soap is a salt of fatty acid and sodium hydroxide.

d) Synthetic detergents can be used in hard water as well.

e) For construction purposes Portland cement is the most commonly used cement.


Question 2: Write answer to the following questions.

a) How does the use of a detergent help to clean soiled clothes ?
Answer : Action of Detergents - When detergent is dissolved in water it forms a colloidal solution. The grease or only particles of dirty clothes get attached to the hydrocarbon end of the detergents. Thus the dirt particles get stuck. On rinsing with water, these particles come outside and the cloth becomes clean.

b) How will you check with the help of soap powder whether water is hard ?
Answer: soap can be used to check whether a sample of water is hard or soft. Soaps do not give lather with hard water Instead form scum . Where as they give leather with soft water . if we do not get leather in water while using soap powder, we can say it is hard water.

c) What are the important ingredients of a toothpaste , and what is the function of each ?
Answer : The ingredients vary slightly from toothpaste to toothpaste but the usual main ingredients: silica or baking soda serve as the dentifrice, fluoride helps to mineralize the enamel, sodium laurel sulfate is the foaming agent. It may also include other minerals like calcium or phosphate that are also remineralizing.

d) What are the ingredients of cements ?
Answer : Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.

e) What will happen if cement is not used in making concrete ?
Answer : Cement is used as a bonding agent with aggregate and sand . Without it neither would stick together and collapse in a heap. Lime was and still is used as a bonding agent but is softer and can be dissolved through water breaking it down .

f) Make a list of detergents that you use.
Answer : Detergents are the most commonly used cleaning agent for fabric – that which we put into the washing machine in powder or liquid form. Soaps. Bleach, Laundry Boosters, Stain removers, Optical Brighteners, Fabric Softener, Shampoo etc .

g) What should be expected from a detergent for delicate garments ?
Answer : Delicates laundry detergent is a gentle washing liquid, especially formulated to take care of your delicate garments in the wash. It can be used for machine washing in the gentle cycle, or for hand washing delicate fabrics such as silk and wool.

h) What is meant by ' surface activity'? Name three chemicals responsible for the surface activity of various detergents.
Answer : Chemicals responsible for the surface activity of various detergents are alkylbenzene sulphonate, alcohol ethoxysulphates and alkyl sulphates.


Question 3 : What  are the  similarities and differences between 
a)  Natural  detergents  and man-made detergents 
Answer : One difference between the two is that natural detergents do not harm the environment. Man-made detergents are the chemically synthesized compounds which are also used as surfactants and are much more efficient than the natural detergents. However, these are harmful to the environment.

b) Soap and synthetic  detergent 
Answer : The main difference between synthetic detergents and soap is the ingredients used during manufacturing. While synthetic detergents are made from petroleum products, manufacturers make soap from lye and natural fat. Unlike natural soap, a synthetic detergent breaks up oil and dirt molecules in hard water.

c) Bath soap and soap for washing clothes 
Answer : Bathing soap are potassium salts of long chain fatty acids. They are usually soft and are also free from unused alkali. Washing soaps, on the other hand, are sodium salts of long chain fatty acids.

d) Modern cement  and ancient cement 
Answer : Roman concrete more durable than what we can make today, but it actually gets stronger over time. Modern concrete is typically made with portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures.


Question 4 : Explain why 
a) Soap cannot be used in hard water.
Answer : Soaps cannot be used in hard water because they forms precipitates. When soaps are used in hard water that contains a lot of calcium and magnesium ions they cause the soap to precipitate and it is unable to form lather.

b) Oil does not mix  in water. However, oil and water become  homogeneous if a sufficient  quantity of detergent is added.
Answer :  because density of oil is less than water.. hence it float on water. Now when detergent is added than it combinates the density difference and makes it homogeneous.

c) Synthetic detergents are superior to soap. 
Answer : Synthetic detergents are better than soaps because if water contains calcium hydrogen carbonate (that is hard water), soaps will react with it and form insoluble salts, which cannot be used to clean clothes.

d)  Often coloured  spots are formed  on clothes  during washing. 
Answer : Yes, sometimes colored spots are formed on clothes during washing. It is due to color breed ( washing bright or dark color clothes with light color clothes ) , it forms color stains. If dark and light color clothes washed separately , chances of color stains will be reduced.

e) Tobacco masheri  should not be used for cleaning  teeth.
Answer : It should not be used for cleaning teeths becuase it can harm th soft and hard tissues of our mouth. Use of tobacco poses serious risks like oral cancer, gum disease, poor healing after surgery, receding gums, and tooth decay.


#Can you tell
1. Which  substances were used earlier,  for cleaning  teeth ? 
Answer : Before toothbrushes, people used rough cloth and water to clean their teeth. They would also rub things like salt and chalk  their teeth to try to get rid of the grime.

2. What do we use today to clean  our teeth ?
Answer : Lightly brush your tongue or use a tongue scraper to help keep your mouth clean. Clean between your teeth with dental floss, prethreaded flossers, a water flosser, or a similar product.

3. What do we use for cleaning our body ?
Answer : water, oil, detergent,  soap, etc.

4. What are the materials  used for construction ? 
Answer : Building material is any material used for construction purpose such as materials for house building. Wood, cement, aggregates, metals, bricks, concrete, clay are the most common type of building material used in construction. The choice of these are based on their cost effectiveness for building projects.

5. Nowadays,  why are the roads made of concrete ?  
Answer : Concrete roads are highly durable and last longer on compare to other because of many reasons.
i) In highly extreme conditions, the shape of the road remains same and the durability of concrete last long.
ii) Maintenance of the road not needed upto long time.

6. What causes the hardness of water ?
Answer : The presence of dissolved magnesium and calcium salts causes hardness in water. Permanent hardness is caused due to dissolved calcium or magnesium sulphates which cannot be removed by boiling. Temporary hardness is caused due to dissolved magnesium or calcium hydrogen carbonates which can be removed by boiling.


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