What if we told you the milk you are drinking is not pure? Maybe you do not want to drink it again. While for us a substance is considered pure if it is not adulterated, it is not the same for a scientist. For them, a pure substance is something that consists of a single type of particle. The chapter titled “Is matter around us pure” in the class 9 science syllabus provides students with an in-depth understanding of what constitutes a pure substance. Struggling with the chapter? Here are some detailed notes and questions to help you ace the chapter on Is matter around us pure?
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What is a Mixture?
Let’s first discuss why milk is not pure for scientists. To put it simply, a pure substance means that all the constituent particles of that substance are the same according to their chemical nature. But we know that milk consists of water, fat, proteins etc., hence it is not pure. As per the chapter on ‘Is matter around us pure’, a substance can only be called as pure when all the constituent particles present in it are chemically the same. The best example of a pure substance is sugar. Based on their chemical composition, substances can be classified either as elements or compounds. According to the chapter, mixtures are formed by more than one kind of matter i.e. a pure form of a substance. Soil and milk can be called as mixtures.
Types of Mixtures
Two types of mixtures have been discussed in the chapter of ‘Is matter around us pure’. These types of mixtures are:
- Homogeneous Mixture: These mixtures have a homogeneous or uniform composition. The best example of a homogenous mixture is a solution of salt and water or sugar and water.
- Heterogeneous Mixture: Such mixtures have a variable composition with physically distinguishable parts. Some examples of heterogeneous mixtures include iron filings and sodium chloride (NaCl) or water and oil.
What is a Solution?
As mentioned in the Class 9 chapter on ‘Is matter around us pure?’, a homogeneous mixture having two or more substances is known as a solution. Some examples are soda water, lemonade etc, in which the particles of two or more substances are evenly distributed, thus forming a solution.
The chapter on ‘Is matter around us pure?’ also discusses the two constituents that form a solution i.e. solute and solvent. In the simplest term, a solution can be explained as – solute + solvent.
- Solute: The component of any solution which easily dissolves with the other constituent such as water (usually the component present in larger amount) in it is called the solvent.
- Solvent: The solution’s component like sugar that is dissolved in the solvent (usually present in lesser quantity) is called the solute.
Now from the above points, we can conclude four properties of a solution. These are:
- It is a homogeneous mixture
- Its particles can not be seen through the naked eye
- In a solution, the path of light is not visible.
- The solution is stable. The separation of solute particles from a mixture cannot be performed using filtration. The undisturbed solute particles do not settle down; which shows the stability of a solution.
What is Suspension?
If we are discussing a solution, then it is important to discuss suspension too. Opposite to solution, suspension simply means a heterogeneous mixture, and its particles can be easily seen by the naked eye. As we discussed four properties of a solution above, it is important to note that all the four properties of suspension are opposite to the properties of the solution. The chapter ‘Is matter around us pure’ is full of experiments and it is important to go through all of them. Also performing all the activities will help a lot to improve your approach to the solutions for NCERT science class 9 exercises.
The Concentration of a Solution
We know that a solution is made with the combination of a solute and a solvent. We can describe the concentration of the solution simply by referring to the quantity of solute present in a given amount (mass or volume) of the solution or the amount of solute dissolved in a given mass or volume of solvent. Thus, the concentration of solution = Amount of solute/ Amount of solution (or Amount of solute/Amount of solvent).
The solution can be divided into three types as per the amount of solute present in it. These are:
- Diluted: Having lower amounts of solute.
- Concentrated: Having higher amounts of solute.
- Saturated: This solution can be made when there is no more solute to dissolve in a solution. The temperature should be constant.
Although there are a number of different ways of finding out the concentration of a solution, the chapter ‘Is matter around us pure’ discusses the three ways mentioned below:
- Mass by mass percentage of a solution
- Mass by volume percentage of a solution
- Volume by volume percentage of a solution
Do you think milk is heterogeneous or homogeneous? Many of you may say that milk is homogeneous because of its uniformity. But actually, it is heterogeneous, which makes it a good example to understand colloids. So the particles of a colloid are consistently present throughout the solution. The mixture appears to be homogeneous due to the relatively small particle size, as compared to the size of a suspension. But the colloidal solution is not homogeneous but a heterogeneous mixture. Hence these colloids can easily scatter the beam of light(Tyndall effect).
How do you Separate the Components of a Mixture?
The chapter ‘Is matter around us pure’ mentions a number of different activities which you have to engage in in order to understand how to separate the components of different mixtures. The discussed activities in NCERT Class 9 are as follows:
|How can we obtain coloured components(dye) from blue/black ink?||This activity will help you to learn about evaporation and how it’s helpful to separate the volatile component (solvent) from its non-volatile component (solute).|
|How to segregate cream from milk?||This activity will help you to learn about centrifugation. Principle: When rotated fast the lighter particles stay at the top, and the denser one’s are pushed to the bottom|
|How can we segregate a mixture of two immiscible liquids?||This activity will help you to learn about separating kerosene oil from water using a separating funnel. The principle used in this activity: The separation of immiscible liquids is in layers that depends on the density of the layers.|
|How can we separate a mixture of salt and ammonium chloride?||This activity will help you to learn about the separation of ammonium chloride and salt by sublimation. Sublimation is used to separate the sublimable volatile liquid from a non-sublimable impurity.|
|How can we separate a mixture of two miscible liquids?||For this, we use distillation, i.e. used for the separation of components of a mixture containing two miscible liquids(having enough difference in their boiling points) that boil without decomposition.|
|How to get several gases from the air?||The air is compressed by increasing the pressure and is then cooled, decreasing the temperature to make liquid air. Then in a fractional distillation column, the liquid air warmed up slowly, where depending upon the boiling points the gases get separated at different heights.|
|How can we obtain pure copper sulphate from an impure sample?||With the help of the crystallization method that is used to get pure solid(like Salt) in the form of its crystals from a solution(like the sea).|
As we have covered all the important points of the chapter on ‘Is matter around us pure?’, now is the time to solve important questions and to go through the solutions of ncert chapter 2 of class 9. Here are some of the important questions that you can practice for this chapter:
- Is matter around us pure?
- Are alloys a solution or not? Give reasons for the same.
- Why is the light path not observable in the solution?
- Why is the same temperature maintained to define saturation? Given the difference between physical and chemical change.
- Why Crystallisation is better than evaporation?
- Explain the Tyndall effect.
- Compare four important properties of colloidal solutions, suspension, and solutions.
- Explain the separation process of black ink.
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