Neutral litmus paper, acid-base

Indicators are chemical dyes that change color with a change of pH. Litmus paper and phenolphthalein are two common indicators used in acid-base reactions. They are chosen because they change color at or very near solution neutrality. Litmus paper is red in acidic solutions and blue in basic solutions. Phenolphthalein is colorless in acidic solutions and turns red in basic solutions. 

It was recognized centuries ago that many substances could be divided into the two general categories. Acids have a sour taste (as in lemon juice), dissolve many metals, and turn litmus paper red. Bases have a bitter taste (as in soaps), feel slippery, and turn litmus paper blue. The chemical reaction between an acid and a base is called neutralization. The products of neutralization reactions are neither acids nor bases. Litmus paper is an example of an acid-base indicator, a substance that changes color when added to an acid to a base. 

AgN03(c9) + HiO(/). 4.113 (a) Check with litmus paper, test reactivity with carbonate, or mix with NaOH(c ) and demonstrate neutralization (use an acid-base indicator), (b) Titrate a known quantity of acid with standard NaOH( ) solution, (c) Visually compare the conductivity of an acid solution with that of a sodium chloride solution of the same molarity. 4.115 (a) The complete reaction is Pb (c9) + 2NO3 (09) -1- Na2S04( )... 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

Acids turn blue litmus to red but have no effect on red. Bases turn red litmus to blue, but have no effect on blue. You can see that it is often necessary to use both to get a conclusive result. The substances that have no effect on either type of litmus paper are neutral. That means they have neither H atoms nor OH... 

You probably knew before you even did the previous section that litmus paper helps determine whether a substance is an acid or abase. If blue litmus paper turns red when dipped in a liquid, the liquid is an acid. If red litmus paper turns blue in a liquid, the liquid is a base. If neither the red nor the blue litmus paper changes color, the liquid is neutral. pH paper takes this a step further and shows how acidic or basic a substance is. 

Red and blue litmus paper is used for testing alkaline and acid solutions. For instance, if we had a solu-tion and wished to know whether it was a base or acid, we would take a piece of red or blue litmus paper and dip it in it. If the red paper turned blue it would indicate a base. If it remained red we would not know whether or not it was neutral or alkaline. Therefore, we would try it with blue litmus paper. If the blue litmus paper would turn red we would then under-... 

A. Acids are likely to dissolve metal and turn litmus paper red. Acids will be neutralized by a base, such as KOH. Bases typically have a slippery feel, will turn litmus paper blue, and are neutralized by acids such as HCI. 

Alkali Any base or hydroxide, as soda, potash, etc., that is soluble in water and can neutralize acid. Turns red litmus paper blue. 

We now come to a consideration of acids and bases. A substance is either acid, base or neutral. Some substances arc strongly base, others only slightly so. Acids have many properties in common, by means of which they can be identified. They all taste sour, redden litmus paper and contain hydrogen as... 

Directions a) Determine by testing with litmus paper whether solutions of the following salts show a neutral, acidic, or basic reaction sodium chloride, potassium nitrate, sodium sulphate, calcium chloiide, sodium sulphide, copper sulphate, ferric chloride, sodium carbonate, and sodium acetate. (1) Tabulate the results and state in the case of each salt the strength (weak or strong) of the acid and the base from which the salt is derived. 

All compounds are either acidic, basic or neutral. The nature of a substance in water is easily determined. Pure water is neutral and has no effect on litmus paper, be it red or blue. A solution of an acid, however, turns blue litmus red, while a basic solution turns red litmus blue. We recognize acids by their sharp taste, and by their ability to dissolve metals such as magnesium, at the same time giving off hydrogen. Bases generally do not do this one characteristic is the soapy feel to their solution. 

Acids can turn blue litmus to red. Litmus does not change color at the exact neutral point between acid and base, but the color change is very close to that point. The compound litmus is often added to paper to make litmus paper. 

Through this reaction, bases can neutralize acids. Finally, bases turn litmus paper blue (Figure 13-2). 

There are substances other than Arrhenius bases that turn litmus paper blue, feel slippery in water solution, or neutralize acids. In 1923, Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, presented a broader definition of acids and bases. The Bronsted-Lowry theory states that an acid is a proton donor and a base is a proton acceptor. This definition of an acid is similar to that in the Arrhenius theory (a hydrogen ion is a proton), and thus all Arrhenius acids are also Bronsted-Lowry acids, and vice versa. However, the Bronsted-Lowry definition of a base is broader. There are substances other than Arrhenius bases that accept hydrogen ions. In other words, the hydroxide ion is a hydrogen ion acceptor, but there are other anions that also do this. 

Litmus is a substance that is extracted from a type of lichen and absorbed into porous paper. There cire three different types of litmus — red, blue, and neutral. Red litmus is used to test for bases, blue litmus is used to test for acids, and neutral litmus can be used to test for both. Here s how the paper reacts to acids and bases ... 

Acids have the following general properties a sour taste, the ability to dissolve many metals, the ability to turn blue litmus paper red, and the ability to neutralize bases. Table 15.1 lists some common acids. 

Bases have the following general properties a bitter taste, a slippery feel, the abihty to turn red litmus paper blue, and the ability to neutralize acids. Because of their bitterness, bases are less common in foods than are acids. Our aversion to the taste of bases is probably an evolutionary adaptation to warn us against alkaloids, organic bases found in plants that are often poisonous. (For example, the active component of hemlock— the poisonous plant that killed the Greek philosopher Socrates—is the alkaloid coniine.) Nonetheless, some foods, such as coffee and chocolate (especially dark chocolate), contain bitter flavors. Many people enjoy the bitterness, but only after acquiring the taste over time. 

Acids generally taste sour, dissolve metals, turn blue litmus paper red, and neutralize bases. Common acids are hydrochloric, sulfuric, nitric, and carboxylic acids. 

Bases generally taste bitter, feel slippery, turn red litmus paper blue, and neutralize acids. Common bases are sodium hydroxide, sodium bicarbonate, and potassium hydroxide. 

Organic acids or bases that have been extracted can be regenerated by neutralizing the extraction reagent. This would be done if the organic acid or base were a product of a reaction rather than an impurity. For example, if a carboxylic acid has been extracted with the aqueous base, the compound can be regenerated by acidifying the extract with 6 M HCl until the solution becomes just acidic, as indicated by litmus or pH paper. When the solution becomes acidic, the carboxylic add will separate from the aqueous solution. If the acid is a solid at room temperature, it will precipitate and can be purified by filtration and crystallization. If the acid is a liquid, it will form a separate layer. In this case, it would usually be necessary to... 

---