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Adjective Test Review - 4th grade
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Adjectives test review What is a rubric? A tool comprising a set of criteria (with possible levels of performance quality on the criteria) developed to assess learnersâ work, from written to oral to visual. It is used tomeasureperformance,suchastheprocess of doing something (e.g.,playing a musical instrument, making a speech) or products of the learnersâ work (e.g., concept map, laboratory report, bookshelf) (Brookhart, 2013). BENEFITS OF USING RUBRICS Support authentic assessment Reflects how well learners are able to apply knowledge inthe real-world context. Communicate expectations Gives learners an idea of what is expected of them. It is especially useful when the rubrics are communicated to the learners before they are assessed. Improve performance Explicit criteria and performance level descriptions allow learners to understand the desired performance. Learners are able to assess themselves by referring to the specific criteria and performance-level descriptions. Provide informative feedback Instructors are able to provide constructive feedback to learners on their weaknesses and strengths. Promote thinking andlearning 4 Provide informative feedback Instructors are able to provide constructive feedback to learners on their weaknesses and strengths. Learners are able to review and revise their work,thus reflecting on their learning experiences. Ensure fairness Learner performance assessed fairly given its objectivity. It helps avoid disputes between learners and instructors about the scores/grades achieved. TYPES OF RUBRIC ANALYTIC It consists of individual criterion with corresponding descriptor of performance. HOLISTIC It consists of performance descriptors that are placed together to refeclet to overalll performance. ANATOMY OF ANANALYTIC RUBRIC Rating scales with corresponding scores or weights The row represents the criteria for the desired performance, while the column represents the evaluation score. Under the rating scale (corresponding weights orscorescanbeassigned),theperformance descriptors are explicitly stated ANATOMYOF AHOLISTICRUBRIC Descriptions: It comprises the rating scale (corresponding weights or scores can be assigned) in the row while the combined desired performance descriptors are placed in the column. Description of the task The purpose of the assignment is to assess learnerâs cognitive and analytic skills in applying knowledge gained and constructed throughout the course Diffusion of Innovation,bywatching the Surrogates movieand writing ananalytical review of the movie in the context of innovation diffusion.Iwant to provide learners with informative feedback on their cognitive and analytic skills such as the following: applying the concepts of innovation diffusion,making judgmentson the scenes related to innovation diffusion identified from the movie,selecting and critiquing theories of innovation diffusion and making connections between the theories,aswell asarguingand proposing necessary solutions to the problemss hown in the movie. ESTABLISHING ALTERNATIVEASSESSMENTINHIGHEREDUCATION VALIDITYAND RELIABILITYOF RUBRICS. Validity Measuring what is supossedto be measured. Reability Yielding consists results. Instruments that are used in the alternative assessment must be aligned to the learning outcomes and measure well what it intends to measure (valid) and produce consistent scores (reliable). The valid instrument will manifest the true ability (latent trait) of learners and permit appropriate inferences to be made about a specific group of people for specific purposes. TYPES OF VALIDITY FACE VALIDITY Simple form of validity thatapplies a superficial and subjective assessment whether the instrument measures what it is supposed to measure. CONTENT VALIDITY Refers to the extent to which the items on a measure assess the same content or how wellthe content material was sampled inthe measure. CONSTRUCT VALIDITY Refers to the extent to which the test may be said to measure a theoretical construct or trait. CONCURRENT VALIDITY Refers to the extent to which scores onanewmeasure are related to scores from a criterion measure administered at the same time. PREDICTIVE VALIDITY Refers to the uses of the scores from the new measure to predict performance on a criterion measure administered ata later time. STEPS TO CONSIDER WHEN ESTABLISHING CONTENT VALIDITY Calculate the level of expert agreeement for the content validity, get expert to verfy. Interview the expert ,make meta contentdata anĂĄlisis from literatura. STEPS TO CONSIDER WHEN ESTABLISHING CONSTRUCCT VALIDITY Administer the instrument for alll learners, revise any item necccesay, run an apropriates statistical analiysis, administerthe instrument to learners as a pilot test . CONSTRUCTMAP Morepreciseconceptthan construct. Ranges from one extreme to another(fromhightolow,small tolarge,positivetonegative,or strongtoweak). Identifiesthepositionofthe respondentsinthisrange. Representativenessofsampling (questions and ability of respondents). EXAMPLEO FACONSTRUCTMAP:AFFECTIVE LEVELOF AFFECTIVE VARIABLES EXAMPLESOFITEMSIN MEASURINGTEAM WORKING SKILLS 5. Characterisation Learnersvolunteerstodothe groupworks. 4. Organisation Learners are willing to help others,althoughitisnottheir scopeoftask. 3. Valuing Learners respect other team membersâopinionwhendoing thediscussion. 2. Responding Learnergivescooperationwhen neededingroupworks. 1. Receiving Learneracceptsthediversityof races and nationalities among groupmembers. EXAMPLEOFACONSTRUCTMAP:PSYCHOMOTOR LEVELOF PSYCHOMOTOR VARIABLES EXAMPLESOFITEMSIN MEASURING DIGITAL SKILLS 7.Origination Learnerscanmodifytheirowndevicesto performbetter. 6.Adaptation Learnerscansolveandtroubleshootthe problemwhileusingthecomputer. 5.ComplexOvertResponse Learnerscanusethecomputercompetently. 4.Mechanism Learners can use the computer independently,butstillmakeminorerrors. 3.GuidedResponses Learnerscanusethecomputer,butstill needguidance. 2.Set Learnersarereadytousethecomputer. 1.Perception Learnerscanobservehowtousecomputer. EXAMPLEOFACONSTRUCTMAP:COGNITIVE LEVELOF COGNITIV E VARIABLES EXAMPLESOFITEMS IN MEASURING THINKINGSKILLS 6. Creating Learners are able to suggest anewmodelorframeworkof learningdigitalcommunity. 5. Evaluating Learners are able to judge the impactofthescenariotowards educationperspective. 4. Analysing Learnerscandifferentiate the factsusingafew theories. 3. Applying Learnerscansolveproblems usingthefactsgiven. 2. Understanding Learnersareabletoexplainthe factsusingtheirownwords. 1. Remembering Learnersonlymemorisethe. Direction of Increasing âXâ Learners Learners with high âXâ Learners with mid range âXâ Learners with low âXâ Responses to Item Item response indicate highest level of X Item response indicate higher level of X Item response indicate lower level of X The construct map shows the lower ability students are in line with the lower level of items. This shows that when educators plan to develop an instrument, it Item response indicate lowest level of X Direction of Decreasing âXâ is crucial to create an item difficulty thatrepresents learnersâ ability. Learnersâ ability Learners who engage in level characterisation Learners who engage in level organisation Learners who engage in level valuing Learners who engage in level responding Learners who engage in level receiving Direction of DecreasingâXâ MEASURINGCONSTRUCTVALIDITY Unlike content validity, this construct validity can be analysed using statistical analysis. Use Exploratory FactorAnalysis [EFA], Confirmatory FactorAnalysis [CFA] or Unidimensionality to confirm all items are measuring the right construct and the raw variance explained for the latent variables is sufficient. Gap initem map also can show accuracy in construct validity. RELIABILITY The degree to which test scores are consistent over repeated administrations of the same/ equivalent test and therefore considered dependable and repeatable for an individual learner.A test thatproduces highly consistent and stable results (i.e. relative free from random error) is said to be highly reliable. TYPESOFRELIABILITY Test-retest demonstrates the stability of a measure over time 01 Internal consistency most of the items within a rating scale of a concept show consistency of scoring. Inter-rater the extent to which two or more independent raters are consistent in observing, recording and scoring data (should be 70% or higher agreement) 04 Intra-rater relies on one rater to rate an object or event twice (70% or higher of agreement) FACTORSAFFECTING VALIDITYANDHOWTO INCREASEVALIDITY? FACTORS AFFECTING VALIDITY HOWTO INCREASE VALIDITY? 1. Inaccuracy of items in measuringtheoutcomes 1. Vetting session to get reviewsfromtheexpert. 2. Pooritemsdevelopment 2. Followtheformatandtips indevelopinggooditems. 3. Unclearinstructions 3. Do pilot testing to measuretheusabilityof thetest. 4. Interveningevents 4. Controltheinternalthreats validityfactors. 5. Itemsdifficultyisnot suitableforthelearners 5. Create a construct map toensurethereisanitem thatrepresentslearners ability. FACTORS AFFECTING RELIABILIT Y HOWTOINCREASERELIABILITY? 1. TestLength 1. Thetestlengthshouldbeappropriate withtestdifficulty. 2. Test retest interval 2. Suggesteddurationisbetween3 weeksto2months. 3. Variability of scores 3. Doconstructmaptoensuretheitems aresuitablewithlearnersâability. 4. Guessing 4. Penalisetheguessinganswers.You alsocandetecteitherthelearnersare guessing or not using the statistical analysis named guessing analysis andpersonfitanalysis. 5. Inconsistency score from different raters 5. Appointtheratertomarkcertain questionsforalllearners(Thisalways happen when you have more than onesectionandhavemorethanone lecturer). CONCLUSION Coming back to the issue of validity and reliability in assessment, there is a need for educators to put an effort to ensurethattheitemsintheformofquestionsorinstructions arenotonlyclearbutalsoabletomeasurewhatitisintended tomeasurebasedontherelatedlearningoutcomes. Establishingvalidityandreliabilityofinstrumentscan provide educators with some indications of the quality of the measuring tools being used. Valid and reliable instruments enabletheeducatorstocontinuouslyusethemeasuringtools withoutreservation. Reliablenot valid Precisenot Accurate Reliableand valid Preciseand Accurate NotReliable butvalid NotPrecisebut Accurate NotReliable butNotvalid NotPrecisebut NotAccurate 94 We found you were making a quiz on the subject of "What is a rubric? A tool comprising a set of criteria (with possible levels of performance quality on the criteria) developed to assess learnersâ work, from written to oral to visual. It is used tomeasureperformance,suchastheprocess of doing something (e.g.,playing a musical instrument, making a speech) or products of the learnersâ work (e.g., concept map, laboratory report, bookshelf) (Brookhart, 2013). BENEFITS OF USING RUBRICS Support authentic assessment Reflects how well learners are able to apply knowledge inthe real-world context. Communicate expectations Gives learners an idea of what is expected of them. It is especially useful when the rubrics are communicated to the learners before they are assessed. Improve performance Explicit criteria and performance level descriptions allow learners to understand the desired performance. Learners are able to assess themselves by referring to the specific criteria and performance-level descriptions. Provide informative feedback Instructors are able to provide constructive feedback to learners on their weaknesses and strengths. Promote thinking andlearning 4 Provide informative feedback Instructors are able to provide constructive feedback to learners on their weaknesses and strengths. Learners are able to review and revise their work,thus reflecting on their learning experiences. Ensure fairness Learner performance assessed fairly given its objectivity. It helps avoid disputes between learners and instructors about the scores/grades achieved. TYPES OF RUBRIC ANALYTIC It consists of individual criterion with corresponding descriptor of performance. HOLISTIC It consists of performance descriptors that are placed together to refeclet to overalll performance. ANATOMY OF ANANALYTIC RUBRIC Rating scales with corresponding scores or weights The row represents the criteria for the desired performance, while the column represents the evaluation score. Under the rating scale (corresponding weights orscorescanbeassigned),theperformance descriptors are explicitly stated ANATOMYOF AHOLISTICRUBRIC Descriptions: It comprises the rating scale (corresponding weights or scores can be assigned) in the row while the combined desired performance descriptors are placed in the column. Description of the task The purpose of the assignment is to assess learnerâs cognitive and analytic skills in applying knowledge gained and constructed throughout the course Diffusion of Innovation,bywatching the Surrogates movieand writing ananalytical review of the movie in the context of innovation diffusion.Iwant to provide learners with informative feedback on their cognitive and analytic skills such as the following: applying the concepts of innovation diffusion,making judgmentson the scenes related to innovation diffusion identified from the movie,selecting and critiquing theories of innovation diffusion and making connections between the theories,aswell asarguingand proposing necessary solutions to the problemss hown in the movie. ESTABLISHING ALTERNATIVEASSESSMENTINHIGHEREDUCATION VALIDITYAND RELIABILITYOF RUBRICS. Validity Measuring what is supossedto be measured. Reability Yielding consists results. Instruments that are used in the alternative assessment must be aligned to the learning outcomes and measure well what it intends to measure (valid) and produce consistent scores (reliable). The valid instrument will manifest the true ability (latent trait) of learners and permit appropriate inferences to be made about a specific group of people for specific purposes. TYPES OF VALIDITY FACE VALIDITY Simple form of validity thatapplies a superficial and subjective assessment whether the instrument measures what it is supposed to measure. CONTENT VALIDITY Refers to the extent to which the items on a measure assess the same content or how wellthe content material was sampled inthe measure. CONSTRUCT VALIDITY Refers to the extent to which the test may be said to measure a theoretical construct or trait. CONCURRENT VALIDITY Refers to the extent to which scores onanewmeasure are related to scores from a criterion measure administered at the same time. PREDICTIVE VALIDITY Refers to the uses of the scores from the new measure to predict performance on a criterion measure administered ata later time. STEPS TO CONSIDER WHEN ESTABLISHING CONTENT VALIDITY Calculate the level of expert agreeement for the content validity, get expert to verfy. Interview the expert ,make meta contentdata anĂĄlisis from literatura. STEPS TO CONSIDER WHEN ESTABLISHING CONSTRUCCT VALIDITY Administer the instrument for alll learners, revise any item necccesay, run an apropriates statistical analiysis, administerthe instrument to learners as a pilot test . CONSTRUCTMAP Morepreciseconceptthan construct. Ranges from one extreme to another(fromhightolow,small tolarge,positivetonegative,or strongtoweak). Identifiesthepositionofthe respondentsinthisrange. Representativenessofsampling (questions and ability of respondents). EXAMPLEO FACONSTRUCTMAP:AFFECTIVE LEVELOF AFFECTIVE VARIABLES EXAMPLESOFITEMSIN MEASURINGTEAM WORKING SKILLS 5. Characterisation Learnersvolunteerstodothe groupworks. 4. Organisation Learners are willing to help others,althoughitisnottheir scopeoftask. 3. Valuing Learners respect other team membersâopinionwhendoing thediscussion. 2. Responding Learnergivescooperationwhen neededingroupworks. 1. Receiving Learneracceptsthediversityof races and nationalities among groupmembers. EXAMPLEOFACONSTRUCTMAP:PSYCHOMOTOR LEVELOF PSYCHOMOTOR VARIABLES EXAMPLESOFITEMSIN MEASURING DIGITAL SKILLS 7.Origination Learnerscanmodifytheirowndevicesto performbetter. 6.Adaptation Learnerscansolveandtroubleshootthe problemwhileusingthecomputer. 5.ComplexOvertResponse Learnerscanusethecomputercompetently. 4.Mechanism Learners can use the computer independently,butstillmakeminorerrors. 3.GuidedResponses Learnerscanusethecomputer,butstill needguidance. 2.Set Learnersarereadytousethecomputer. 1.Perception Learnerscanobservehowtousecomputer. EXAMPLEOFACONSTRUCTMAP:COGNITIVE LEVELOF COGNITIV E VARIABLES EXAMPLESOFITEMS IN MEASURING THINKINGSKILLS 6. Creating Learners are able to suggest anewmodelorframeworkof learningdigitalcommunity. 5. Evaluating Learners are able to judge the impactofthescenariotowards educationperspective. 4. Analysing Learnerscandifferentiate the factsusingafew theories. 3. Applying Learnerscansolveproblems usingthefactsgiven. 2. Understanding Learnersareabletoexplainthe factsusingtheirownwords. 1. Remembering Learnersonlymemorisethe. Direction of Increasing âXâ Learners Learners with high âXâ Learners with mid range âXâ Learners with low âXâ Responses to Item Item response indicate highest level of X Item response indicate higher level of X Item response indicate lower level of X The construct map shows the lower ability students are in line with the lower level of items. This shows that when educators plan to develop an instrument, it Item response indicate lowest level of X Direction of Decreasing âXâ is crucial to create an item difficulty thatrepresents learnersâ ability. Learnersâ ability Learners who engage in level characterisation Learners who engage in level organisation Learners who engage in level valuing Learners who engage in level responding Learners who engage in level receiving Direction of DecreasingâXâ MEASURINGCONSTRUCTVALIDITY Unlike content validity, this construct validity can be analysed using statistical analysis. Use Exploratory FactorAnalysis [EFA], Confirmatory FactorAnalysis [CFA] or Unidimensionality to confirm all items are measuring the right construct and the raw variance explained for the latent variables is sufficient. Gap initem map also can show accuracy in construct validity. RELIABILITY The degree to which test scores are consistent over repeated administrations of the same/ equivalent test and therefore considered dependable and repeatable for an individual learner.A test thatproduces highly consistent and stable results (i.e. relative free from random error) is said to be highly reliable. TYPESOFRELIABILITY Test-retest demonstrates the stability of a measure over time 01 Internal consistency most of the items within a rating scale of a concept show consistency of scoring. Inter-rater the extent to which two or more independent raters are consistent in observing, recording and scoring data (should be 70% or higher agreement) 04 Intra-rater relies on one rater to rate an object or event twice (70% or higher of agreement) FACTORSAFFECTING VALIDITYANDHOWTO INCREASEVALIDITY? FACTORS AFFECTING VALIDITY HOWTO INCREASE VALIDITY? 1. Inaccuracy of items in measuringtheoutcomes 1. Vetting session to get reviewsfromtheexpert. 2. Pooritemsdevelopment 2. Followtheformatandtips indevelopinggooditems. 3. Unclearinstructions 3. Do pilot testing to measuretheusabilityof thetest. 4. Interveningevents 4. Controltheinternalthreats validityfactors. 5. Itemsdifficultyisnot suitableforthelearners 5. Create a construct map toensurethereisanitem thatrepresentslearners ability. FACTORS AFFECTING RELIABILIT Y HOWTOINCREASERELIABILITY? 1. TestLength 1. Thetestlengthshouldbeappropriate withtestdifficulty. 2. Test retest interval 2. Suggesteddurationisbetween3 weeksto2months. 3. Variability of scores 3. Doconstructmaptoensuretheitems aresuitablewithlearnersâability. 4. Guessing 4. Penalisetheguessinganswers.You alsocandetecteitherthelearnersare guessing or not using the statistical analysis named guessing analysis andpersonfitanalysis. 5. Inconsistency score from different raters 5. Appointtheratertomarkcertain questionsforalllearners(Thisalways happen when you have more than onesectionandhavemorethanone lecturer). CONCLUSION Coming back to the issue of validity and reliability in assessment, there is a need for educators to put an effort to ensurethattheitemsintheformofquestionsorinstructions arenotonlyclearbutalsoabletomeasurewhatitisintended tomeasurebasedontherelatedlearningoutcomes. Establishingvalidityandreliabilityofinstrumentscan provide educators with some indications of the quality of the measuring tools being used. Valid and reliable instruments enabletheeducatorstocontinuouslyusethemeasuringtools withoutreservation. Reliablenot valid Precisenot Accurate Reliableand valid Preciseand Accurate NotReliable butvalid NotPrecisebut Accurate NotReliable butNotvalid NotPrecisebut NotAccurate 94 ". Would you like to continue making it or start afresh?Adjective TestAdjective Test!Comparative and Superlative Adjective TestMatch the word to its synonym level B1 CEFR. Use the vocabulary exactly adverb precisely except that aside from exist verb to be real existing adjective real, current Example: Flying cars are not practical with existing technology. existence noun reality Example: The existence of black holes has been confirmed by indirect observation. extraordinary adjective unusual feature noun important part of something Example: The Ramon Crater is a unique feature of the Negev Desert. feedback noun reaction figure noun shape Example: I canât tell if that figure in the shadows is a man or a woman. figure out verb understand Example: I just canât figure out how the magician did that amazing trick. financial adjective related to money Example: Her family is having financial problems so they canât travel overseas this year. finance verb pay for Example: If I canât get a loan from the bank, I wonât be able to finance a new apartment. finance noun money Example: An expert in finance predicts a global recession. finding/findings noun discoveries; results of a study Example: According to the findings of the police investigation, this is the gun which fired the fatal bullet. flexibility noun willingness to change flexible adjective adjusts easily Example: Iâd prefer to meet on Monday morning but I can be flexible depending upon your schedule. flood noun a lot of water flood verb to cover with too much water flu noun type of sickness focus on/upon verb pay attention to Example: You should focus on your schoolwork if you want to improve your grades. focus noun attention People with attention deficit disorder lose focus easily. frequency noun how often frequent adjective very often Example: Hanah is a frequent customer and everyone at the store knows her. fresh adjective new Example: We need some fresh ideas if weâre going to solve this problem. frighten verb scare from preposition position, starting point gain verb make an increase, profit, earn Example: I have nothing to gain by choosing sides so I shall remain neutral. gain noun profit, amount earned generate verb create, make Example: Chat GPT can generate text written in any style you choose. guidance noun help, advice hopeful adjective optimistic, having a positive outlook Example: The farmers are hopeful that we will have rain this winter. hopefully adjective with luck ideal adjective best, most preferable Example: Nuclear power may not be an ideal solution to global warming, but itâs certainly worth considering. illness noun sickness, disease illustrate verb draw pictures illustration noun picture, image Example: Childrenâs storybooks have colorful illustrations. image noun picture, especially on film or television Example: The mother of the pop singer cried when she first saw her daughterâs image on television. in preposition within, inside, into in terms of regarding Example: That company makes a great product but theyâre lacking in terms of customer service. in actual fact in truth Example: The mayor says the city is a safe place to live, but in actual fact the violent crime rate is very high. in connection with about Example: Police arrested four men in connection with the robbery. in that case if that is true Example: Billy Bob: âTraffic could be heavy tomorrow.â Peggy Sue: âIn that case, we better leave early.â in the meantime while, during Example: The new computers wonât arrive until next week, but we can keep using the old ones in the meantime. initial adjective first Example: Her initial reaction to that song was negative, but over time sheâs come to like it. initially adverb at first instruction noun teaching, order Example: Most new electronic devices come with a set of instructions. intelligence noun smartness Example: Since you have a degree from a good university, I assume you have sufficient intelligence to understand this problem. intelligent adjective smart Example: Joe isnât very intelligent, but he is a kind person with a warm heart. interest noun attraction Example: Yossi has little interest in politics, whereas his wife goes to all the protests and demonstrations. interest verb to attract Example: Sports donât really interest me, but my brother is a big basketball fan. introduce verb to show something new Example: Today in class I will introduce the basic concepts of literary analysis. invest verb to put money into something in order to earn money Example: Joe invested in cryptocurrency and lost a lot of money. investor noun one who puts money into something in order to earn money Example: Venture capitalists are investors who put money into risky start-up businesses. investment noun putting money into something in order to earn money Example: Buying real estate in Israel is a very safe investment because the value never goes down. investigate verb research, study Example: The police collected evidence to investigate the murder. investigation noun study Example: The police donât have a suspect for the murder as the investigation isnât finished yet. investigator noun detective Example: Detective Schmendrick is the lead investigator for the murder case. just about almost Example: Iâm just about done here so Iâll be there shortly. keep on doing verb continue Example: Youâre crazy if you keep on doing the same thing and expect different results. kind of type of Example: What kind of dog is that, a poodle? knowledge noun awareness Example: John failed the test due to lack of knowledge of the material. lack verb not having, missing Example: John failed the test due to lack of knowledge of the material. landscape noun the view of the land likely adjective, adverb probably Example: When we learn from our mistakes, weâre not likely to forget. limited adjective restricted Example: We should go to the store today because the sale is for a limited time only. limitation noun restriction little adjective small, not a lot Example: She always tells the truth. I have little reason to doubt her. look at verb see Example: People used to read newspapers on the train. Nowadays they just look at their phones. low adverb to a small amount or level Example: I have to charge my phone because the battery is running low. material noun documents, information Example: We have a lot of material to cover before the end of the semester. meaning noun significance mean verb to have significance or purpose means noun form of, by the use of Example: They communicate by means of radio. measure noun step Example: The teacher took measures to prevent cheating during the test mention verb to say, point out Example: The coach said the team played very well today but didnât mention any player specifically. miss verb (1) fail to catch (2) wishing to see somebody Examples: (1) The football player kicked the ball but missed the goal. (2) Wow, itâs good to see you! Iâve missed you so much! misunderstand verb understand incorrectly Example: Iâm afraid I misunderstood the instructions. Could you repeat them please? more or less approximately, somewhat, to a varying degree Example: This is more or less a religious neighborhood, though there are a few secular families. must modal verb have to naturally adverb as expected, normally nature noun (1) open air (2) character Examples: (1) We like to go hiking in nature reserves. (2) Pit bulls are aggressive by nature.A solution is a mixture in which one or more substances are uniformly distributed in another substance. Solutions can be mixtures of liquids, solids, or gases. For example, plasma, the liquid part of blood, is a very complex solution. It is composed of many types of ions and large molecules, as well as gases, that are dissolved in water. A solute (SAHL-YOOT) is a substance dissolved in the solvent. The particles that compose a solute may be ions, atoms, or molecules. The solvent is the substance in which the solute is dissolved. For example, when sugar, a solute, and water, a solvent, are mixed, a solution of sugar water results. Though the sugar dissolves in the water, neither the sugar molecules nor the water molecules are altered chemically. If the water is boiled away, the sugar molecules remain and are unchanged. Solutions can be composed of various proportions of a given solute in a given solvent. Thus, solutions can vary in concentra- tion. The concentration of a solution is the amount of solute dis- solved in a fixed amount of the solution. For example, a 2 percent saltwater solution contains 2 g of salt dissolved in enough water to make 100 mL of solution. The more solute dissolved, the greater is the concentration of the solution. A saturated solution is one in which no more solute can dissolve. Aqueous (AY-kwee-uhs) solutionsâsolutions in which water is the solventâare universally important to living things. Marine microorganisms spend their lives immersed in the sea, an aqueous solution. Most nutrients that plants need are in aqueous solutions in moist soil. Body cells exist in an aqueous solution of intercellu- lar fluid and are themselves filled with fluid; in fact, most chemical reactions that occur in the body occur in aqueous solutions. Copyright © by Holt, Rinehart and Winston. All rights reserved. Liquid water Solid water Ice (solid water) is less dense than liquid water because of the structure of ice crystals. The water molecules in ice are bonded to each other in a way that creates large amounts of open space between the molecules, relative to liquid water. FIGURE 2-12 solvent from the Latin solvere, meaning âto loosenâ Word Roots and Origins CHEMISTRY OF LIFE 43 ACIDS AND BASES One of the most important aspects of a living system is the degree of its acidity or alkalinity. What do we mean when we use the terms acid and base? Ionization of Water As water molecules move about, they bump into one another. Some of these collisions are strong enough to result in a chemical change: one water molecule loses a proton (a hydrogen nucleus), and the other gains this proton. This reaction really occurs in two steps. First, one molecule of water pulls apart another water molecule, or dissociates, into two ions of opposite charge: H2O â H OH The OH ion is known as the hydroxide ion. The free H ion can react with another water molecule, as shown in the equation below. H H2O â H3O The H3O ion is known as the hydronium ion. Acidity or alkalin- ity is a measure of the relative amounts of hydronium ions and hydroxide ions dissolved in a solution. If the number of hydronium ions in a solution equals the number of hydroxide ions, the solution is said to be neutral. Pure water contains equal numbers of hydro- nium ions and hydroxide ions and is therefore a neutral solution. Acids If the number of hydronium ions in a solution is greater than the number of hydroxide ions, the solution is an acid. For example, when hydrogen chloride gas, HCl, is dissolved in water, its mol- ecules dissociate to form hydrogen ions, H, and chloride ions, Cl, as is shown in the equation below. HCl â H Cl These free hydrogen ions combine with water molecules to form hydronium ions, H3O. This aqueous solution contains many more hydronium ions than it does hydroxide ions, making it an acidic solution. Acids tend to have a sour taste; how- ever, never taste a substance to test it for acidity. In concentrated forms, they are highly corrosive to some materials, as you can see in Figure 2-13. Bases If sodium hydroxide, NaOH, a solid, is dissolved in water, it dissociates to form sodium ions, Na, and hydroxide ions, OH, as shown in the equation below. NaOH â Na OH Copyright © by Holt, Rinehart and Winston. All rights reserved. Eco Connection onnection Acid Precipitation Acid precipitation, more commonly called acid rain, describes rain, snow, sleet, or fog that contains high levels of sulfuric and nitric acids. These acids form when sulfur dioxide gas, SO2, and nitrogen oxide gas, NO, react with water in the atmosphere to produce sulfuric acid, H2SO4, and nitric acid, HNO3. Acid precipitation makes soil and bodies of water, such as lakes, more acidic than normal. These high acid levels can harm plant and animal life directly. A high level of acid in a lake may kill mollusks, fish, and amphibians. Even in a lake that does not have a very elevated level of acid, acid precipitation may leach aluminum and magnesium from soils, poisoning water- dwelling species. Reducing fossil-fuel consump- tion, such as occurs in gasoline engines and coal-burning power plants, should reduce high acid levels in precipitation. Sulfur dioxide, SO2, which is produced when fossil fuels are burned, reacts with water in the atmosphere to produce acid precipitation. Acid precipitation, or acid rain, can make lakes and rivers too acidic to support life and can even corrode stone, such as the face of this statue. FIGURE 2-13 44 CHAPTER 2 This solution then contains more hydroxide ions than hydronium ions and is therefore defined as a base. The adjective alkaline refers to bases. Bases have a bitter taste; however, never taste a substance to test for alkalinity. They tend to feel slippery because the OH ions react with the oil on our skin to form a soap. In fact, commercial soap is the product of a reaction between a base and a fat. pH Scientists have developed a scale for comparing the relative con- centrations of hydronium ions and hydroxide ions in a solution. This scale is called the pH scale, and it ranges from 0 to 14, as shown in Figure 2-14. A solution with a pH of 0 is very acidic, a solution with a pH of 7 is neutral, and a solution with a pH of 14 is very basic. A solutionâs pH is measured on a logarithmic scale. That is, the change of one pH unit reflects a 10-fold change in the acidity or alkalinity. For example, urine has 10 times the H3O ions at a pH of 6 than water does at a pH of 7. Vinegar, has 1,000 times more H3O ions at a pH of 3 than urine at a pH of 6, and 10,000 times more H3O ions than water at a pH of 7. The pH of a solution can be measured with litmus paper or with some other chemical indicator that changes color at various pH levels. Buffers The control of pH is important for living systems. Enzymes can function only within a very narrow pH range. The control of pH in organisms is often accomplished with buffers. Buffers are chemi- cal substances that neutralize small amounts of either an acid or a base added to a solution. As Figure 2-14 shows, the composition of your internal environmentâin terms of acidity and alkalinityâ varies greatly. Some of your body fluids, such as stomach acid and urine, are acidic. Others, such as intestinal fluid and blood, are